Marine mammals and seabirds are sensitive indicators of changes in ocean environments. Springer (1998) concluded that synchrony in extreme fluctuations of abundance of marine birds and mammals across the North Pacific and Western Arctic were a response to physical changes, including climate warming. The linkages with climate change were compelling enough for Springer to suggest that fluctuations in marine bird and mammal populations in the North Pacific are entirely related to climate variations and change.

The climate variations beginning in the 1990s and associated with El Niño conditions (Trenberth and Hoar, 1996), in combination with overfishing, have been linked to behavioral changes in killer whales. These changes drastically reduced sea otter abundance along the Aleutian Islands, which in turn changed the ecology of the kelp forests (Estes et al., 1998). The changes in prey resulting from persistent changes in climate appear to be one of the important impacts of a changing climate on the marine mammals that feed from the top of the food chain.

Climate change also may have an effect on access to prey among marine mammals. For instance, extended ice-free seasons in the Arctic could prolong the fasting of polar bears (Ursus maritimus), with possible implications for the seal population (Stirling et al., 1999). Reduced ice cover and access to seals would limit hunting success by polar bears and foxes, with resulting reductions in bear and fox populations. This dynamic could have negative effects on the lifestyle, food, and health standards of some indigenous peoples (Hansell et al., 1998). Because global climate change is likely to have profound impacts on sea-ice extent and duration, it is in this habitat where the initial impacts on marine mammals may be first evident. Reductions in sea ice have been predicted to alter the seasonal distributions, geographic ranges, migration patterns, nutritional status, reproductive success, and ultimately the abundance of Arctic marine mammals (Tynan and DeMaster, 1997). Studies recognizing multi-year to decadal variability in marine biotic systems include Mullin (1998) on zooplankton over 5 decades in the eastern Pacific (with connections to El Niño), and Tunberg and Nelson (1998) on soft bottom macrobenthic communities in the northeast Atlantic (with connections to the North Atlantic Oscillation). Sagarin et al. (1999) have argued that changes in the distribution of intertidal macroinvertebrates on rocky shores in California over the past 60 years have been caused by climate change.

Seabirds are an integral part of marine ecosystems, where they may consume vast amounts of fish. It has been estimated that seabirds consume 600,000 t yr^-1 of food in the North Atlantic (Hunt and Furness, 1996). Modeling studies have shown that in several marine ecosystems, seabirds eat 20-30% of the annual pelagic fish production. The dependence on some species of fish, particularly during breeding, and their large abundance make seabirds a good indicator of ecosystem change. Where changes in breeding success or mortality occur, however, distinguishing the climate impact from fishing impacts can be difficult (Duffy and Schneider, 1994). Very few decadal-scale studies of seabirds are available to assess the impacts of long-term variations in climate, however.

In general, seabirds have evolved to adapt to weather patterns (Butler et al., 1997). The ability of a species to alter its migration strategy appears to be important to survival in a changing climate. Food resources appear to be critical to general survival, especially for young seabirds. Dolman and Sutherland (1994) proposed that feeding rate affects the ability of individuals to survive winter. The change in marine ecosystem described by Roemmich and McGowan (1995) was associated with a mortality resulting in a 40% decline in seabird abundance within the California current system from 1987 to 1994 (Veit et al., 1996). The decline was largely related to a dramatic (90%) decline of sooty shearwaters (Puffinus girseus), but the response in the ecosystem was not characterized only by declines. There was a northward movement of some species, and in offshore waters the abundance of the most common species, Leach's storm petrels (Oceanodroma leucorhoa), increased over the same period. The authors were careful to note that the changes in abundance they described could not be related directly to population dynamics because of complex migratory patterns and the size of the habitat.

Such changes are evidence of the sensitivity of seabirds to climate-ocean changes and that survival and distribution impacts will occur as climates shift. The anomalous cold surface waters that occurred in the northwest Atlantic in the early 1990s changed the fish species composition in the surface waters on the Newfoundland shelf. These changes were readily detected in the diets of northern gannet (Sula bassana). The sensitivity of the distribution patterns of the pelagic prey of fish-feeding and plankton-feeding seabirds imply to Montevecchi and Myers (1997) that small changes in the ocean environment resulting from climate changes could affect seabird reproductive success. Changes in fish-feeding seabird abundance in the eastern Bering Sea are related to the abundance of juvenile pollock (Springer, 1992). It has been argued that long lifespans and genetic variation within populations enable seabirds to survive adverse short-term environmental events, as evidenced by the response to El Niño and La Niña events in the tropical Pacific (Ribic et al., 1997). However, small populations tied to restricted habitat, such as the Galapagos Penguin (Spheniscus mendiculus), may be threatened by long-term climate warming (Boersma, 1998).

Changes in precipitation, pH, water temperature, wind, dissolved CO₂, and salinity can affect water quality in estuarine and marine waters. Some marine disease organisms and algal species are strongly influenced by one or more of these factors (Anderson et al., 1998). In the past few decades there has been an increase in reports of diseases affecting closely monitored marine organisms, such as coral and seagrasses, particularly in the Caribbean and temperate oceans. The worldwide increase in coral bleaching in 1997-1998 was coincident with high water temperatures associated with El Niño, but Harvell et al. (1999) suggest that the demise of some corals might have been accelerated by opportunistic infections affecting the temperature-stressed reef systems. Talge et al. (1995) report a new disease in reef-dwelling foraminifera, with implications for coastal sedimentation.

ENSO cycles and increased water temperatures have been correlated with Dermo disease (caused by the protozoan parasite Perkinsus marinus) and MSX (multinucleated spore unknown) disease in oysters along the U.S. Atlantic and Gulf coasts. In addition to affecting marine hosts, several viruses, protozoa, and bacteria affected by climatic factors can affect people, by direct contact or by seafood consumption. Many of the reported cases of water-borne diseases involve gastrointestinal illnesses; some can be fatal in infants, elderly people, and people with weakened immune systems (ASM, 1998).

The bacterium Vibrio vulnificus, which is found in oysters and is potentially lethal to humans with immune-system deficiencies, becomes more abundant as water temperature increases (Lipp and Rose, 1997). The incidence and severity of cholera (Vibrio cholerae) epidemics associated with marine plankton also has been linked with prolonged elevated water temperature. Annual epidemics of cholera in Bangladesh have been correlated with increased SST and sea-surface height (Harvell et al., 1999).

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