Worldwide, the marine environment is characterized by increasing temperatures and decreasing pH. In some (subtropical) areas, increased intensity and frequency of tropical storms also play a major role. These changes cause loss or decline of ecosystems, or large changes in their geographic distribution.
The majority of tropical coral reefs are affected widely by mortality due to 'bleaching' (expulsion of their symbiotic algae) which has been linked to rapid warming of sea water. There is further damage to some coral reefs due to increased hurricane force in some, mostly subtropical regions. Ocean acidification is expected to affect reefs as well, but the evidence for this is not yet conclusive. In Antarctic waters, warming has led to a decline in krill populations and, as a further consequence, to declining seabird and seal populations. Virtually all fish populations of commercial interest are heavily affected by overexploitation and in many areas at risk oftotal failure, irrespective of climate change. In some cases, however, part of their decline is attributed to climate-caused impacts on fish larvae, or on organisms at lower levels of the food web, for example, through changes in marine plankton which is a food source of cod larvae in the N. Atlantic.
Major changes in geographic distribution can be observed for several marine organisms, such as intertidal communities (e.g., kelp forest fish) in several regions. Subtropical fish are observed more frequently in northern waters. There appears to be a general northward move of planktonic communities in the N. Atlantic, by about 10° latitude over the past 40 years, which is much more pronounced than anything documented from terrestrial ecosystems. Different species and functional groups change their seasonal cycle in different ways as response to warming, leading to a disruption of the interactions between different trophic levels.
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