Continuous plankton records: Phytoplankton in the Nnorth Sea, 1958–1980, with special reference to 1980

Synoptic surveys of the plankton at a depth of 10 m in the North Sea have been carried out at monthly intervals using Continuous Plankton Recorders. Fluctuations in abundance of all diatoms and some dinoflagellates from one year to another have shown a downward trend from 1958 onwards and all taxa, except Thalassiosira spp. were much below average in 1980. Interpretation of these results has been attempted through empirical associations with biological and physical environmental factors; the percentage incidence of rare species; zooplankton biomass as an index of grazing pressure; sea-surface temperature and salinity; westerly weather type as an index of climatic change; wind strength as an estimate of mixing of the surface layer; hours of bright sunshine at Lowestoft as an estimate of radiant energy. Principal component analysis was used to extract the main patterns of changes in the plankton and correlation analysis for assessing the time-series of results for the phytoplankton in relation to environmental factors. In general, the values of the first principal component of variability from one year to another of the phytoplankton, zooplankton biomass, the occurrence of rare species of phytoplankton, the westerly weather index and wind strength are correlated. Seven out of a possible ten correlations between these parameters are significant at the 5% level, at least. Empirical associations between the plankton and the environment emphasize the point that the abundance of plankton is controlled by both biological and physical factors. All conditions which are thought to affect the production of plankton and for which data were available were below average in 1980; their effects may have been additive and responsible for the low abundance in that year.