The open ocean changes slowly, but once changes do occur, they are not likely to be reversed quickly nor modified easily by humans.
From the point of view of protection of living resources in the marine environment, the ecological effects of pollution are of the most vital concern. Perhaps the most important effect of pollutants in the marine environment is ecological disruption, such as the imbalance created between organisms and their environment, and between communities of organisms of different species. This is often an insidious, long-term effect which can lead to large changes in populations of commercially important fish. So far, there has been no conclusive evidence that populations of marine fish have been seriously affected by pollution alone. However, this may be related in part to our inability to clearly identify cause and effect in certain fisheries' problems. A whole ecosystem may be modified by input of a particular pollutant, because certain species are reduced in number or eliminated. Other hardier species may fill an ecological niche vacated by a sensitive species eliminated by the pollutant.
There is a host of ecological effects due to disturbances to the marine environment, particularly arising from activities in coastal waters. Habitats of marine organisms may be adversely affected by solids settling on the bottom and by materials leached from them. Sedimentation from coastal mining operations may alter tropical waters unfavourably, especially coral reefs, where the light that is vital for photosynthesis is reduced. Erosion from improper land management may affect coastal spawning grounds as well as those in rivers. Organic substances in both dissolved and solid form decompose and remove oxygen from the water. This can be a serious problem in partially confined areas, such as bays and fjords, in which the water is not frequently replaced. It may even occur in basins on the exposed continental shelf where there is little or no flushing action by bottom currents. In areas where the volume waste is very large compared to the amount of water available to dilute it, even the salt composition of the sea water may be significantly changed. Normally, this is not a problem in most coastal waters if there is adequate flushing action. Such problems as pH change, due to the input of highly alkaline or acidic wastes, which can be a serious complication in fresh waters, do not commonly occur in the sea because of the buffering action of sea water. The inflow of fresh water over coastal sea water causes a stratification to take place in the absence of tidal and wind mixing. This in effect reduces vertical mixing, and such processes as aeration of deeper water tend to be minimized. Moreover, the comparatively fresh surface layer may have low buffering capacity and can be affected by a pollutant in much the same way as a river or a lake.
Pollutants all exhibit toxicity to aquatic organisms in various degrees. Some may be acutely toxic even in low concentrations and kill aquatic organisms over a short period of exposure. Others may have a debilitating effect. It is perhaps the latter which is most important from a long-term ecological point of view. The sub-lethal, chronic effects of pollutants may include retardation of growth, alteration of chemoreception in food-finding and mating, aberrant behaviour, physiological stresses affecting the vigour of organisms, and reproductive failure. There are pollutional effects of substances introduced into the environment which are not necessarily characterized by toxicity to aquatic organisms. For example, coloured and suspended particulate materials may retard the penetration of sunlight and thereby inhibit photosynthesis. An excess of nutrients may cause dense algal growths that adversely affect higher forms of life such as fish and shellfish. The input of heat with cooling waters may not necessarily destroy organisms in the water, but again, the conditions in which they live may be adversely altered. One species may be encouraged at the expense of another. The migrating behaviour of certain fish species may be altered by temperature gradients introduced by cooling waters.
The oceans are the largest ecosystems on Earth. They are as rich and diverse as any terrestrial ecosystem yet are still largely unexplored. While the deep ocean is mainly unpolluted, evidence is emerging of environmental degradation in some areas, and a decline in many marine species.