There are various types of oils released from oil spills. Microbes can biodegrade only a small fraction of this. The rest of it remains in water as it is. It takes thousands of years to clean up these oil spills. This left over non-biodegradable oil can disrupt the food chain and food web continuously over years to continue.
The first level of a marine ecosystem is phytoplankton. Phytoplanktons continue the process of photosynthesis and as a result, release oxygen into the marine environment. Due to oil, many phytoplanktons die. As a result, the overall dissolved oxygen of the marine environment is depleted.
Less oxygen further results in the death of marine flora and fauna. These include many seaweeds, photosynthetic bacteria, bryozoan thickens, bryozoan thickets, zooplanktons, phytoplanktons, sea stars, small and large fishes, red cods, corals etc.
Phytoplanktons are consumed by zooplanktons, which are further consumed by small fishes; small fishes are eaten up by large fishes, dolphins, red cod, crabs, bivalve mollusks, sea stars, arrow-squid and birds. Reduction in the number of phytoplanktons results in reduction in zooplanktons also. So, the entire fauna of euphotic zone will be disrupted. This disruption further affects the flora and fauna of bathyal, abyssal and benthic zone; the effects of this disruption are long and lasting. These can be viewed even years afte the occurrence of oil-spills. Many sea animals have become extinct because of the same, and many are endangered. The most important animals of the endangered list are North Atlantic Blue Fin Tuna, Sea Turtles, Sharks, Dolphins, Brown Pelican, Oysters, and migratory songbirds. Some plants which are badly affected are cordgrasses and seaweeds.
It is important to note that phytoplanktons and zooplanktons contribute to the lowest trophic levels in the ocean food chain. If these are disrupted, then the higher trophic levels will automatically be affected.
Now, suppose we know that oil spills are affecting zooplanktons in a particular part of the sea. Via the aid of food web, we can find out which sea animals are dependent on zooplanktons. So, we can find out which animals will be directly and indirectly affected by the reduction of zooplankton population.
The plants and animals of bathyal, abyssal and benthic zone contribute a lot to the world’s economy. These are a source of food to many. Sea animals are a source of recreation and tourism. This business will be affected drastically if sea animals reduce in number.
One good example is the oil disaster of Gulf. Because of the oil spills, a thick coating of oil was embarked all over the bodies of Brown pelicans residing in this part of the sea; this resulting in extinction of this species.
Industries like Oyster, dolphin and shark industry are a million dollar sea industries in present times. Through food web we can easily find out that which part of food web will be drastically affected by oil spills. We can further find out how much time (in years or months) it will take to disrupt these industries. The bad effects of oil spills occur years after the accident has occurred.