1.3.4.2 Changes in marine ecosystems

There is an accumulating body of evidence to suggest that many marine ecosystems, including managed fisheries, are responding to changes in regional climate caused predominately by warming of air and sea surface temperatures (SSTs) and to a lesser extent by modification of precipitation regimes and wind patterns (Table 1.5). The biological manifestations of rising SSTs have included biogeographical, phenological, physiological and species abundance changes. The evidence collected and modelled to date indicates that rising CO2 has led to chemical changes in the ocean, which in turn have led to the oceans becoming more acidic (Royal Society, 2005). Blended satellite/in situ ocean chlorophyll records indicate that global ocean annual primary production has declined by more than 6% since the early 1980s (Gregg et al., 2003), whereas chlorophyll in the North-east Atlantic has increased since the mid-1980s (Raitsos et al., 2005).

Table 1.5. Examples of changes in marine ecosystems and managed fisheries.

In the Pacific and around the British Isles, researchers have found changes to the intertidal communities, where the composition has shifted significantly in response to warmer temperatures (Sagarin et al., 1999; Southward et al., 2005). Similar shifts were also noted in the kelp forest fish communities off the southern Californian coast and in the offshore zooplankton communities (Roemmich and McGowan, 1995; Holbrook et al., 1997; Lavaniegos and Ohman, 2003). These changes are associated with oceanic warming and the resultant geographical movements of species with warmer water affinities. As in the North Atlantic, many long-term biological investigations in the Pacific have established links between changes in the biology and regional climate oscillations such as the ENSO and the Pacific Decadal Oscillation (PDO) (Stenseth et al., 2002). In the case of the Pacific, these biological changes are most strongly associated with El Niño events, which can cause rapid and sometimes dramatic responses to the short-term SST changes (Hughes, 2000). However, recent investigations of planktonic foraminifera from sediment cores encompassing the last 1,400 years has revealed anomalous change in the community structure over the last few decades. The study suggests that ocean warming has already exceeded the range of natural variability (Field et al., 2006). A recent major ecosystem shift in the northern Bering Sea has been attributed to regional climate warming and trends in the Arctic Oscillation (Grebmeier et al., 2006).

The progressive warming in the Southern Ocean has been associated with a decline in krill (Atkinson et al., 2004) and an associated decline in the population size of many seabirds and seals monitored on several breeding sites (Barbraud and Weimerskirch, 2001; Weimerskirch et al., 2003). Some initial observations suggest that changes to the ice habitat via the total thickness of sea ice and its progressively earlier seasonal break-up in the Arctic and Antarctic caused by regional climate warming has had a detrimental impact on marine mammal and seabird populations (Forcada et al., 2005, 2006; Stirling and Parkinson, 2006).

In the North Atlantic, changes in both phytoplankton and zooplankton species and communities have been associated with Northern Hemisphere temperature (NHT) trends and variations in the NAO index. These have included changes in species distributions and abundance, the occurrence of sub-tropical species in temperate waters, changes in overall phytoplankton biomass and seasonal length, changes in the ecosystem functioning and productivity of the North Sea, shifts from cold-adapted to warm-adapted communities, phenological changes, changes in species interactions, and an increase in harmful algal blooms (HABs) (Fromentin and Planque, 1996; Reid et al., 1998; Edwards et al., 2001, 2002, 2006; Reid and Edwards, 2001; Beaugrand et al., 2002a, 2003; Beaugrand and Reid, 2003; Edwards and Richardson, 2004; Richardson and Schoeman, 2004). Over the last decade, numerous other investigations have established links between the NAO and the biology of the North Atlantic, including the benthos, fish, seabirds and whales (Drinkwater et al., 2003) and an increase in the incidence of marine diseases (Harvell et al., 1999). In the Benguela upwelling system in the South Atlantic, long-term trends in the abundance and community structure of coastal zooplankton have been related to large-scale climatic influences (Verheye et al., 1998).

Recent macroscale research has shown that the increase in regional sea temperatures has triggered a major reorganisation in calanoid copepod species composition and biodiversity over the whole North Atlantic Basin (Figure 1.3) (Beaugrand et al., 2002a). During the last 40 years there has been a northerly movement of warmer-water plankton by 10° latitude in the North-East Atlantic and a similar retreat of colder-water plankton to the north. This geographical movement is much more pronounced than any documented terrestrial study, presumably due to advective movements accelerating these processes. In terms of the marine phenological response to climate warming, many plankton taxa have been found to be moving forward in their seasonal cycles (Edwards and Richardson, 2004). In some cases, a shift in seasonal cycles of over six weeks was detected, but more importantly the response to climate warming varied between different functional groups and trophic levels, leading to a mismatch in timing between different trophic levels (Edwards and Richardson, 2004).

Figure 1.3. Long-term changes in the mean number of marine zooplankton species per association in the North Atlantic from 1960 to 1975 and from 1996 to 1999. The number of temperate species has increased and the diversity of colder-temperate, sub-Arctic and Arctic species has decreased in the North Atlantic. The scale (0 to 1) indicates the proportion of biogeographical types of species in total assemblages of zooplankton. From Beaugrand et al., 2002b. Reprinted with permission from AAAS.