A barrel of oil contains many components that interact with the atmosphere and with the water after an oil spill. Compounding the problems are the chemicals that are used to break down the oil, which then travel with the broken down components of the oil. This creates an oil/chemical mass that can move in two ways: by hydraulic action or by atmospheric action.
Depending on how close the mass is to land and on the formations of the land, many things can happen. If the land/sea interface is rocky, with lots of violent wave action, components of the mass can be sprayed and forcibly washed well inland. Estuaries and channels concentrate the force of tides due to their narrow formations.
A combination of violent weather and large areas of sea level or below sea level land mass can force the oil/chemical mass well inland, past intertidal zones, through brackish zones, and into freshwater riparian zones and lands that are seasonally dry and used for farming.
When the lighter and more volatile components of the oil mass vent off or are separated, there are two components of the oil mass: the tarry globules of residual oil that form and sink and the lighter “slick” that rides the water surface. Again, with tides and violent weather, the tarry globules will move to the intertidal zone, where they may come and go, incorporate with rock and soil structures and become a permanent problem that saturates, then periodically seeps out. The lighter components will travel farther.
As a result, all of the lifeforms in all of the aquatic zones will be affected. The benthic zone (or floor) of the ocean, intertidal, brackish and even freshwater zones will undergo changes as the lighter components and tarry globules settle and incorporate into or on to smother the areas where life lives on or under the benthic soil.
If the oxygen levels of the water are affected, then oxygen dependent lifeforms will attempt to migrate to better areas, abandoning suffocating and non viable larvae, eggs and microscopic life forms that make up the food chain. The life forms that are able to, will migrate to already balanced areas to create overpopulation.
Exposure will affect metabolic and reproductive functions, as well as poison, physically incapicate, and sicken to make life vulnerable to disease or predators.
Aquatic plant life will struggle to synthesize light and to produce oxygen, plant metabolism and reproduction will be challenged. In some cases, if components of the oil act as nutrients, then overgrowth of aquatic plants will cut the available oxygen in the water.
In the riparian zone, the water table that supports surrounding plants will be contaminated, challenging plants, shrubs and trees. The supporting soil structure can be compromised as the oil/chemical mass saturates surrounding soil.
Land and aquatic animals that either live in or look for food in ocean, brackish and freshwater riparian zones will find aquatic food to be either scarce, poisoned or poisoning, and will be compromised by contact with the tarry globules, which are incredibly sickening,incapacitating and impossible to remove when they contact fur, feather scales and skin. The water, itself, might be poisoned and poisoning, affecting mammal and invertebrate metabolism and reproductive capacity, as well as sickening and making land animals vulnerable to disease and predators.
Birds in all zones that interact with oil infested waterways will be covered with the tarry globules, where they will be incapacitated, suffocated and die. Those who drink the water or feed on the compromised food chain may be poisoned and sickened, with reproduction and metabolisms compromised by the toxins in the oil/chemical mass. Offspring and adults may not remain viable enough to survive predation or the rigors of migration, which will affect the entire chain of life on the migratory routes and endpoints.
Open ocean life will be affected by the “slick”, which can interfere with temperature, oxygen/carbon dioxide exchange and possibly light penetration. But open ocean life is far less abundant than coastal or intertidal life. It is in the shallower waters where light can penetrate that life is abundant.
In the abyssal ocean, the process might be beneficial to creatures that are essentially scavengers of whatever it is that falls from above or seeps from below. Many of these creatures are capable of thriving on and in the half of oil that enters the water naturally through seepage.
In summary, wherever the oil or the components of the oil and chemical dispersant travels, life will be affected in one way or another. There might be effects that will not show up for months or even years. The tarry oil from the Exxon Valdez and the Exxon Houston spills, is still running back and forth or seeping and sinking, even decades after those events. The unique nature of the BP spill is that the land mass is low lying and subject to violent movement of water and oil from hurricanes and even possibly tsunamis from Caribbean earthquake activity. This might mean that even inland brackish and freshwater riparian zones will be affected, threatening all land and air animals that interact with the water for hydration and food.