Temporal variability and community composition of zooplankton at station L4 in the Western Channel: 20 years of sampling

Zooplankton community composition and diversity have been analysed for the period 1988-2007 at the coastal station L4 off Plymouth in the Western Channel. The seasonal cycle of the total zooplankton was characterized by two peaks. The total zooplankton abundance did not show any long-term trend and its interannual variations were not directly related to changes in environmental conditions. However, different periods of change in total zooplankton abundance were observed that appeared to be consistently related to changes in sea surface temperature (SST) and wind. The number of taxa (22) contributing 80-90% of the total zooplankton abundance was consistent over the 20 years and their cumulative contribution to the community showed only small interannual variations. A significant long-term increase in species richness of the copepod community was detected and its maximum annual value was significantly related to annual average SST. There was evidence of long-term changes: numbers of Oncaea spp., Cirripede larvae, Calanus helgolandicus and Chaetognaths have increased, whereas numbers of Pseudocalanus elongatus, Temora longicornis, Acartia clausi, Evadne nordmanni, Appendicularians, Podon spp. and Ctenocalanus vanus have decreased. The use of indicator species to monitor hydroclimatic changes in the Western Channel is discussed.