Plankton, jellyfish and climate in the North-East Atlantic.

Extensive changes in plankton ecosystems around the British Isles over the last 60 years, including production, biodiversity and species distributions, have had effects on fisheries production and other marine life. This has been mainly driven by climate variability and ocean warming. These changes include:

• Extensive changes in the planktonic ecosystem in terms of plankton production, biodiversity, species distribution which have effects on fisheries production and other marine life (e.g. seabirds).

• In the North Sea, the population of the previously dominant and important zooplankton species, (the cold-water species Calanus finmarchicus) has declined in biomass by 70% since the 1960s. Species with warmer-water affinities (e.g. Calanus helgolandicus) are moving northwards to replace the species, but are not as numerically abundant.

• There has been a shift in the distribution of many plankton and fish species around the planet. For example, during the last 50 years there has been a northerly movement of some warmer water plankton by 10° latitude in the Northeast Atlantic and a similar retreat of colder water plankton northwards (a mean poleward movement of between 200-250 km per decade).

• The seasonal timing of some plankton production has also 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.

• The decline of the European cod stocks due to overfishing may have been exacerbated by climate warming and climate-induced changes in plankton production (Beaugrand et al., 2003). 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.

• Future warming is likely to alter the geographical distribution of primary and secondary pelagic production, affecting ecosystemservices such as oxygen production, carbon sequestration andbiogeochemical cycling. These changes may place additional stresson already depleted fish stocks, as well as have consequences formammal and seabird populations. Additionally, melting of Arcticwaters may increase the likelihood of trans-Arctic migrations ofspecies between the Pacific and Atlantic oceans.