Fish can be affected directly by oil spills through ingestion of oil or oiled prey, through uptake by the gills, through effects on eggs and larval survival, or through changes in the ecosystem that support the fish.
Benthic and Intertidal Invertebrates
Contaminated sediments may impact bottom-dwelling organisms feeding and living
in and on the bottom. However, generalizations about the vulnerability of
benthic and intertidal invertebrates are difficult because there is a great
deal of variation among organisms as well as various life cycle stages of
any one species.
Intertidal benthic invertebrates may be especially vulnerable to oil spill impacts because oil coming ashore during spills may become highly concentrated in a narrow band along the shoreline. Additionally, in low-energy shoreline areas, fine sediments tend to act as sinks for the spilled petroleum.
Oil spills present both physical and physiological hazards to benthic invertebrates. Sessile species such as barnacles are vulnerable to smothering by heavy oils, and even the more mobile invertebrates may come immobilized by toxic effects of the hydrocarbons. The broad range of impacts can include (but are not limited to) mortality due to smothering, altered population composition, altered metabolic rates and feeding rates, and alteration in shell formation.
The type of spilled oil will determine the type of chronic impacts on intertidal and subtidal organisms. For example, benthos surveyed after a heavy crude spill were found to be contaminated, but apparently unaffected. However, studies following the 1978 Amoco Cadiz spill (light oil) determined that there were chronic and deleterious effects on benthic invertebrates.
Some research investigations at spill sites and in laboratory studies have shown that certain benthic macroinvertebrates (such as the fiddler crab and some clam species) can survive in highly oiled sediments. While the reasons are not fully understood, scientists speculate that this result is due to several factors such as feeding habits of certain invertebrates that preclude uptake directly from contaminated sediments.
Delaware's riverine and estuarine areas are used by a variety of birds --
those most likely to be affected by an oil spill are wading birds, waterfowl,
shorebirds, bulls, and rails. Problems for these species include direct contact
resulting in oil-covered feathers and loss of food sources.
Oil causes immediate and serious external harm to birds by impacting their plumage. The direct effect of oil on a bird is to clog the fine structure of its feathers, which is responsible for maintaining water-repellence and heat insulation. Feathers that have been in direct contact with oil are likely to be matted down, losing their ability to trap air and provide the bird with warmth and buoyancy. This increases the risk of drowning because the complex structure of feathers that allows the bird to fly or float has been damaged.
Birds may ingest oil while eating contaminated food or while trying to clean their oiled feathers. Ingestion of oil results in harmful internal effects. Autopsies of oiled seabirds have revealed abnormal conditions in the lungs, kidneys, liver, and other internal injuries.
For insight into the treatment of oiled birds, see "Saving Oiled Birds" and "What You Can Do to Help."
Examples of mammals that may be affected by the oil spill include muskrats,
raccoons, and river otter. These mammals tend to move repeatedly to and from
the marsh-river interface, making it likely for them to come into contact
with the spilled oil. Individual animals will likely come in direct contact
with oil (via the waterway, sediments, or oil-soaked vegetation) or may consume
Research has demonstrated that the sensitivity of mammals to spilled oil is highly variable. The amount of damage is related to the impacts of oil on matting the animals' fur and the role of fur in keeping the animal warm (thermoregulation). Additional impacts result from ingestion of oil that can cause internal damage.
For marine mammals and sea turtles:
In the Delaware Bay, the most vulnerable marine mammals to this oil spill
are seals. Four species of seals (harbor, harp, hooded, and gray) are regular
winter visitors. Seals migrate south to more temperate waters and move into
Delaware Bay in late fall and winter. Direct contact with oil or consumption
of contaminated food can cause a variety of life threatening ailments including
hypothermia, internal damage, and infections. While seals are less frequently
sited in the upper bay, MERR
are maintaining vigilance along Delaware Bay sites.
At this time of year, our two most frequent marine mammal visitors -- bottlenose dolphins and harbor porpoises -- are generally not present. Bottlenose dolphins have migrated to warmer water and harbor porpoises arrive in late winter. Other species that could be affected in the lower bay include common, striped, and white-sided dolphins. Large whales are possible in the lower bay.
At this time of year, sea turtles (loggerhead, Kemp's Ridley, leatherback, and green) have migrated to warmer waters, so it is unlikely that the oil spill will have immediate effects. Oil deposits could have an indirect effect, contaminating the water column and bottom sediments where food sources occur.
The MERR Institute in Delaware is prepared to respond to marine mammal and sea turtle strandings associated with the oil spill. See http://www.merrinstitute.org/. Call 302-228-5029 for marine mammal or sea turtle sitings.
Reptiles and Amphibians
A variety of species of turtles, frogs, and salamanders use the Delaware River and estuary ecosystem and may be exposed to direct or indirect oil impacts.
Contaminated shorelines may act as a secondary source of exposure by direct contact to organisms using it such as wading birds, crabs, mammals, and reptiles. Although oil can soak into sand and gravel (depending on physical characteristics of the spilled oil) few organisms live full-time in this habitat, so the risk to animal life or the food chain is less than in other habitats, such as tidal flats.
Persistence of the oil material in the intertidal zone also provides a pathway for long-term impacts to intertidal organisms feeding in this area such as blue crabs and larval fish (depending on the seasonality of the spill). If oil is buried in the sediment, it may persist for a long period of time, and latent impacts may be variable and subtle. Oil on beaches is a form of oil pollution most obvious to the public. Most of the oil can be removed by natural processes and modern cleaning techniques, but some oily residues, if buried, can persist for long periods of time. The process of natural "cleansing" is slow on sandy beaches because the oil tends to become buried in the sediments.
In bays, estuaries, and marshes where fine sediments occur, natural cleansing by wave action is even slower, and any buried oil may remain for long periods of time.
Impacts to emergent and intertidal wetland vegetation may result from surficial oiling of shoots and leaves. Additionally, oil may have a direct detrimental effect on the physiology of wetland via contamination of marsh grass root and rhizome systems. Scientific studies have demonstrated that marsh vegetation is easily damaged by fresh light oils (as opposed to heavy crude oil).
"Oils vary in their toxicity to plants with the content of low-boiling compounds, unsaturated compounds, and aromatics. In general, the higher the concentration of these constituents, the more toxic the oil (Baker 1970). The amount of oil and frequency of oiling will affect the degree of damage. The viscosity of the oil is also important. Light oil has a physical limit to the amount that will "stick" to plants and the substrate. Tides easily redistribute light oil. In contrast, the viscosity of heavy oils often allows retention of large amounts on plant tissue and the substrate, causing mortality, not through toxicity but through smothering. Heavy oils also aggregate into globules which are more persistent, rolling around in the intertidal zone. Emergence of vegetation may be prevented by smothering or, if emergence takes place, foliage can become coated with oil which reduces the capacity for photosynthesis. In addition, seed may come in contact with oil which may inhibit reproduction." (RCG/Hagler, Bailly, Inc., 1989, pg. 3-5).
Observations from a spill with No. 6 fuel oil in the lower Chesapeake Bay "indicate that many of the contaminated fringe marsh areas recovered almost immediately and exhibited stable healthy shoots during the growing season (Rolandetal, 1977). A few heavily-coated areas in the Chesapeake, where oil lay thickly on the root system, were thinned out and somewhat stunted during the growing season." (RCG/Hagler, Bailly, Inc., 1989, pg. 3-5).