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(March 8, 2002)
This is the first of a series of analytical papers designed to provide clarification about issues concerning offshore oil and gas production.
Ž Oil in the Santa Barbara Channel
Ž Natural Seeps
Ž Oil Spills
Ž Biological Effects of Natural Seeps and Spills
Ž Benthic organisms
Ž Marine animals
Ž Intertidal zone, saltmarshes, estuaries
Ž Summary of Biological Effects
A comparison of the impacts of natural oil seeps versus oil spills involves much more than determining the volume of oil released. Natural oil seeps in the Santa Barbara Channel introduce substantial volumes of hydrocarbons into the marine environment. Seepage rates may be on the order of 100 barrels of oil per day. Most spills associated with oil production offshore of Santa Barbara County have been small during the years since the catastrophic 1969 Santa Barbara oil spill. The Minerals Management Service estimates that total combined spill volume for the 841 reported spills between 1970 and 1999 was about 830 barrels. However, a comparison of the impacts of seeps and spills based solely on volume would be misleading. The evidence is clear that, far from being invisible against a background of seeps, major spills can have far greater and qualitatively different impacts on the environment than do seeps.
Natural seeps occur extensively in offshore waters along Santa Barbara County’s south coast, and to a lesser extent north of Point Conception. Seep hydrocarbons are released gradually throughout the marine environment, including sea floor, water column, sea surface, and shoreline. Seeps are a potential source of chronic, low-level stress to many organisms, including invertebrates, fish, marine mammals, and birds. Yet, the environment is able to keep pace with the influx of oil at the rate it enters the environment via seeps. Seeps do not result in major mortality of marine animals, nor do they lead to massive accumulations of tar along the shore, though to be sure seeps are responsible for some fatalities of birds and do leave weathered tar residue on rocks in some areas and intermittent tarring of beaches.
Major offshore oil spills are unlikely at any particular place and time, but they do occur regularly in U.S. waters. Spill response is unable to prevent major impacts from large spills. Spills differ from seeps in many ways, but the greatest difference is probably the rate of influx of oil into the environment. Spills release large volumes of oil, usually from a specific location in a short time. The sudden injection of oil into the environment can overwhelm the natural mechanisms for dispersing and degrading the oil, so that large amounts are deposited on shore, many animals are killed, and sensitive habitats are impacted, sometimes with long-term consequences. Hence, impacts on the environment from a spill are not simply an incremental addition to the impacts of natural seeps, but can be far more destructive.
Since the inception of oil drilling offshore Santa Barbara County, citizens and decision-makers have worried about the effects of oil spills on the environment. The blowout and resulting 80,000-100,000 barrel oil spill in 1969 from Union Oil’s Platform ”A” formed the foundations for a lasting public opposition of the offshore oil industry. That opposition has been reinforced by subsequent offshore oil spills worldwide, most notably the 1989 Exxon Valdez mega-spill.
The oil industry has implied, based on its spill record offshore Santa Barbara since 1969, that effects of oil spills are of little consequence in comparison to natural oil seepage from the seafloor, since estimated seep volumes far exceed recorded spill volumes during that period.  In recent years, the hypothesis that seep volume is inversely related offshore oil and gas production has been bolstered by research at U.C.S.B. and is being promoted by industry.  The idea is that offshore oil production might actually benefit the environment by reducing oil reservoir pressures in subsea oil-bearing formations, resulting in decreased oil and gas seepage into the marine environment, and less shoreline tarring. Some research has also examined the possibility that organisms near natural seeps in Santa Barbara Channel might have adapted to centuries of exposure to hydrocarbons and, therefore, have developed enhanced tolerance for oil in the environment.
This paper gives a brief overview of the nature and environmental effects of natural seeps and offshore spills and discusses some of their similarities and differences. Although both seeps and spills deposit oil in the environment and affect marine organisms, the processes and consequences differ greatly. As an examination of the distinctions between them will show, the idea that oil spills are insignificant against the background of natural seeps is unfounded. Analysis of the potential environmental impacts of major oil spills is complicated by the presence of natural seeps in the area. It is the purpose of this paper to sort out and help clarify some of the important issues.
Santa Barbara’s petroleum seeps have been much studied since the early 1970s. The most extensive survey was lead by Peter Fisher,  whose team documented some 2,000 natural seeps offshore from Point Conception to Rincon. While the most active seep areas are directly south of Coal Oil Point, several concentrations of seeps are found along the coast. As summarized by Quigley, whose Master’s thesis includes a good background review on seeps:
“Seep trends align with dominant east-west structural features which parallel the tectonic grain of the Western Transverse Ranges. Faulted east-west trending anticlines correlate with tar seeps from fractures in Miocene siliceous shales near Point Conception, and elongated seepage structures in the Coal Oil Point Area also parallel anticline crests. Dense networks of crestal fractures within the anticline axes are indicated by the absence of seismic reflections , and inferred to be the seep conduits for vertical fluid migration.” 
In other words, seeps emerge through fractures in the crests of folds in rock formations beneath the sea floor that contain oil and gas deposits. Oil and gas tend to rise and become trapped in anticlinal folds in subsea rock strata. Seepage occurs through fracture zones where the folds are truncated at the sea floor. The seeps are located mainly in bands that are oriented roughly parallel to the coastline. Maps showing the locations of seep trends may be found at the website of the UCSB Hydrocarbon Seeps Project.
The densest concentrations of seep clusters are within about three miles of the shore. Fisher et al.  describes the appearance of seeps as follows:
“Seeps may emanate from a single point or as many as 3´104 [30,000] individual seepage signals may be merged onto a high resolution profile record. The vents are commonly about one-half centimeter in diameter, but may reach as large as a meter in diameter. Divers describe high seepage area as looking ‘like a bunch of gopher holes’.”
Viewed at the sea surface, seeps range from being so diffuse that they are undetectable to appearing on the surface as areas of effervescence or boiling, measuring one to ten meters in diameter. 
A large amount of seepage takes the form of gas bubbles that emerge from the seafloor, carrying a thin coating of oil on their surfaces. Seepage also occurs as discrete oil droplets and as tar that oozes out and forms tar mounds on the seafloor. Tar mounds off Point Conception reach dimensions of eight to twelve feet in height and one quarter square mile in area.  Mechanisms believed to drive seeps include pressure in the underlying hydrocarbon reservoirs and buoyancy of gas and oil in the water that penetrates into seep fracture zones. The composition of seeps varies greatly along a continuum of varying hydrocarbon molecular weights. Seeps range from almost entirely gas (primarily methane) to oil, to heavy tar. Hence, the term “oil seep” is a rather slippery concept, that represents a diverse assortment of natural gas, oil and tar releases. Composition of seep oil may also differ from that of oil produced from wells. Analyses of seep hydrocarbons in the 1970s indicated that sulfur content of seep oil exceeded that of oil produced from wells.  There is also evidence that seep hydrocarbons have lower sulfur content than is present in the underlying reservoir, possibly as a result of biological activity within the seep fractures.  Several methods have been used to distinguish or “fingerprint” oil from different sources, including analyses of polycyclic aromatic hydrocarbons, carbon and sulfur isotopes, and trace metals. The U.S.G.S. recently developed a fingerprinting technique based on ratios of heavy, long-lived constituents of oil and tar, termed “biomarkers.” The method appears well-suited for tracing the origins of beach tar. 
As seep bubbles rise to the ocean surface, substantial amounts of hydrocarbons dissolve in the water column, forming a subsurface gradient of dissolved hydrocarbons, principally methane.  As the hydrocarbon-rich zone spreads out, methane concentrations decrease due to dilution and probably outgassing to the atmosphere.  Part of the dissolved fraction may be carried hundreds of kilometers out to sea.  The remaining seep bubbles that do not dissolve continue to the surface and burst, releasing the gaseous components to the atmosphere. The more volatile liquid seep constituents soon enter the atmosphere by evaporation. Oil slicks of varying thickness form on the sea surface and spread out under the influence of wind and currents. As the oil loses its lighter fractions and undergoes weathering, some of it sinks to the ocean floor, some is dispersed by wave agitation into the water column, and some eventually washes up on shore or sticks to rocks near the high tide line. As any local beachgoer knows, beach tar ranges from minute flecks to “tar balls” and “tar patties” that can be several inches across, occasionally forming mats a foot or more across. The amount of tar found on the beach varies greatly, day-to-day and seasonally, due to variations of sea state, wind, and currents, which affect both the dispersion of the floating oil and the trajectory of the slick.
Rates and patterns of seepage are also variable. Seeps have been observed to stop, start, grow, shrink or migrate, over both the short-term and long-term. Researchers sometimes observe a seep to be bubbling at the ocean surface one week, and yet show no signs of activity when they revisit exact same site the following week. Recently, seep rates have been shown to vary with the tides, being greatest at low tide.  Some of the observed changes undoubtedly reflect natural alterations beneath the sea floor, such as opening or obstruction of seepage paths.
Other changes may be attributable to offshore oil production. Several investigators have inferred that withdrawal of oil from the reservoirs near Platform Holly (about two miles southwest of Coal Oil Point) is responsible for decreases in seep activity in the vicinity of the platform.  Localized seepage rates are reported to have declined by more than 50% over a 22 year period.  Oil production methods, which may involve injection of gas, steam, produced water, or other materials into subsea formations, may also be associated with increases in seep activity. For example, a vigorous seep approximately 6,000 feet east of Platform Holly became active in 1973 during a time when gas was being injected into the production zone.  (The causal relation of gas injection to seepage in this case was not established positively, but the case illustrates the potential for such effects.) In addition to variations in volume, seeps also vary in hydrocarbon composition; differences are found not only among different seeps, but in a single seep zone over time. 
The Western States Petroleum Association cites a figure of 150-170 barrels of liquid oil seepage from the sea floor in the Coal Oil Point area each day.  In a discussion paper on natural seeps, the Minerals Management Service (MMS) used a seepage rate estimate of 11-160 barrels per day.  Quigley cites previous estimates, ranging from 10-100 to 25-400 barrels per day.  Quigley’s own estimate is 130-190 barrels per day, and other recent estimates fall into the same range.  The Hydrocarbon Seeps Project at U.C.S.B. puts the figure at 100 barrels (4,200 gallons) per day.  Fischer, extrapolating seep volumes for the whole Santa Barbara basin based on the distribution of mapped seeps, estimated total seep volume in the basin to be about 70% greater than for the Coal Oil Point area alone.  Seeps are known from anecdotal reports to be active in the Santa Maria Basin, but have been less studied, and seep volume estimates do not exist for north of Point Conception.  Active seeps are also known adjacent to the Channel Islands, including a large seep at Frazer Point, at the western end of Santa Cruz Island. 
In addition to their impacts on coastal water, natural seeps constitute a major source of air pollution and of atmospheric methane (a greenhouse gas). Recent estimates put total seep gas volume offshore Coal Oil Point at 100,000-200,000 cubic meters per day.  In 1982, ARCO installed two massive seep tents, 100 feet on a side, over a major seepage zone 1.6 kilometers southeast of Platform Holly. The seep tents were installed as part of an agreement with Santa Barbara County Air Resources Board, in which ARCO captured the seep gas to offset emissions of nitrogen oxides from planned future offshore drilling.  Seep gas collection peaked at over 1.5 million cubic feet (42,000 cubic meters) per day in 1985-1989, and has since been in decline. 
Estimating oil seepage is a difficult task, made all the more knotty by changeable seepage rates. As a result, the estimates are quite uncertain and could err by an order of magnitude (i.e., off by a factor of 10).  Current estimates have been obtained roughly as follows: Surveys of the Coal Oil Point seep zone were conducted using underwater acoustic profilers (sonar systems) that record acoustic return signals reflected off streams of rising seep bubbles. From this data, estimates of gas seepage are made. The volume of seeped oil is then computed indirectly from the gas seep data, by assuming a nominal ratio of seeped gas to oil. (The ratio actually varies among seeps and over time.) The resulting estimate may be extrapolated to the region as a whole, based on estimated spatial distribution of seeps.
Using a different approach, a 1976 study estimated the amount of oil floating on the surface by means of aerial reconnaissance and air photo interpretation.  Overflights covering a 36 square mile study area (from More Mesa to Gato Canyon, and extending 3 miles offshore) took place on three dates. Oil volume was estimated by combining the area of the observed oil slicks with oil film thickness estimates that were based on the distinctive appearance of oil films of varying thickness. Total volume of the slicks was estimated to be 30 barrels on October 6, 45 barrels on October 27, and 95 barrels on October 29. The build up of oil during the month was attributed to calm weather and glassy seas, which would tend to minimize the rate of oil dispersion and evaporation. The oil slicks covered 18.6 square miles, 95% of which was judged to have a thickness of about 10-5 (0.00001) inches or less, appearing from the air as “iridescence” or a “silvery sheen.” About 50 barrels of oil accumulated over two days between October 27 and 29, implying a seepage rate of at least 25 barrels per day. This is not inconsistent with other current seep rate estimates, considering the uncertainties involved.
A closely related part of the seeps puzzle, which has not yet been adequately explained, concerns the fate of seeped oil, i.e., the “sink” budget. If the environment absorbs 25 or 100 barrels of oil per day, where does it go? What fraction of it a) evaporates, b) enters the water column, c) is incorporated into sediments, or d) is deposited on or near the shore. A better understanding and quantification of the sink processes that remove seep-originated oil from the sea surface at a rate equal, on average, to the rate of seepage would provide a more meaningful context in which to discuss seeps and spills.
Oil spills in Santa Barbara’s coastal waters include possible surface releases from offshore platforms, spills from the barge Jovalon that transports oil from Venoco’s Ellwood processing facility, or spills from offshore vessels or tankers. They also include possible subsurface spills from well blowouts and pipeline leaks or ruptures. The oil produced offshore Santa Barbara is medium to heavy crude, but vessels or tankers far offshore might spill other types of crude or petroleum products. The local crude is light enough to float in sea water, and gains buoyancy by being warm, but it may sink after some weathering. Most offshore oil is piped to shore as emulsion, containing a substantial fraction of water (except for the Pt. Arguello Project, which separates the produced water at the platform). In addition to crude emulsion, the pipelines may contain diesel or anti-corrosion compounds.
Oil spills are by nature probabilistic. Individual spills are unpredictable, but the lesson learned from decades of offshore oil operations is that inevitably some spills are going to happen somewhere. Spills result from diverse causes, such as well blowouts, pipeline corrosion, pipeline damage from anchor drag, severe weather, human error, or a combination of factors. Spills range from tiny, unarguably insignificant releases, which number in the thousands, to environmentally disastrous releases, which are relatively infrequent. For the period 1969-1999, a total of 841 oil spills appear in the MMS data base for Pacific Outer Continental Shelf (OCS) operations alone.  Estimated spill volumes were less than or equal to 1 barrel for 796 of these spills, between 1 and 50 barrels for 40 spills, and equal to or greater than 50 barrels for 7 spills. The 7 largest spills include the two 1969 Santa Barbara spills, which the MMS estimated to have a combined volume of 80,900 barrels. No spills exceeding 50 barrels were recorded from 1970 to 1989, but from 1990 to 1999, there were 5 spills over 50 barrels. The largest of these (Torch Operating Company’s 1997 spill from a ruptured pipeline off Point Pedernales) is officially recorded as163 barrels. Broadening the context to include all platform and pipeline spills on the United States OCS (including Pacific and Gulf of Mexico regions), during the period 1971-1999 there were 2,125 spills more than 1.5 barrels recorded, of which 106 were in the range of 50-999 barrels.  From 1964 to 1999, there were a total of 11 platform spills and 16 pipeline spills of 1000 barrels or greater.  No platform spills of this magnitude have occurred since 1980, but the rate of pipeline spills has not slackened. Crude oil spills more than 1000 barrels from barges and tankers into U.S. waters have been larger and more frequent than platform and pipeline spills. From 1974 to 1999 there were 46 tanker spills and 26 barge spills more than 1000 barrels (187 barge spills if petroleum other than crude oil is included).  Of the 72 tanker and barge crude oil spills, 30 were over 10,000 barrels.
To fold the above data into a broad generalization, over the past three decades there have been an average of more than 4 oil spills of 50 barrels or greater from platforms and pipelines on the OCS, and about once in two years there has been a spill more than 1000 barrels. The fact that most of the spills have been in the Gulf of Mexico OCS region is to be expected, because far more oil exploration and production takes place in the Gulf than in the Pacific region. By the same token, the incidence of spills offshore Santa Barbara can be expected to increase if there is an increase of offshore exploration and production here. The MMS model for estimating oil spill occurrence rates and impacts uses estimated future “volume of oil handled” as the predictor for future spill probabilities.  The same 1982 study that introduced this model determined that the risks of an oil spill contacting coastal or marine resources is considerably greater for the Pacific OCS than for the Atlantic and Gulf regions. For the OCS as a whole, probabilities for spills of 1000 barrels or more are 0.32 platform spills and 1.33 pipeline spills per billion barrels of oil handled. This translates to one spill (of 1,000 barrels or more) offshore Santa Barbara during the next 20 years, at the current production rates of roughly 30 million barrels per year.
How to interpret such numbers, and how to evaluate impacts of improbable but possible spills, are difficult issues of public policy. Some environmental studies carried out for offshore platforms and pipelines in the past have used a probability-consequence matrix to guide risk interpretation. In this approach, the less likely a spill scenario is, the more damage is acceptable, so that if an oil spill were sufficiently improbable, its impact would be considered insignificant even if the consequences of such a spill were of major proportions. For many, such an appraisal may be intuitively troublesome, for it denies the existence of the most calamitous spills. The Energy Division is currently developing an Oil Spill Thresholds policy intended to provide guidance on the impacts of oil spills, with appropriate consideration of their probabilistic nature.
A question often arises as to what constitutes a “major spill.” The blowout on Union Oil’s Platform A in 1969 released an estimated total volume of 80,000 to 100,000 barrels of oil at rates of 500-5,000 barrels per day.  The Exxon Valdez spill released about 260,000 barrels.  At the other end of the spectrum are the innumerable very small micro-spills, typically a few gallons and usually less than a barrel. Approximately 94% of offshore spills are 1 barrel or less, and about 99% are less than 10 barrels.  The smallest spills, though they contribute cumulatively to the chronic, background oil pollution of the ocean, may not individually have a significant environmental effect. However, at some level between a few gallons and, say, 1,000 barrels an individual spill must be considered significant. That critical threshold is not a fixed number of barrels, but varies depending on the circumstances of the spill. The location of the spill in relation to sensitive environments, oil characteristics, spillage rate, weather and sea state, currents, effectiveness and rapidity of spill response, and many other factors affect spill impact.
The Outer Continental Shelf Lands Act (OCSLA) defines “major oil spillage” as “any spillage in one instance of more than two hundred barrels of oil during a period of thirty days.”  This may be a reasonable benchmark for spills far offshore and away from sensitive marine environments, but smaller spills close to shore can have major impacts. The Environmental Protection Agency considers a “small discharge” to be the equivalent of the U.S. Coast Guard’s “average most probable discharge” for determining response capabilities for oil transportation facilities regulated by both agencies; in this case, small is defined as “the lesser of 50 barrels (2,100 gallons) or 1 percent of the volume of the worst case discharge.” 
A good example is the 1997 Torch pipeline spill. Spill volume is officially estimated to be 163 barrels.  (However, by another well-founded estimate, spill volume may actually have been over 1242 barrels.  ) The spill occurred approximately two miles offshore of Vandenberg A.F.B., in 122 feet of water.  A broken flange in the 20-inch crude oil emulsion pipeline caused the accident, and operator error exacerbated it. Spill response was rapid. The accident occurred about 11 PM, and by 11 AM the next morning three Clean Seas response vessels were on scene and had deployed 3,500 feet of spill containment boom. An intense offshore clean-up effort ensued, involving over 25 vessels, 4 aircraft, and 350 personnel. The response was aided by calm wind and seas. During the next two days an estimated 27 barrels of oil were recovered from the ocean, and subsequently another 28 barrels were recovered in on-shore cleanup, leaving an estimated 108 barrels of oil in the environment.  Some of the impacts of this spill were as follows:
“Stranded oil attributed to the Spill was documented along approximately 40 miles of shoreline to the northeast, east, and southeast of the pipeline break. Also documented were a variety of oil-impacted natural resources, including plants, animals, and physical structures on sand and gravel beaches; rocky shores and seawalls; and estuaries and lagoons.” 
Of the 40 miles of shoreline affected, approximately 1.8 miles was moderately to heavily oiled. One California sea lion died as a result of the spill, and it is likely that others observed in the vicinity were exposed and suffered sub-lethal effects.  An estimated 635-815 seabirds were oiled, including at least 13 snowy plovers and 14 brown pelicans.  Stress attributable to the spill is expected to impact black abalone, a species already in decline.
“… the Trustees estimate that black abalone resources suffered a 10-15% loss in the Spill area. Other rocky intertidal organisms likely suffered similar injuries due to exposure to Torch oil.” 
The Torch example leaves little doubt that an offshore spill of on the scale of 100-200 barrels is capable of causing significant environmental damage. Smaller spills, in the 10-100 barrel range, may also have potential to cause significant damage, especially if the oil were to enter a sensitive estuarine environment.
There is a common misconception that rapid oil spill response with modern spill clean-up technology, such as provided by the Clean Seas fleet, offers an effective antidote for oil spills. Such is not the case; once in the ocean, the oil is difficult to contain and recover. Even under the best conditions of calm weather and rapid response, mechanical recovery methods (i.e., booms, skimmers, sorbent materials) are generally unable to recover more than 10-20% of the spilled oil.    The only other response option currently viable for oil spills in our coastal waters is to apply chemical dispersants. Dispersion of floating oil into the water column can be effective under some conditions as a way to reduce oiling of sea birds and marine mammals and to prevent oil from reaching the shore and impacting sensitive intertidal or estuarine habitats. But dispersants and dispersed oil have impacts of their own to organisms that live in the water column. Dispersants cannot be used in shallow water or near shore. Also, dispersants may be ineffective with heavy, local crude oils. (The “Area Committee” with primary responsibility for oil spill response offshore Santa Barbara has conducted a Net Benefit Analysis for dispersant use, and is currently finalizing a protocol specifying the circumstances under which dispersant use is appropriate.) The point here is not that oil spill response is futile, but rather that one should not be under the illusion that spill impacts can be greatly reduced by a well-planned and executed response.
As was mentioned above, small, everyday micro-spills cannot be counted as individually significant. They blend into the background level of hydrocarbons in the ocean, including inputs from both point sources and non-point sources, and including natural hydrocarbon seeps. It would be worthwhile to further clarify the distinction between background pollution and spills.
Oil enters the ocean through urban runoff, rivers and creeks, municipal wastewater discharge, marine vessels and recreational boating, permitted platform discharges, and natural seeps. The National Pollutant Discharge Elimination System (NPDES) and the California Ocean Plan both seek to minimize pollution from point sources. NPDES imposes strict standards for effluent discharge from municipal effluent and oil platforms. For example, under proposed NPDES requirements, the maximum permitted releases of oil in produced water from all of Santa Barbara’s offshore oil platforms combined is about 85 barrels of oil annually.  Actual releases could be much smaller. Although no estimates of oil inputs to the ocean from urban runoff and stream flow are available, sampling conducted by Santa Barbara’s Project Clean Water indicates that the amounts are very minor compared to natural seepage.  Seeps are most likely the largest ongoing contributor of oil to Santa Barbara’s offshore waters, at least within the Channel. (For the Southern California coastal ocean taken as a whole, contributions from anthropogenic point sources and non-point sources are believed to exceed those from seeps.  )
All of the sources listed above introduce oil over wide areas of the ocean and do so on an ongoing basis. These regular inputs of oil (seeps in particular) often cause thin oil slicks to form on the sea surface and elevated levels of dissolved and dispersed oil in the water column. Wind and wave action continuously removes the surface slicks. Major oil spills differ in that large amounts of oil are released in a small area over a short time, thus exceeding the ocean’s capacity for natural dispersion. Given the large and uncertain volume of oil from natural seeps, a 10-barrel oil spill would probably be lost “in the noise” of daily log of estimated oil inputs into the ocean, whereas a 100-barrel spill would stand out on the record as a sharp spike above the background levels.  This is not to say that a 10-barrel spill would cause no impacts. However, it does suggest that the class of smallest spills, especially micro-spills less than 1 barrel, are likely to be “below the radar” as discrete spills, unless they occur near shore. Therefore, it may be best to place micro-spills in the same category as other, ongoing oil inputs to the ocean, which have combined, cumulative impacts, but which do not stand out individually as major pollution events.
Both natural seeps and major oil spills affect sea birds, marine mammals, and aquatic organisms and ecosystems. However, because spills are much more concentrated, their impacts on marine organisms are more intense, and in many respects qualitatively different than those of seeps. Santa Barbara’s seeps release oil over tens of square kilometers every day, and the atmosphere and water column take it up at about the same rate. Toxic constituents are released steadily, but gradually, over the region, allowing currents and natural mixing to dilute their concentrations. Tar mounds on the ocean floor are colonized by bacteria, forming the basis of productive meiofaunal communities. Seep oil does not accumulate on the surface in very thick layers, nor does it cause oiling of many birds or result in heavily tarred beaches. Rocks and cliff faces in some areas show localized deposits of weathered tar.
Major spills, however, may blanket the sea surface of a large area with fresh oil. A thick, gooey water-in-oil emulsion, or “mousse,” often forms on the surface after oil spills, eventually falling to the ocean floor in large amounts or fouling the intertidal zone, beaches, rocky shores, and salt marshes. Organisms including larvae may have no opportunity to escape the sudden influx of oil and high concentrations of its dissolved toxic fractions. Spills often kill large numbers of animals including sea birds and marine mammals. For these reasons, Burger states that, “in any given area, the amount of oil from a catastrophic spill far overshadows the oil coming from natural seeps.” 
Petroleum distribution in sediments is patchy throughout the seep-affected region, ranging from background levels below 100 mg/l to 90,000 mg/l.  As a result, the impact of seeps on benthic organisms (those found on the ocean floor) varies greatly. Near seeps, bacterial mats thrive at the interface between oil-rich, anaerobic sediments and the oxygen-containing water above. “White mats of Beggiatoa spp., the sulfur oxidizing bacterium, may be found together with large populations of nematodes.”  Several studies have demonstrated that benthic productivity and biomass is elevated adjacent to the seeps. More moderately oiled areas at a distance from the seeps are correspondingly less affected, and benthic populations resemble those typical of the southern California shelf.  In a general sense, communities near natural seeps have adjusted to a hydrocarbon-rich environment. Oil-degrading organisms utilize the hydrocarbons as a source of energy. Organic enrichment stimulates the growth of these organisms, potentially leading to increased abundance of the organisms that feed upon them. 
Studies indicate that species diversity and community structure of infauna near the Coal Oil Point seeps are highly similar to the typical fauna elsewhere in the region, the main difference being a greater abundance of oligochaetes in the near-seep environment.  Recent work carried out at a finer spatial scale showed densities of nematodes to be positively correlated with the amount of oil present, whereas all other major taxa were negatively correlated. There was evidence of a “halo effect,” described as follows: “infaunal densities are very low in the most contaminated microsites within a seep area (due to toxic effects), reach a maximum at ‘clean’ sites within the seepage area (due to enrichment), and are low again at distant sites (50 to 1000 m from the seeps) that are unaffected by oil seepage (i.e., where there is no enrichment).” 
Palmer et. al. found that colonization by meiofaunal organisms (organisms smaller than 0.5 mm) occurred more rapidly at seep sites than at non-seep sites. Despite this faster recovery rate, meiofauna are unlikely to survive in areas near natural seeps where oil settles on the sediment surface. Survival is achieved only by moving to nearby areas where no oil has accumulated or where the oil has been dispersed.  In this study, Palmer et. al. viewed seeps as natural disturbances, because the location and activity of seep sites change frequently and unpredictably. Spies and Davis, on the other hand, characterized seeps as sources of persistent hydrocarbon exposure, that provide an opportunity to study sub-lethal effects of hydrocarbons on benthic ecosystems.  Spies and Davis contrasted gradual, chronic inputs of oil from seepage with major oil spills, which, with a “single injection” of oil into sediments, can kill most of the infaunal organisms, as was well documented for the West Falmouth oil spill of 1969. 
Oil spill impacts on benthic organisms may result from smothering or oil toxicity, with possible long-term consequences. Hydrocarbon exposure is known to cause a broad range of lethal and sublethal effects on bottom-dwelling organisms, including reduced reproductive success, and may perturb entire populations.  Notwithstanding the studies of ecology of Coal Oil Point seeps, there are many reports of oil spills and low-level oil pollution causing changes in benthic community structure.  In some cases recovery takes years. For example, five years after the 1969 spill from the barge Florida the normal balance of benthic invertebrates was not yet restored.  In this case, diversity of subtidal organisms decreased immediately following the spill. An opportunistic polychaete worm species underwent a population explosion and subsequent decline, as other native species began to reestablish themselves. There are also many reports of species surviving in highly oiled sediments and of rapid recolonization. Palmer et. al. noted that, “meiofauna may survive oil spills and, for some species of meiofauna, colonization of oiled sediments may occur at rates comparable to the colonization of untreated azoic and natural sediments.”  Thus, some of the smallest benthic organisms that make up the base of the food web may be able to recolonize areas affected by oil spills fairly quickly, though the community may be impacted for a long time.
Exposure to whole crude oils as well as specific compounds can have immediate and pronounced impacts on rates on photosynthesis and growth of phytoplankton, including blue-green algae, green algae, diatoms, dinoflagellates, and chrysophytes. Many studies show reductions in photosynthesis and growth of phytoplankton exposed to oil.  Growth lag or lethality has generally been observed at hydrocarbon concentrations of 1-10 mg/l.  Effects of dissolved or dispersed oil on phytoplankton appear to be complex and species-dependent. In laboratory experiments, both inhibition and stimulation of phytoplankton growth have been observed in response to oil exposure; oil exposure has had no measurable effect on some species.  Sensitivity also varies by type of oil, toxicity of crude oil generally being lower than that of refined oils. Increases in flagellate populations have been observed following oil spills in a few cases; this may be a secondary effect, possibly resulting from relaxation of grazing pressure by zooplankton (which themselves are impacted by the oil) or from shifts in the balance and dominance of species in the phytoplankton community related to nutrient availability. Low-level oiling has been shown to induce replacement of large phytoplankton species with smaller ones; it is speculated that this might lead to replacement of larger zooplankton species with smaller species, which are less favorable food for juvenile fish.  Spies cites an experiment in which depressed respiration and productivity of phytoplankton was observed two weeks after oiling of estuarine ponds. However, the scenario is different if oil is introduced into marine waters with adequate dilution and turbulent mixing. Phytoplankton regenerate in as little as 9 to 12 hours.  They may immigrate rapidly and in large numbers from adjacent unaffected waters. Regeneration occurs rapidly once toxicity levels have decreased due to weathering of the oil.  Therefore, many authors conclude that the phytoplankton population can be expected to rebound to pre-spill levels rapidly once the contamination levels have decreased.  
The evidence for impacts of oil on zooplankton is substantial. Lethal oil concentrations are in the range of 0.05 to 9.4 mg/l. “A whole array of sublethal effects have been seen below 1 mg/l and these include feeding and other behaviors as well as reproduction and development, which seem to be particularly sensitive.”  Effects may extend to concentrations as low as 10 mg/l. Similar lethality thresholds of soluble hydrocarbons (0.1-10 mg/l) have been estimated for fish eggs, larvae, and pelagic crustaceans.  Observations of impacts on of zooplankton following spills have been variable. Abundance and species composition did not change dramatically following several large oil spills, including the Torrey Canyon, the Santa Barbara blowout, the Argo Merchant, or the Kurdistan; however, in these cases limitations of sampling precluded detection of less-than-drastic effects.  A more careful investigation following the Bravo spill found no acute effects on zooplankton, although hydrocarbon levels up to 256 mg/l were measured in the water column. Conversely, biomass of zooplankton decreased dramatically following the Tsesis spill in the northern Baltic Sea.  After the Amoco Cadiz spill, high mortality and a widespread metabolic disorder in zooplankton persisted for 15 days in offshore areas and 30 days near shore. 
As with phytoplankton, high reproductive rates of zooplankton, coupled with immigration, can repopulate the spill area fairly quickly following a spill. Concerns other than total plankton biomass remain. Changes in structure of the planktonic community may take place as a consequence of oil pollution.  Very low levels of pollution (0.2 mg/l) may affect fish larvae, thus impacting fish populations.  Contaminated plankton are ingested by fish, and contaminated planktonic feces settle into the benthic zone. We are not aware of any evidence that dissolved hydrocarbons from natural seeps affects plankton productivity, populations or community structure. Seeps visibly do affect individual organisms in the planktonic community, such as jellyfish, which have been found dead, floating in seep oil slicks.  It would be premature to conclude, based on a lack of evidence of chronic low-level impacts and on the rapid regeneration of plankton populations following a spill, that there are no long-term effects involving plankton. Spies notes that, “…because of turbulent mixing in the sea pollution biologists consider the impact of petroleum on plankton as an extremely difficult problem to assess in field studies.” 
Exposure to oil in the sea is detrimental to marine mammals, turtles and other animals. Refined petroleum products are more toxic than crude oil, and fresh oil more toxic than weathered oil, with toxicity varying in relation to concentrations of the most volatile, soluble, and toxic constituents. Causes of damage include inhalation and ingestion of hydrocarbons, surface contact, oral obstruction in young turtles, and possibly fouling of baleen in whales.  Severity of effects depend on toxicity and concentration of the oil, exposure time, and other factors, including susceptibility of the species, and age and condition of the individual animal. Inhalation has variable effects on lungs, liver, nervous system, and other organs; more intense exposures lead to death.  Oil can be ingested directly, through grooming of oiled fur, or through eating contaminated prey. Ingestion of oil can damage stomach and intestines, and toxic constituents may be absorbed into the blood, leading to systemic damage.  Severe irritation of mucus membranes may occur. Contact with oil compromises insulating properties of fur, leading to hypothermia in sea otters, particularly young animals that lack insulating fat.  Sea turtles that have come in contact with oil suffer from a variety of ailments, including skin sores, digestive tract inflammations, and eye and nose injuries. Weakened immune systems and lower hatching success of eggs is common in turtles that have been exposed to oil. 
In the aftermath of the Exxon Valdez spill, scientists discovered that seal carcasses had elevated levels of hydrocarbons in their mammary tissue and milk. Some developed brain lesions in the thalamus region, making it difficult for them to perform normal tasks. This doubtlessly led to a reduction in predator evasion and food procurement. As a result, seal numbers, 34 percent lower on the oiled beaches three years after the spill, remain far below prespill numbers even today.  Inhalation or ingestion of oil can cause internal lesions in the liver and kidney. Interestingly, “seals can metabolize and excrete some of the oil that ends up in the liver and kidney, but this mechanism is less effective in young than adults, and is not effective with high exposure to oil.”  Thus, seals may have adapted to low levels of hydrocarbons in their environment, such as might be found at natural seeps, yet may be unable to deal with the large doses of toxins present in oil spills.
Most marine animals do not appear to have adapted to hydrocarbon releases. The long life cycles of these animals makes long-term, multi-generational adaptation unlikely, and scientists have hypothesized that “only animals in the area where the highest concentrations occur are likely to adapt to petroleum.”  However, behavioral adaptations have been observed. It appears that some animals are able to detect and avoid floating oil from oil spills, while others are not. Pinnipeds, sea otters and turtles do not seem able to avoid oil, whereas some evidence indicates dolphins do.  There are anecdotal reports that sea lions are attracted to seeps, and they have been observed lounging in the slicks.  Following contact with oil, sea otters have been observed to spend most of their time under water, grooming to remove oil from their fur. Although whales do not avoid oil,  gray whales have been observed to spend less than normal time on the surface when oil is present. Animals such as harbor seals and sea lions that swim through oil slicks and haul themselves out on rocks that may be covered with oil can be especially hard hit by oil spills. Unlike fish that can swim out from underneath an oil spill, marine mammals and sea turtles must come to the surface for air, thereby increasing their contact with floating oil. It should not be assumed that marine animals behave the same toward slicks from natural seeps as they do toward spills. For example, otters may avoid slicks occurring in locations they know and may avoid food sources near seeps, and yet be vulnerable to oil that appears suddenly in their surroundings from oil spills. 
Following Exxon Valdez, which caused the deaths of thousands of marine mammals, there have been several studies of oil impacts on marine mammals, including killer whales, harbor seals, sea otters and sea lions.  The studies leave no doubt that a massive oil spill in a sensitive environment is a recipe for disaster. The same conclusion is evidenced by observations following many previous spills, including the 1969 Santa Barbara blowout. The list of animals probably killed in oil spills includes gray seals, harbor seals, northern fur seals, northern elephant seals, harp seals, California sea lions, sea otters, gray whales, harbor porpoises, fin whales and minke whales.    While large spills are predictably devastating, it is more difficult to understand or predict effects of smaller spills on marine mammals, for it is a matter of chance, as opposed to near-certainty, that some animals will encounter heavy slicks or oiled shores. Thus, in the 1997 Torch spill, “one California sea lion died as a result of the Spill,… It is likely that other pinnipeds observed in the proximity of oil were exposed and suffered sub-lethal effects.” 
In contrast to small benthic organisms and phytoplankton, organisms at the higher trophic levels are less numerous, live longer lives, and produce fewer offspring. Thus, their populations are less able to rebound from larger insults to the environment. For example, the number of sea otters in Prince William Sound following the Exxon Valdez oil spill declined 35 percent, “and show no recovery in the areas hit hardest.” 
There is little evidence that natural seeps are responsible for deaths of marine mammals. Cases of fatalities attributable to seep oil are rare, as are findings of oiled animals. The BeachCOMBERS program, organized by the Otter Project, began collecting data on dead birds and marine mammals on Santa Barbara County beaches in August, 2001.  Pairs of trained volunteers walk nine beaches once per month, making careful observations of dead animals. No attempt is made to determine cause of death, but oiling is noted, if present. During the first seven months of observations (August, 2001, to February, 2002), 26 pinnipeds, one whale, and no otters were found on the beaches. Of these 27 mammals, 24 were not oiled and three were too decomposed to identify any possible oiling.  Previous records are scant. The carcass of one old male otter washed up near Coal Oil Point on July 27, 1998. The animal was externally oiled, and analysis of the oil pointed to natural seep origins.  However, the otter’s teeth were badly worn down, indicating the animal’s survival was marginal, independent of oiling.  (Otters must eat approximately one quarter of their body weight daily, so bad teeth are seriously debilitating.) Oiling may have been a contributing factor in the otter’s death.
Although seep-related fatalities are rare, low-level hydrocarbon exposure might be a significant stressor for animals living in seep areas. Systemic poisoning from chronic exposure could weaken the animals, making them more vulnerable to disease and other perils, including oil spills. Studies on mink, a semiaquatic mammal, indicate that ingestion of oil has toxic effects on reproduction, though hydrocarbons are undetectable in the animals’ tissues on necropsy.  Litter size decreased from an average of five to two when the food was contaminated with crude oil, and to less than one when the contaminant was bunker oil. The offspring had below normal reproductive success, though they ware not exposed to oil. It is possible that marine animals living offshore Santa Barbara suffer sublethal effects from chronic exposure to seep oil, but there is no direct evidence.
Barred sand bass, kelp bass, sanddabs and several species of surfperch are some of the more commonly observed fish near Coal Oil Point.  In a study by Montagna et. al., the distribution and abundance of these fish and meiofauna at seep sites and non-seep sites were compared. Although the study attempted to analyze the impact of fish predation on meiofauna, the authors established the similarity of average abundance, distribution, and diversity of fish at both sites.  Thus, it would appear that the presence of small amounts of seep hydrocarbons in the water does not affect fish numbers, although the transient nature of fish makes this difficult to establish factually. Some fish may even be attracted to seeps, as illustrated in photographs displayed on the UCSB Hydrocarbon Seeps Project website. 
While numbers of fish observed may not be affected by hydrocarbon exposure, the health of individual fish may: “both species of surfperch from the seep environment had significantly elevated concentrations of hydrocarbons (and their matabolites) in their bile compared to fish from the reference areas.”  Rainbow surfperch living in proximity to seeps were found to have high incidence and severity of gill and liver lesions compared to non-seep fish, whereas rubberlip surfperch did not have abnormal lesions. Rainbow surfperch have a benthic feeding habit and more limited movement than rubberlip surfperch, which may explain the difference in symptoms between the species. The similarity of numbers of fish in non-seep areas and unhealthful seep areas can easily be explained: any fish that eventually die from complications of hydrocarbon exposure may be replaced by other fish looking for the abundant benthic food found near seeps. Both species of fish had napthalene concentrations of 59 to 111 mg/g and phenanthrene concentrations of 74 to 136 mg/g in fish near a seep site. These levels exceed those in fish from non-seep sites, but are less than those observed in the bile of white sturgeon (approximately 200 mg/g for each compound) in the Columbia River, Washington after an oil spill. 
There are many studies demonstrating lethal and sublethal effects of oil on fish.   Oil may be taken in by ingestion of oil or contaminated prey, or through the gills or other epithelial tissues. Developing eggs and larvae are more vulnerable to hydrocarbon exposure than are juveniles and adults.  Viability of larval fish may be compromised at the very low concentrations of 2-10 mg/l, and sublethal effects been observed as low as 0.2 mg/l.  Eggs of pink salmon and Pacific herring were both adversely affected by concentrations of toxic oil constituents (polynuclear aromatic hydrocarbons or PAHs) less than 0.5 mg/l, resulting in malformations, genetic damage, decreased size, inhibition of swimming, and mortality.   The salmon eggs accumulated PAHs to concentrations more than two orders of magnitude greater than in the surrounding water,  making them vulnerable to extremely low levels of dissolved PAHs. Contrary to expectations, the PAH molecules leached into the water from well-weathered oil proved more toxic than lower molecular weight PAHs from less weathered oil.  This suggests that that low concentrations of PAHs gradually released from weathered oil that persists in sediments following a spill or from natural seepage, could accumulate in fish eggs, to the detriment of fish populations.
Adult fish experience increased heart rate and respiration due to hydrocarbon exposure; effects on growth and gonad development have also been observed.  Other effects include damage to liver, gills, gut, stomach, brain, olfactory organs, and various physiological abnormalities.  The resulting stress can increase susceptibility of fish to disease, impair their ability to cope with other natural stresses, and lead to dysfunctional behavioral responses that may impact survival or reproduction.  Fish can accumulate, metabolize, and excrete oil constituents and their metabolites via liver and gills. The ability of fish to eliminate toxic hydrocarbons may lessen the toxic effects of exposure to oil.  It also implies that adult fish might suffer stress and complications from hydrocarbon exposure without necessarily retaining high levels of hydrocarbons in their tissues.
There is much evidence that oil spills kill fish in their developmental stages.  “Contact with oil on the surface or with dissolved or dispersed hydrocarbons in the upper water column may kill large numbers of embryos and larvae.”  Losses of salmon fry and herring eggs from the Valdez spill in 1989 were considerable; an estimated forty percent of the 1989-year class of herring of Prince William Sound were exposed at toxic levels.  Natural mortality rates for developing fish are extremely high, and populations vary greatly from year to year, which could lead one to conclude that loss of one generation will have little overall effect on the adult population;  the opposite conclusion (i.e., that loss of a generation is a significant impact) seems more tenable, particularly in view of already declining fish populations in coastal waters. In contrast to eggs and larvae, adult fish are usually able to move away from the area of an oil spill in open ocean; at least they do not remain in highly oiled areas for long enough to suffer immediate mortality. Spies notes that “Extensive fish kills have been associated with only a few spills. In the Florida spill large numbers of scup (Stenotomus chrysops) and tomcod (Microgadus tomcod) washed up on silver beach, North Falmouth. After the Amoco Cadiz spill large numbers of fish washed up on the beach at Brest. Unfortunately the doses of oil causing these kills are not known.” [references omitted] 
Long-term effects of oil spills on fish populations may be a more serious concern, because the immediate death toll of adult fish is usually small, except for the most catastrophic spills. Residual oil in the environment, on rocky shorelines, in sediments, in the food web, may continue to affect fish populations for years. In addition to the toxic accumulation of PAHs in fish eggs, discussed above, fish are potentially exposed to hydrocarbons in their food sources, including benthic organisms and nearshore invertebrates. “Elevated hydrocarbon levels in nearshore invertebrates would be likely, leading to increased stress and potential decreases in growth and reproduction in fish feeding on the invertebrates. These effects are expected to be short-term under normal conditions; however, oil may become sequestered in the sediments of low-energy embayments and persist for several years.” 
Following Valdez, several species showed reduced growth rates in the following season.  This led to higher death rates for juvenile fish due to decreased ability to avoid predation and to endure other environmental stresses.  The persistent effect of slowed growth rates that were observed in the years following the spill indicated that many fish were exposed to hydrocarbons by eating contaminated prey since levels of oil in the water column had declined to sub-lethal levels.  In 1993, the adult Pacific herring population of Prince William Sound collapsed as a result of disease, that may be linked to aftereffects of the spill.  Pink salmon showed increased egg mortality through 1993. Salmon migration returns were also impacted for several years.  One study suggested that egg mortality in 1991 may have resulted from genetic damage to the parent fish, which had hatched in oiled streams.
Stress experienced by fish following an oil spill would be additive to stresses from other sources, natural and anthropogenic. Hydrocarbons in the environment from natural seeps constitutes a potential source of stress to some fish. In addition, as noted in the recent Delineation Drilling Draft EIS, “EFH [essential fish habitat] in the Southern California Bight is stressed due to overfishing, and degraded water quality in estuaries south of Point Conception.”  Although the authors go on to say that impacts from an open ocean spill would be short term, “not expected to last more than a few days,” the lesson from Valdez is that effects of a really big spill can persist for years.
The effects of oil on marine birds are concisely summarized by Hunt:
“Major spills can directly destroy large numbers of adults and indirectly result in the starvation of nestlings deprived of food. Less dramatic long-term, chronic pollution or disturbance may also have detrimental effects on marine birds or their food supplies. Low levels of pollution may increase adult and juvenile mortality through fouling or ingestion, and sub-lethal amounts of ingested oil may lower reproductive success. Finally, disturbance of birds at colonies may reduce reproductive success or cause desertion.” 
From another perspective, birds exposed to oil may suffer from direct contact, toxic effects, damage to habitat, and contamination of food sources.  Types of damage include loss of buoyancy, loss of insulation, and physiological effects of oil ingestion, which may occur directly from water or prey, or as the birds try to clean their plumage.  Research, mainly laboratory studies of domestic or captive birds, point to several physiological effects: exposure of females to oil prior to egg laying reduces number and viability of eggs and leads to chick deformities; exposure of eggs to oil smeared on them (to simulate contact with oiled plumage) reduces hatching rate; chick growth and survival may be reduced when they ingest oiled food provided by the parents.  “Birds that receive lethal doses succumb to a host of physiological dysfunctions (e.g., inflammation of the digestive tract, liver dysfunction, kidney failure, lipid pneumonia, and dehydration).”  What constitutes a lethal dose depends on many factors, including amount and toxicity of the oil. Wasting of fat and muscle tissues, and many pathological conditions have been observed in oiled adult sea birds, but it is uncertain whether these are effects of stress or toxicity; the actual cause of death is most often hypothermia or drowning. 
Some birds may be adept at avoiding oil slicks from natural seeps.  Adult and young shearwaters completely avoid seep areas. Adult pelicans and sea gulls are less often oiled than juveniles, indicating that for those species seep avoidance is a learned behavior. Little is known about the effects of natural seeps on bird populations; however, floating seep oil does take a toll. The Santa Barbara Wildlife Care Network recovers an average of about fifty oiled birds from the beaches of Santa Barbara and Ventura Counties each year. Some are treated and released, but the majority die. No attempt is made to determine the cause of death. Oiling may be the cause in some cases, but is probably only a contributing or incidental factor in others. Most of the birds are presumed to have encountered oil slicks from seeps. Unfortunately, there are few statistics on oiled dead birds. The BeachCOMBERS program, described in the above section on marine mammals, found 158 dead birds on nine area beaches during the first six months of beach monitoring. Eight of the carcasses showed some degree of oiling, which could have occurred either before or after death.
In dramatic contrast to natural seeps, which are an ongoing cause of death of relatively small numbers of birds, oil spills kill birds en masse. “Birds are probably the most conspicuous casualties of oil pollution in the sea.”  Thousands to tens of thousands of birds have died in each of many documented oil spills.  The number of dead is difficult to estimate, as only a fraction of the carcasses are recovered and tallied. Also, continuing, sub-lethal effects on exposed birds, and their eggs and young, can be expected to continue until long after the visible evidence of a spill is gone. Spill cleanup activities can exacerbate the impacts, by disturbing sensitive habitats and flushing birds out into oiled water.  Estimates of dead birds following the Exxon Valdez spill range from 250,000 to 645,000.  In Torch Operating Company’s relatively small (163 barrel) spill in 1997, an estimated 635-815 seabirds were oiled, including at least 13 snowy plovers and 14 brown pelicans.  Post-spill censuses of snowy plover populations on affected beaches showed marked decreases, as compared with pre-spill counts. Depending on the time of year and spill location, the degree of impact and the bird populations most seriously affected would vary. For example,
“ If the spill approaches the mainland coast, especially if it occurs during the winter months when many nearshore species are in the project area, the birds that would most likely be affected include loons, western grebes, California brown pelicans, cormorants, and surf scoters.” 
Depending on the oil spill trajectory, shorebirds, waterfowl, and marshbirds might be affected; probably the most serious impacts would occur if a spill reached the Channel Islands during the breeding season.  Even a spill of less than 200 barrels offshore has potential for significant impacts to threatened and endangered birds, including the California brown pelican, California least tern, light-footed clapper rail, and western snowy plover. A large spill of, say, a few thousand barrels, would have far greater impacts. A helpful review of effects of oil on birds may be found in the National Wildlife Healthcare Center’s Field Manual of Wildlife Diseases.
The previous sections focussed on particular classes of marine life, comparing effects of oil seeps with effects of spills. This section will touch on effects of oil at the tidally influenced zones where sea meets shore. As summarized in the 1985 National Research Council report,
“The margins of the sea are particularly susceptible to the impact of oil pollution. They are subject to heavy oiling when a large spill drifts ashore, with a fraction of the oil becoming sequestered in sediments and persisting in some cases for years. This is in marked contrast to conditions in the open sea, where currents and diffusion usually rapidly reduce the concentration of petroleum, making it less toxic and most likely more amenable to degradation processes.” 
The shoreline is also where most people encounter the effects of oil seeps and spills, and therefore where impacts are most noticed.
Tar from the natural seeps off Coal Oil Point travels north to Jalama Beach and beyond, and south as far as Santa Monica Bay.  Deposits are found on rocks in many areas; tar balls and flecks of weathered oil are found intermittently on area beaches, marking the high tide line. Shoreline tarring is heavier near seeps, where tar may persist for years. If any seep tar enters estuaries, lagoons, or saltmarshes, the quantities are probably inconsequential.
There is little reason to believe natural seep oil deposited in the rocky intertidal zone significantly affects the organisms that live there. An extensive study by Straughan compared condition and abundance of a number of intertidal and subtidal species living near Coal Oil Point to animals in carefully chosen control areas outside the influence of seeps.  The results indicate that chronic hydrocarbon exposure from natural seeps probably has little adverse impact on major intertidal species. Mussels showed no adverse effects; in fact the animals from Coal Oil Point apparently had greater tolerance for Santa Barbara crude oil than did the same species collected elsewhere. (Tolerance was also observed in larvae of sea urchins.) Two species of barnacle showed no difference in brooding rate between seep locations and control sites. A stalked barnacle had a reduced brooding rate at the seep location, which the authors attributed to increased fluctuations in body temperature at low tide, due to the heat absorption by tar adhering to the animals’ carapaces.
The current understanding is that the impacts of natural seeps on intertidal habitats is quite limited. Natural tarballs that adhere in the barnacle zone may persist for extended periods. For example, “Barnacles covered by natural tar die; however, barnacles will recruit and establish new populations on top of residual tar (Raimondi, 2000). Impacts from natural seeps are patchy and chronic and represent low impacts.”  Because the rocky intertidal zone is subject to intense washing, and the influx of seep oil is gradual and intermittent, the environment is able to keep pace with the gradual influx of seep oil. However, the intertidal zone is vulnerable to oil spills. Each high tide potentially carries a new load of fresh oil. Raimondi offers a useful perspective:
“We believe that in the short term the upper zones (Endocladia and barnacles) are most likely to be affected [by an oil spill]. However results from our other shoreline studies suggest that these are likely to be the species most able to quickly recover via recruitment. By contrast, mussels and fleshy algae are more resistant (for different reasons) to oiling but are much less likely to recover quickly if damaged via an oil spill.” 
A major oil spill, as opposed to natural seepage, can cause severe damage to intertidal ecosystems. Extensive stretches of intertidal habitats may be heavily oiled, resulting in death to plant and animal populations through toxic effects, clogging of feeding systems, respiratory damage and outright smothering.  Differences in sensitivity among species may throw the community out of balance, as opportunistic species dominate the affected areas.  Hydrocarbons enter the food chain through sand crabs, mussels and other organisms. Birds and mammals that utilize the intertidal zone are impacted both directly and through contaminated food. Mussels were heavily impacted following Exxon Valdez; years later mussels in the spill area showed high levels of hydrocarbons in their tissues and continued to be a source of contamination in the food chain.  For the rocky intertidal zone, “The primary concern would be direct contact with long-lived animals, such as seastars, limpets, abalone, and important communities such as algal assemblages and mussel beds.”  Algae and surf grass may also suffer long-term impacts.  Oil may penetrate deeply and be buried in sandy beaches, leading to slow releases of toxic hydrocarbons into the environment. Clean-up activities may also have serious impacts to buried organisms. Sandy beaches may recover in weeks, for lightly oiled beaches, but recovery can take from two to seven years in heavily oiled areas. 
Oil from the 1997 Torch spill, though the volume was fairly small in comparison to other plausible scenarios, affected an estimated 21 miles of sand and gravel beaches, 18 miles of rocky shores and seawalls, and 1 mile of estuaries and lagoons. Oiling was moderate to heavy for 1.8 miles of sand and gravel beaches, and light to very light in the remaining areas.  In heavily oiled areas, the beaches were covered with huge blobs of oil. Analysis of the effects on rocky intertidal organisms, which were not heavily oiled, revealed local decreases in abundance of barnacles, mussels and sea grass.  However, conclusions about the causes of the observed changes were confounded by other factors, particularly effects of El Nińo enhanced storms during the winter of 1997. The cause of barnacle impacts were indeterminate, while mussel and sea grass impacts were attributed to storm damage. Impacts on black abalone were also suspected, could not be separated from effects of a disease that has brought about a decline in the population since 1992.  Unquestionably, had the spill been larger, say 2000 barrels, environmental damages would have been far greater.
Estuaries and saltmarshes are considered to be the marine environments most sensitive to oil spills for several reasons.  They contain rich and diverse flora and fauna, serve as nurseries for many fish species, and are habitat for waterfowl. They are sheltered from waves and strong currents, and lack the natural mechanisms for vigorous scrubbing, dispersion, dilution, and flushing of oil that are present on exposed shorelines.  Marsh vegetation is easily coated with oil, which blocks water and nutrient uptake and gas exchange, leading to death. Recovery can be very slow, sometimes measured in decades.  Spilled oil may directly foul organisms, become incorporated in sediments, cause widespread toxic impacts, and enter the food web. As the tide rises and falls, the vegetation and bottom sediments are alternately dosed with oil. A well studied case is Exxon’s 1990 pipeline spill into the Arthur Kill, a fifteen mile long estuary between Staten Island and New Jersey.  An estimated 13,500 barrels of crude oil entered the estuary, oiling tidal creeks and marshes. Immediate impacts included 20% loss of vegetation, high rates of mortality for a number of species, including fiddler crabs, grass shrimp, and mussels, and also large numbers of deaths among blue crabs, Diamondback terrapin, gulls, ducks, herons and muskrats. Some effects on vegetation, populations, and reproduction persisted for at least six years.
One of the priorities for oil spill response would be to protect estuaries and wetlands by deploying oil containment booms, however containment would almost certainly be ineffective, except in the case of a relatively small spill during a period of atypically calm winds and smooth seas.
1. Benthic community
Bacteria on the ocean floor colonize seeps and gradually degrade hydrocarbons deposited there, boosting productivity of benthic communities. Organisms around seeps are influenced by both hydrocarbon toxicity and the increased food supply, which affect their spatial distribution. Oil spills, in contrast, may smother or poison seafloor communities. Meiofauna, appear able to recolonize impacted areas quickly, but it can take years for benthic populations to come back into balance following a spill.
Reaction of phytoplankton to oil in the water column is variable, whereas zooplankton may be killed by very small concentrations of hydrocarbons. There is no evidence that the plankton community is adversely affected by natural seep oil. Major spills may decimate zooplankton populations, but populations rebound quickly, due to immigration and rapid regeneration. Larvae coexisting with plankton in the water column are killed and recover less quickly. Subtler effects of massive plankton kills from spills include disturbance to community balance and injection of large quantities of hydrocarbons into the food chain.
3. Marine animals
There is little evidence of fatal impacts of natural seeps on sea otters, pinnipeds, whales, and turtles, although sub-lethal effects are possible. Some species may avoid seep areas, and slicks formed by natural seeps may not be thick enough to cause frequent, serious oiling. Major oil spills, on the other hand, can have devastating impacts. The heavy concentrations of oil can create floating traps of gooey “mousse.” Animals may ingest toxic quantities of hydrocarbons or may suffer other effects of contact or physical fouling. Sea otters and seals may be heavily oiled, compromising the insulating properties of their fur, and leading to death through hypothermia. When major spills impact areas populated by marine animals, dozens to thousands may be killed.
Some evidence exists of sublethal effects of seep hydrocarbons on some fish species, and dissolved seep hydrocarbons might conceivably impact fish eggs. Although large fish kills have been documented following a few oil spills, in most cases adult fish escape apparent injury. The main vulnerability of fish is to their eggs and larvae, which are very sensitive to the toxic PAH constituents of oil. In some cases, impacts on fish populations persist for years after a spill, due to residual hydrocarbons in the physical environment and in the food chain.
Sea birds suffer some limited impacts from natural seep oil. Oiled birds are recovered from Santa Barbara County beaches at a rate of about 50 per year; many additional birds undoubtedly die or are adversely affected, however, the numbers are not known. Sublethal effects and decreased viability of eggs are likely. Birds are among the animals hardest hit by oil spills. Hydrocarbon poisoning by ingestion, loss of insulating properties and buoyancy of feathers, and damage to eggs that come in contact with oiled plumage are some of the effects. Death tolls range from the hundreds (approximately 600-800 in the case of the 1997 Torch spill) to the hundreds of thousands for catastrophic spills like the Exxon Valdez.
6. Intertidal zone, saltmarshes, estuaries
Seep oil, which reaches the shore mainly in a partially weathered state, has little impact on intertidal organisms. Some barnacles die when random tar balls adhere to their shells, but others soon colonize the tarred surfaces. The slow pace of shoreline fouling from seep oil does not overwhelm the intertidal communities. Some evidence exists for increased oil tolerance by the local mussel sub-population, but evolved tolerance is unlikely in general, and does not protect the animals from smothering or other consequences of major spills. The amount, if any, of natural seep oil entering estuaries and saltmarshes is thought to have insignificant impacts. In contrast, major spills can devastate intertidal habitats, estuaries, and saltmarshes, as demonstrated in many historic spills. Long term effects of spills on both flora and fauna can persist for years, or even decades.
From the foregoing, it is obvious that the effects of seeps and spills differ hugely. As one planner put it, “If seeps and spills are the same, why aren’t all the beaches covered with mounds of fresh tar and dead birds?” The key difference has to do with release rates and spatial concentration of the oil. Seeps release large amounts of oil over large areas of the ocean gradually throughout the year. Spills release large amounts of oil from a point source in a short time. Natural seeps and spills differ in that seep rates do not, on average, exceed the marine environment’s capacity to digest the oil, whereas spills may exceed its capacity. Major spills overwhelm nature’s mechanisms for processing the oil, in the short term. The consequences include severe oiling of shorelines and mortality to organisms that are ill-prepared to live in an oil-soaked environment.
Other factors, in addition to release rate, could influence distribution and impact of oil in the marine environment. Seep oils vary in composition, which may affect toxicity of the dissolved and dispersed fractions, as well as the residual tar. Oil spill impacts depend not only on spill size, location and conditions, but also on oil composition and toxicity, whether the oil is in emulsified form, and possibly how it is released (e.g., tanker spill, well blow-out, sub-sea pipeline rupture). However, these complexities do not alter the basic thesis that impacts from seeps are relatively minor and chronic, whereas major spills can be disastrous.
It has been speculated that Santa Barbara’s sea life has physiologically adapted to oil in the environment. While there is some evidence that sea urchin larvae and mussels living in waters near seeps have somewhat greater tolerance for oil than do their counterparts living elsewhere, there is no evidence for tolerance in other invertebrate species. Some fish and seals are able to metabolize and excrete small amounts of hydrocarbons. There is also evidence that some species avoid floating oil, instinctively or through learned behaviors. However, neither avoidance nor increased hydrocarbon tolerance would protect the organisms if they were inundated with oil from a major spill. On the other hand, there is reason to believe that birds, marine mammals, and fish exposed regularly to seep hydrocarbons experience chronic, sub-lethal effects, which may weaken them and increase their vulnerability to effects of oil spills and other natural and anthropogenic stresses.
Since as early as 1973, researchers have suggested that oil and gas production offshore of Coal Oil Point may have reduced the pressure in oil reservoirs beneath the sea floor and decreased seepage rates.  It was proposed that pumping oil out of the reservoirs might be a good way to reduce the rate of oil seepage into the environment. Another approach to reducing seep impacts has been to capture seep hydrocarbons (principally methane gas) as they emerge from the seafloor vents. Collection of gas in the ARCO seep tents has significantly reduced methane emissions to the atmosphere from the South Elwood seeps. Initially, following installation of the seep tents in 1982, appreciable amounts of liquid oil were recovered along with the gas. However, liquid oil seepage declined rapidly, and currently only gas is recovered from the tented seeps. 
Two recent papers by researchers affiliated with the Hydrocarbon Seeps Project at the University of California, Santa Barbara, support the hypothesis that a decrease in reservoir pressure due to oil and gas production has led to decreases in seep activity in the vicinity of Platform Holly.   There is substantial evidence, based on sonar data, that seepage rates near Holly have declined significantly since 1973.  Measured pressures in the sub-sea oil reservoirs near Holly have declined since about 1983. The rate of methane capture in the seep tents (located about a mile from Platform Holly) increased initially between 1983 and 1985, plateaued at over 1.5 million cubic feet per day between 1985 and 1989, and has declined to about one third of that rate in recent years.  The seep process is described as being driven primarily by reservoir pressure, so that:
oil and gas extraction > reservoir pressure drop > lag time > reduced seepage rate
As a generalized concept, it “stands to reason” that oil taken out of the ground can no longer seep out; in the long term, extraction could reduce the amount of seepage that is possible. However, the seep mechanisms are complex, and the pressure driven model does not account for some observations. The following illustrate the complications:
Ų First of all, the reservoir pressures were at or below hydrostatic pressure during the entire 1973 to 1995 study period, which suggests that seep flow is being driven mainly by buoyancy of the oil and gas or other effects, and not so much by reservoir pressure. 
Ų Second, the reservoir pressure measured directly under the seep tents (as opposed to near Platform Holly) was below hydrostatic pressure and remained fairly constant during the study years, while the flow of seep gas collected in the tents decreased to one third of previous rates. This shows that reservoir pressure and seep rates are not closely linked. 
Ų Third, seep vents have opened up near the seep tent, so hydrocarbons that might previously have been captured in the tents now escape around the edges; in any case, output of seeps is known to fluctuate naturally. Hence, the observed changes in seep rates might be attributable to alterations in seep pathways rather than decreases in reservoir pressure.
Ų Fourth, the ratio of liquid oil to gas varies among seeps and changes over time for a given seep. So, for example, if there is a fifty percent decrease in gas seepage, one cannot infer there would be a corresponding fifty percent decrease in oil seepage. The seep tent data demonstrates that gas and oil seep rates do not vary in unison.
Ų Fifth, some petroleum geologists have considered it unlikely that extraction of oil and gas from the relatively deep reservoirs tapped for production could reduce seepage from the relatively shallow strata from which seeps are thought to arise. Possible mechanisms of Santa Barbara seeps are discussed theoretically in Quigley (1997) and Boles et. al. (2001).
Thus, while seep rates around Holly seem to be related (at least indirectly) to withdrawal of oil and gas from producing reservoirs, the production-seep connection is not well understood or proven. Rather, the connection is inferred from the observed decline in seepage in the vicinity of Platform Holly. Based on the current state of knowledge, there is no way to know whether or not additional production from the South Elwood field (under Venoco’s “Extended Field Development” proposal) might result in decreased in seep rates over a larger area over the long term. It is unikely that oil and gas production in areas farther offshore on the Outer Continental Shelf (OCS) will affect seepage rates. Existing OCS platforms and wells proposed for further exploration, tap into petroleum reserves that are generally deeper than the Elwood reserves that were produced in the past decades, whereas origins of seeps are believed to be relatively shallow. Furthermore, although little is known about seep rates on the OCS and in state waters north of Point Conception, seep rates are probably much lower there than offshore of Coal Oil Point. Therefore, any potential decreases in seep rates in those areas would be of little importance.
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1. Links to section headings within the document are listed on page 1 below the title.
2. Emboldened keywords and associated links are in order of occurrence in the paper.
1969 Santa Barbara oil spill - [Energy Div. web page]
UCSB Hydrocarbon Seeps Project - http://seeps.geol.ucsb.edu/
Fundamentals of Physical Geography - http://www.geog.ouc.bc.ca/physgeog/contents/10l.html
Weathering of Oil - http://www.api.org/oilspills/weather.htm
Area Committee - [Energy Div. web page]
Field Manual of Wildlife Diseases - http://www.nwhc.usgs.gov/pub_metadata/field_manual/chapter_42.pdf
Extended Field Development - [Energy Div. web page]
exploration - [Energy Div. web page on OCS delineation drilling]
 Fischer, 1977.
 Quigley, 1997, 17. (References omitted from quote.)
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 Egland, 18.
 Calif. SLC, vii.
 Calif. SLC, xiii.
 Pers. comm., James Boles, Jan. 2002.
 Fischer 1976, 58; Quigley, 1999.
 Quigley, 1999.
 Calif. SLC, iv.
 Pers. comm., Bruce Luyendyk, 2001
 WSPA, 8.
 Quigley, 1997, 20-21.
 Homafius, Egland.
 [http://seeps.geol.ucsb.edu], accessed 3/8/02.
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 From meeting with UCSB Seeps Group, Feb. 2001.
 Pers. comm., Mary Elaine Dunaway, MMS, Feb. 2002.
 Quigley, 1997, 56.
 Quigley, 1997.
 From meeting with UCSB Seeps Group, Feb. 2001.
 MMS Delineation, 5-16. (These numbers do not include spills in state waters.)
 MMS Delineation, 5-18.
 Anderson. (These numbers do not include spills in state waters.)
 Quigley, 1997, 15.
 43USC Sec. 1348(d)(1).
 Code of Federal Regulations, 40 CFR Part 112, Appendix E, 3.2.1.
 State of California, Department of Fish & Game, Fact Sheet, 5/22/98 (PCA Number 7800-65679)
 Analysis by Santa Barbara County’s engineering consultant. Pers. comm., Alice McCurdy, Jan. 2002.
 “Investigation of Pipeline Break and Hydrocarbon Spill: Pt. Pedernales Submarine 20-inch Oil Emulsion Pipeline,” Report by U.S. Dept. of the Interior, MMS, Dec. 9, 1997.
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 Draft NRDA, “Preliminary Injury Determination for Marine Mammals,” 10/9/98.
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 MMS Delineation, 5-100.
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 MMS Delineation, 5-100.
 MMS Delineation, 5-103.
 Hunt, 543.
 MMS Delineation, 6-43.
 MMS Delineation, 6-43. (The text cites Hartung and Hunt, 1966, as source.)
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 Burger, 166.
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 MMS Delineation, 6-44.
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 MMS Delineation, 6-32.
 MMS Delineation, 6-33.
 MMS Delineation, 6-34.
 MMS Delineation, 6-34.
 Draft NRDA, “Release and pathway of Oil in the Environment,” 4/16/99.
 Draft NRDA, “Monitoring of Rocky Intertidal Resources along the Central and Southern California Mainland,” 10/30/98.
 Draft NRDA, “Resource Equivalency Analysis of Abalone Injury and Restoration,” 4/26/99.
 For a synopsis on sensitivity of different shoreline types, see Calif. Dept. of Fish and Game, Office of Spill Prevention and Response website, [http://www.dfg.ca.gov/Ospr/scientific/field/esi.html], accessed 3/8/02.
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 Pers. comm., James Boles, Jan. 2002.
 Pers. comm., James Boles, Jan. 2002.