Impact on sessile animals
Marine life in coastal areas will be affected sooner and more intensely than that in the deep ocean. Those species that move little or not at all, such as clams, mussels, and oysters, will not be able to follow a changing environment. However, it is possible that their progeny may flourish in new areas. If suitable areas do not develop, perhaps because there is no suitable sea-bottom, or currents to carry the eggs, the species may be severely restricted or eliminated.
Food web changes
Changes are always occurring in the oceans at many scales, and marine life has evolved with them. Changes in oceanographic features would alter marine habitats. However, the global warming forecasts are for relatively fast changes that would endure for centuries. This would likely lead to some discontinuities in the oceanic food web, particularly if some aspects of an ecosystem move but others are constrained by factors such as salinity or bottom topography. Environmental changes also can lead to different advantages for predator and prey species. Strong winds or currents can lead to more mixing of water layers. Prey that stratify at a certain depth may become more dispersed when there is mixing and thus less available to predators. Their abundance may increase while that of the predators may decrease. There are many such interactions and complexities. For example, a cod egg or larva that is not consumed by a filter-feeding herring may one day grow large enough to eat herring as a major part of its diet.
As in any complex situation there are always exceptions. Among the most notable in this subject are species that exist in eastern boundary currents and in the Northwest Atlantic. Displacement toward the equator (not poleward) may occur in eastern boundary currents as a result of upwelling and ecological processes related to tradewind acceleration. In the Northwest Atlantic, there may be no warming and cooling may occur due to changes in wind and currents. This may have negative effects on species that do better when it is warm.
Increased storminess that may come with climate change affects coastal ecosystems in several ways. For example, increases in water runoff from more intense precipitation can reduce salinity along the coast, bring more nutrients in the runoff, and increase sediments in the water. The changes can lead to changes in production and distribution of species, including changes in migratory paths. When combined with the expected warming of coastal waters, there may be considerable ecological changes. For example, turbidity, by itself, provides protection of some prey species. Changes in sedimentation can lead to important changes throughout the ecosystem. Similarly, a rush of freshwater will kill several predators of oysters, which are less tolerant of low salinity than are oysters.
Temperature changes alone do not account for changes in marine ecosystems and fisheries. One must consider all the diverse elements of the marine environment. Species have many ways to deal with changes in their habitat and migrating predators and prey. However, most species depend on near-surface layers and shallow waters in estuaries at some point in their lives. These are the areas that will have the greatest changes. There is a need to particularly monitor these species in order to ensure their wise stewardship.
Similarly, environmental changes cannot be considered in the absence of human impacts, such as from pollution and fishing. There is likely a synergistic effect. For example, a cooling environment in the 1990s quite likely led to reduced cod recruitment off eastern Canada. However, it came at a time of heavy fishing pressure that had led to a small biomass and few older, more productive fish. Together the factors caused the stock to become vulnerable to collapse. Climate change is happening at a time when many stocks are under heavy fishing pressure. For those stocks where the climate impact may be negative, resource managers should be particularly vigilant.