In order to understand the ecological impacts, we need to get a sense of what oil spills are and what they can do to an ecosystem over time. When oil enters marine environments through a spill, its composition undergoes continuous change due to a process known as ‘weathering’. As oil begins to evaporate, emulsify, form sediment it begins to weather; as it is continuously broken down with the combination of sunlight and microorganism. Oil is mainly derived from biological materials; which takes over millions of years to be modified as complex mixtures of hydrocarbons. These hydrocarbons take a long time to breakdown.
The extent of an oil spill impact depends on the oil’s chemistry and how toxic it is; the capacity of microorganisms to degrade oil and reduce its toxicity, and how benthos — which is the flora and fauna found on the seabed or in the sediments respond to the spill. Oil, sinks or floats depending on its density. For example, oils like petroleum, diesel and kerosene might float on the water column, whereas thick and high density lubricating oils or crude oil can sink to the seabed. Weathering of oil in sea water forms a viscous mixture, which becomes resistant to further rapid weathering and is difficult to remediate.
Near-shore ecology
Near-shore ecosystems are shallow zones where water is warmed by sunlight. These warm waters are preferred by many breeding fish as the temperatures are perfect enough to incubate their eggs. Fish use a range of near-shore habitats through their life cycle, feeding on plankton, which are basically microscopic plant and animal organisms that float or drift in water. Nutrients that are required by plankton are flushed in and out with tides from estuaries. Unlike high energy beach ecosystems where waves crash and help in mixing of water, estuaries are comparatively low energy ecosystems. Estuaries are contained wetlands – with one or more outlets to the sea. During high tide water enters from the sea into the estuary, where it submerges land and most of its vegetation. This is when high microbial activity takes place where dead organic matter is broken down further. When the seawater flushes out during low tide it carries all the rich organic matter into the sea on which several fish depend on.
Salt marshes, sea grass and mangroves are important estuarine vegetation which are a foundation for structure and functioning an entire ecosystem. Animals not only forage on water surface – but there are also bottom feeders such as crabs, shrimp mussels, clams and oysters. These bottom feeding animals are the most crucial for proper functioning of the ecosystem and are the most susceptible to an oil spill. For instance, oysters are suspension feeder, consuming leaf litter and other organic matter broken down by microorganisms. They not only filter the water but also provide food through their excreta to other bottom feeder. ‘Oyster reef’ formed by both live and dead oysters are important spawning and shelter areas for fish.
Ecological impact of oil on near-shore ecosystems
Oil causes severe environmental damage. Oil ingested and absorbed by marine organisms affect respiratory organs such as gill or skin. Damage to the gills impair respiration and this could have lethal effects. Coating or smothering of oil on the animals’ body affects gas exchange, resulting in the loss of temperature regulation and other critical life supporting processes. Animals such as crabs, shrimp and other fish which mainly feed on seabed deposits are exposed to ingestion of oil in weathered states. As we know fish are mobile animals that may be able to avoid heavily oiled near-shore areas. But if their primary habitat is limited they will be forced to remain in the same region without other alternatives.
Surface waters and marsh sediments contain high levels of microorganisms. Microbial degradation depends on the concentration of hydrocarbons in the oil and several other environmental factors. When ecological conditions are already in a compromised state microorganisms will find it difficult to degrade oil. On the other hand, microorganisms could deplete dissolved oxygen in a local area as they undertake the process of breaking down oil particles. Oil is capable of inducing oxygen deficiency and stress in already oxygen low areas. Therefore oil deposits could alter whole microbial process – and also bring biological and physical changes to marine ecosystems.
Estuarine habitat such as salt marshes and mangroves are submerged either completely or partially during high tide. Even if oil becomes less toxic due to weathering, it can cover the plants and prevent photosynthesis. Most importantly, spawning or breeding ground of fish are susceptible to damage by oil. If fish had laid eggs or were supposed to lay eggs after an oil spill event then the successful hatchling of next generation will be severely jeopardised. These are some of the direct effects on oil in marine environments; but the indirect effects of oil spreads through the food webs in unforeseen ways.
Some possible long-term solutions
Oil spills have long-term impacts which require long-term monitoring. Given the lack of knowledge and our understanding of near-shore ecosystems, we can only speculate the impacts of an oil spill of such gravity. It is unfortunate that we do not have data on near-shore fisheries in oil spill region, which is small-scale or artisanal in nature. Therefore, it will become difficult to assess the impending ecological, economic and livelihood impacts faced by fishing communities dependent on near-shore seas.
The first best long-term solution would be securing our already vulnerable coast from further development with hazardous and polluting industries. Secondly, dissolving oil with chemical dispersants will create more problems than the oil itself. Other than the use of dispersant which might have adverse effects on both biologically as well as physically complex tropical marine ecosystem like ours, we do not have other means to safely dispose the oil. Thirdly, we need a shift in the attitudes of governing agencies. Measures implemented to deal with the present oil spill stems from ignorance of how complex near-shore ecosystems are and how they function. Our governing agencies are focused on hiding the problem rather than pondering upon long-term solutions.