Monitoring marine plankton ecosystems. II: Long-term changes in North Sea calanoid copepods in relation to hydro-climatic variability

Recently, a framework has been proposed to monitor plank-ton ecosystems in the North Atlantic and adjacent seas using calanoid copepod species. In this study, we use this framework to investigate, at the community structure (calanoid copepod) level, the long-term changes in plankton ecosystems related to hydro-climatic variability in the North Sea during the period 1958-1999. A chronology of ecological events that occurred in the North Sea is outlined. In addition to the long-term and year-to-year variability, this study reveals that North Sea plankton ecosystems had 2 dynamic regimes during the period 1958-1999: a cold-biological (1962-1982) and a warm-biological dynamic regime (1984-1999). The impact of the regime shift on the community structure of calanoid copepods and total diversity (as mean number of calanoid copepod species per continuous plankton recorder sample) is detectable in the stratified regions of the North Sea after ca. 1983. This study reveals that the regime shift resulted from the conjunction of both local and regional hydro-climatic forcing and a change in the location of an oceanic biogeographical boundary in the north-east Atlantic Ocean, Results indicate a strong dependence of ecological processes in the North Sea to both hydro-climatic and biological variability in the north-east Atlantic Ocean. If the current climate warming persists, results suggest that this may continue to alter the structure of North Sea ecosystems and lead to other regime shifts, thus making it very challenging to predict future responses of North Sea pelagic ecosystems to climate change.