Mary Elaine Helix, Biologist, Pacific OCS Region

Most beachgoers are unaware that the tar they see washed up on the sand comes from a natural source of oil -- seeping up through cracks in the rock and sediment below the ocean floor. Yet, the existence of natural marine oil and gas seeps has been known since prehistoric times. In California, the earliest written records of the European explorers (c.1500) document the widespread nature of seeps in the coastal region. In the late 1800's, studies of the marine seeps were initiated in an effort to locate commercial sources of oil and gas. Since then, more than 2,000 active submarine seeps have been mapped along the California coast.

Only in the last 50 years has the ecology of the natural marine seeps been studied. This research seeks to answer basic questions about the plants and animals that live near seeps and how they live in this hydrocarbon-rich environment. Marine biologists are beginning to understand whether biological communities associated with seeps are fragile or robust, and how the petroleum is used to sustain life.

One of the most intensively studied seepage area lies off Coal Oil Point, in Santa Barbara County. Seeps in this area release an estimated 11 to 160 barrels (450 to 6,700 gallons) of oil per day, along with a large volume of natural gas. At Mobil’s (formerly ARCO’s) innovative Coal Oil Point sea floor containment project (a large concrete structure placed over a large seep on the seafloor), over 1 million cubic feet of natural gas is collected each day.

Studies show that seeps have a patchy distribution and are characterized by increased production caused by the added organic material. Information learned about the types of plants and animals found in these areas and some interesting findings are summarized below.

Patchy Distribution

Research on biological communities inhabiting active seep areas has led to some surprising discoveries. The Minerals Management Service (MMS) and others have sponsored studies adjacent to Coal Oil Point. These studies have revealed that the presence of petroleum, even in this active seep area, is very patchy over time and area. Individual oil and gas seeps are not constantly active; in fact, the volume of oil and gas released varies considerably throughout the day and from day to day. Tidal fluctuations, as well as seasonality, appear to influence the activity of individual seeps.

The biological community in the seep area is exposed to varying volumes and types of petroleum. Only small parts of the community are actually exposed to “fresh” oil at any one time. Since fresh oil contains the fractions of the oil that are generally the most toxic, many plants and animals are not exposed to the more toxic components of the oil. Parts of the community are exposed to weathered oil (tar), and some plants and animals within the seep area have little or no exposure to petroleum. This is true of the plants and animals in the tidepools and those found in deeper water.

Increased Microbial Production

The variable amount of oil released complicates the study of the effects of the seeps on the numbers of organisms as well as the numbers of different species. Most biologists agree that the community responds in an organic enrichment pattern where the added organic matter acts like a fertilizer to increase productivity and increase the number of oil-degrading organisms. This pattern is similar to other communities exposed to sources of natural or man-made organic matter, such as municipal sewage. Here, though, the source of organic material is flowing up from the ocean bottom rather than down from the water surface. As such, the most affected groups of organisms are found in or on the ocean bottom.

Scientists differ over the cause of this pattern of enrichment. Studies have found that chronic petroleum sources stimulate microbial production (production of carbon compounds by bacteria). Studies suggest that this greater bacterial production may allow increases in abundance of the animal and plant species that feed upon these bacteria. For example, seep areas were found to be associated with increased numbers of nematodes (roundworms) and increased reproductive abilities in some female copepods (a shrimp-like animal). This last result was surprising since copepods are very sensitive to petroleum exposure and are believed to be good indicators of impacts from oil.

Exposure Level Studies

Studies also show that different species survive best at specific levels of exposure to oil. Off Coal Oil Point, these studies found that nematodes attained their greatest abundance in the highest exposure areas, whereas copepods were more abundant at low levels of oil exposure than at either the high or no exposure areas. Evidence from the study of mussel beds indicates that mussels in the lower beds generally show a positive response to low levels of petroleum exposure because of the added organic material.

Parts of the biological community seem to show no response to oil exposure. For example, studies of kelp beds and kelp bed fishes have not identified any population changes related to exposure to natural oil and gas seepage.

Adaptation

It is unclear whether animals living in oil seep areas adapt to the higher levels of oil and gas found there. Laboratory experiments on a common mussel found offshore California (which has a naturally high tolerance for oil) showed that individual mussels taken from the Coal Oil Point area had a higher tolerance for oil than those taken from other areas. However, many scientists believe it is unlikely that long-term, multi-generation adaptation is present in marine animals living near seeps. This belief is based on the fact that many of marine animals found close to the source of the seep release their young up into the water column where exposure to the oil is considerably less. It would be difficult, therefore, for these animals to evolve to be more tolerant of the hydrocarbons released from the natural seeps. It is theorized that only animals in the area where the highest concentrations of oil occur are likely to adapt to petroleum.

Another possible means of adaptation is through behavioral response. A study funded by MMS of birds off Coal Oil Point found that adult gulls and pelicans were less likely to be oiled than the younger birds. Interestingly, both young and adult shearwaters (a common diving bird) completely avoided the oil seep areas. In the case of the pelicans and gulls, this behavior may be due to experience--the birds apparently "learned" to avoid feeding in active seeps. It is believed that the shearwater's keen sense of smell contributed to their avoidance behavior.

Chemosynthetic Organisms

Perhaps the most exciting biological discovery in this field occurred in 1984 in the Gulf of Mexico when unique communities of previously undescribed large mussels, snails, and tubeworms were discovered near gas vents. Since that time, many other rich seep-associated assemblages have been found in the Gulf of Mexico. Surveys suggest that widespread populations of chemosynthetic organisms occur in these seep assemblages in the deep sea. Chemosynthetic communities refer to the assemblages of marine organisms that depend upon specialized bacteria for their primary food source. These bacteria use a chemical process called chemosynthesis to grow. Chemosynthesis is similar to photosynthesis except for two important distinctions: oxygen is consumed rather than produced, and the process occurs without light energy. Various bacteria use carbon (generally from CO2) and oxygenated water to produce new organic compounds. In petroleum seep areas, specialized bacteria have been found that use methane as their carbon source.

Chemosynthetic organisms have been located in several places off California. However, it is not known whether these organisms obtain their carbon source from oil and gas seeps. One possibility is a species of clam found in Monterey Canyon near a potential source of hydrocarbons. Scientists are conducting studies to learn whether a seep-based community exists there. Other chemosynthetic animals collected in trawls off the Pacific coast are thought to be either associated with sewage outfalls or the source of their carbon is unknown.

A special association between two organisms, or symbiotic relationship, was first identified between mussels and a type of methane-eating bacteria in 1986. The methane-eating bacteria found in the mussel's gills produce organic compounds from the methane and provide them as a food source to the mussel. The mussel, therefore, is able to use methane as its sole carbon source.

While symbiotic relationships were not found in seep-associated communities off California, this relationship is consistent with studies of the bacterial mats found on active seeps at Coal Oil Point. In these studies, investigators found that bacteria were feeding on the petroleum and sulfide found in the sediments and tiny invertebrates living in the bottom sediments were grazing on the bacteria. The petroleum-based carbon reached the food chain through invertebrates eating the bacteria, and, in turn, being eaten by larval fish.

Studies in the Gulf of Mexico have significantly expanded the geographic area of the deep ocean where oil seeps and gas vents are known and where dense groups of these unusual biological communities are found. Here, plants and animals are thriving in an otherwise adverse set of environmental conditions. Research on these types of communities is ongoing; a MMS-sponsored study was recently completed which surveyed the communities at five stations in deep waters in the Gulf of Mexico.

The biology of natural oil and gas seeps is only partially understood. Evidence suggests that this naturally occurring organic material plays a locally important role in the food chain. We expect that more clues to the robustness and the processes used to sustain life in this community will come from further investigation of chemosynthetic communities.

References

Allen, A. and Shelueter, R., 1970. "Natural Oil Seepage at Coal Oil Point, Santa Barbara, California. Science, Vol. 170: 974-977.

For more information: Mary Elaine Helix