Temporal patterns in the surface abundance of Calanus finmarchicus and C. helgolandicus in the northern North Sea (1958–1996) inferred from Continuous Plankton Recorder data

Patterns in the temporal abundance of Calanus finmarchicus and C. helgolandicus in 3 sub-regions of the northern North Sea are described from Continuous Plankton Recorder data, updating earlier studies with information from 3 more years (1994, 1995 and 1996). Abundances are considered using time-series techniques that allow the annual seasonal cycles observed within each sub-region to change over the long term. It is demonstrated that, as long-term trends of C. finmarchicus and C. helgolandicus fluctuated in the northern North Sea between 1958 and 1996, simultaneous changes also occurred in the annual seasonal cycles of both species. The results are discussed in relation to local sea-surface temperatures and the North Atlantic Oscillation index.