Edwards, M. and Heath, Michael and McQuatters Gollop, A.; (2010) Plankton. In: Annual Report Card 2010-11, Science Review. Marine Climate Change Impacts Partnership.

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Major changes have taken place in both the plant (phyto-) and animal (zoo-) plankton of the seas around the British Isles over the last few decades. They include: The important decadal climate indicator for the North Atlantic, the North Atlantic Oscillation (NAO), has been generally rising (with occasional negative NAO years) along with Northern Hemisphere Temperatures over the past 30 years and the surface waters of the European Continental shelf have been warming. This has caused extensive changes in the planktonic ecosystem in terms of plankton production, biodiversity, and species distribution which have had effects on fisheries production and other marine life (e.g. seabirds) (Reid & Edwards, 2001; Edwards et al., 2001, 2002; Beaugrand et al., 2003, 2004; Richardson & Schoeman, 2004; Southward et al., 2004; Alheit et al., 2005; Heath, 2005). In the North Sea the population of the previously dominant and important zooplankton species, (the cold water copepod Calanus finmarchicus) has declined in biomass by 70% since the 1960s. Species with warmer-water affinities (e.g. Calanus helgolandicus) are moving northward to replace the species but are not numerically as abundant (Beaugrand et al., 2004; Edwards et al., 2007). There has been a northward shift in the distribution of many plankton and fish species by more than 10o latitude over the past fifty years. This shift is particularly associated with the shelf edge current running north along the European continental margin (Beaugrand et al., 2002; Brander et al., 2003; Genner et al., 2004). The seasonal timing of plankton production has also been altered in response to recent climate changes. This has consequences for plankton predator species, including fish, whose life cycles are timed in order to make use of seasonal production of particular prey species (Edwards & Richardson, 2004). The decline of the European cod stocks due to overfishing may have been exacerbated by climate warming and climate induced changes in plankton production. It is hypothesised that the survival of young cod in the North Sea depends on the abundance, seasonal timing and size composition of their planktonic prey. As the stocks declined they have become more sensitive to the effects of regional climate warming due to shrinkage of the age distribution and geographic extent (Brander, 2005). The effects of an abrupt ecosystem shift in the late 1990s were most pronounced in regions of the north-eastern Atlantic near the 9-10°C sea surface temperature isotherm, a critical thermal boundary between 'warm' and 'cold' water ecosystems. As waters warm this ecosystem boundary has moved northwards. Future warming is likely to alter the geographical distribution of primary and secondary pelagic production, affecting ecosystem services such as oxygen production, carbon sequestration and biogeochemical cycling. These changes may place additional stress on already-depleted fish stocks as well as have consequences for mammal and seabird populations.