Temporal patterns in the surface abundance of Calanus finmarchicus and C-helgolandicus in the northern North Sea (1958-1996) inferred from ContinuousPlankton Recorder data

Patterns in the temporal abundance of Calanus finmarchicus and C. helgoland icus in 3 sub-regions of the northern North Sea are described from Continuo us Plankton Recorder data, updating earlier studies with information from 3 more years (1994, 1995 and 1996). Abundances are considered using time-ser ies techniques that allow the annual seasonal cycles observed within each s ub-region to change over the long term. It is demonstrated that, as long-te rm trends of C. finmarchicus and C. helgolandicus fluctuated in the norther n North Sea between 1958 and 1996, simultaneous changes also occurred in th e annual seasonal cycles of both species. The results are discussed in rela tion to local sea-surface temperatures and the North Atlantic Oscillation i ndex.