Effect of petroleum on fauna
Introduction
Aquatic organisms and organisms whose life closely tied to the ocean are especially vulnerable to chronic as well as acute water pollution. The effects are manifested on individual, community, and population levels. Sublethal concentrations cause physiological, carcinogenic and cytogenic damage. At the population level, the oil contamination results in changes in abundance, age structure, population genetic structure, and reproduction rates. Community level consequences involve modified interactions between competitors, predator/prey balances and mutualistic relationships. Disruption of trophic chains produces most dramatic alterations in natural assemblages of organisms.
The important consideration here is which individual species are affected. In each community there are particular species that by direct and indirect interactions with other species contribute significantly to structuring and maintaining the community. Elimination of those species (sometimes referred as keystone species) will have the most severe impact on the community as a whole.
Catastrophic oil spill typically causes initial massive mortality across all taxa of a given marine community followed by disappearance of most vulnerable species and developmental stages. Community's diversity decreases sharply leading to instability and extreme fluctuations in populations of opportunistic organisms. Over the time, the oscillations slowly subside and diversity eventually may reach sufficient level. However, there is always the risk that the ecosystem will never return to the original state (i.e. species composition remain altered for indefinite time).
Many organisms die from oil contamination due to toxicity, hypothermia, or smothering. Sublethal effects of oil are witnessed by accumulation of unchanged hydrocarbons in tissues of aquatic organisms such as mussels and seals. Oil detoxification products can be also found in bile and liver of many vertebrates. Lesions occur in brain of some animals.
Cleaning operations also can cause substantial mortality due to disturbance of breeding, toxicity of dispersants, high-pressure hot-water washing, and other human activities.
Invertebrates
Invertebrates, especially those that live subtidally (continually submerged) and are not commercially important, are rarely seen and don't enjoy same sympathy during oil spills as do birds and mammals. However, many of them, being herbivores and detritivores, form the foundation of marine ecosystem's food pyramid.
Heavy coating typically kills all taxa indiscriminately. Physical smothering by petroleum (exclusive of toxicity) will prevent respiration, restrain movement, and create excess weight and/or shearing forces on mobile organisms to which oil sticks.
Larval stages are especially sensitive to oil pollution and mortality may occur at concentrations of soluble oil components as low as 0.1-1.0 ppm (parts-per-million).
Oil pollutants in sublethal concentrations often cause cytogenetic damages such as aberrant chromosome arrangements during mitosis, polyploidy, aneuploidy and chromosomally mosaic embryos. The most common physiological functions affected are reproduction, growth, respiration, excretion, chemoreception, feeding, movement, resistance to diseases. Toxicity of oil is directly correlated with the proportion of aromatic hydrocarbons.
For many intertidal invertebrates with long-distance planktonic larval stages, recruitment (settlement of larvae on substrata) rather than reproduction has the greatest influence on recovery. A standard method for assessing recruitment is to clear the substrate or attach settling plates and observe what appear on these bare areas.
Snidaria
Some studied jellyfish and hydroid species, for example, Aurelia and hydroid Tubularia, respectively, responded to oil contamination by various morphological abnormalities and neurological disorders that interfered with feeding and reproduction. These organisms play important roles in food chain by feeding on small planktonic organisms, shrimps and small fish, and in their turn, being eaten by sea turtles, fish, and crabs. Some fish find shelter in jellyfish tentacles.
In contrast, some Cnidaria (anemones and some jellyfish) seem especially resistant to oil. The anemones Anthopleura and Actinia are species found surviving in heavily contaminated waters.
Corals and coral-based ecosystems are adversely affected by oil spills.
Crustacea
It was found that Water Soluble Fractions (WSF) of crude oil affected food-searching abilities in kelp crab Puggetia and rock crab Cancer that largely depend on the organisms' chemoreception. Larvae of King crabs (Paralithodes) and shrimp (Eualus) exposed to WSF slowed their movements and died within hours to days.
Crabs and shrimp constitute a significant part of diet of marine vertebrates such as fish, mammals, and birds.
Most barnacles seem to have a great tolerance for oil. Unless they are completely covered, and thereby die from suffocation, they survive well. Even larvae are resistant and manage to settle and continue development.
Echinodermata
Echinoderms are especially sensitive to the toxicity of oil, probably because of the large area of exposed epidermis. Both sea stars (Pisaster) and sea urchins (Strogylocentrotus) were eliminated for several years following the 1957 wreck of the Tampico Maru off the coast of Baja California.
Sea urchins (grazers) as well as other echoderms such as sea stars (predators) is an example of keystone organisms.
Mollusca
In general, for bivalves and gastropods, increase in energy expenditure and a decrease in feeding rates associated with oil, ultimately result in lower survival and reproduction rates.
Marine worms
Polychaeta (annelid worms) and Echiura (spoon worms) species are differentially affected by oil. For example, Capitella capitata proliferates after oil spills, whereas lugworm Arenicola is usually driven to the surface which exposes them to predation.
Fish
Laboratory research showed that concentration of soluble oil components as low as 1 ppb (parts-per-billion) can adversely affect fish eggs and larvae. In long-lived fish species such as bluefin tuna, swordfish, marlin, and wahoos, the adverse effects may remain unnoticed for a few years.
Dispersants may increase toxicity of oil and increase its accumulation in adult fish causing tumors, mutations in germline, and decreased survivability.
Other effects of oil on fish populations include habitat destruction and decrease of food availability.
Birds
Cormorants, various species of seagulls, and pelicans die from hypothermia after oil coating damaged insulative properties of their plumage and interfered with other activities (feeding, floating, flying, breeding).
Mammals
Charismatic sea otters suffer great losses from oil spills. Thousands of sea otters died as a result of Exxon Valdez oil spill (1989). River otters also were affected to some degree: increased mortality, latrine site abandonment, weight loss were detected.
Otters is an example of keystone species on which harmony within ecosystem hinges: the absence of sea otters in a localized area would allow sea urchins proliferate uncontrollably, destroying kelp beds, which lead to reduction in sedimentation rates and gradually can result in drastic changes in habitat of many other resident organisms.
Increased annual losses (19 and 21%) were detected in orca (Orcinus orca) population within 2 years after Exxon Valdez oil spill in Prince Williams Sound, Alaska, but because small sampling size and because disappeared animals haven't been found dead, attributing the disappearances to oil spill remains speculative.
Hundreds of harbor seals died after Exxon Valdez oil spill. Hydrocarbons and oil metabolites were detected in bile, brain, and blubber. Brain lesions were also found.
References
- SUCHANEK TH. Oil Impacts on Marine Invertebrate Populations and Communities. AMER. ZOOL., 33:510-523 (1993)
- Paine RT et al. TROUBLE ON OILED WATERS: Lessons from the Exxon Valdez Oil Spill Annu. Rev. Ecol. Syst. 1996. 27:197–235 (.pdf)
