PLANKTON COMMUNITY STRUCTURE AND GEOGRAPHICAL-DISTRIBUTION IN THE NORTH-SEA

Continuous Plankton Recorder (CPR) data from July 1984 to June 1987 co llected in the North Sea were re-analysed. These data were sampled mon thly throughout the period and the most abundant species and taxa of p lankton, 35 phytoplankton and 36 zooplankton, were selected for analys es. Multidimensional scaling ordination, cluster analysis and principa l component analysis were used to identify the phytoplankton and zoopl ankton assemblages which showed persistence in time and space and to d efine their geographical boundaries. The three techniques separated ma jor geographical goupings based on species similarity and composition. These groupings were associated with: (1) shallow permanently mixed w aters of the southern North Sea, (2) transitional waters in the centra l area of the North Sea, and (3) seasonally thermally stratified water s of the northern North Sea and off the north-east coast of the UK. Th e zooplankton species showed two well defined groupings with the speci es associated with shallow tidally mixed neritic waters distinct from species associated with stratified shelf waters under oceanic influenc e. The first component of the principal components analysis described over 30% of the variance in this group. The demonstration of persisten t and definable plankton assemblages in the North Sea was a necessary requirement prior to equipping CPRs with new instrumentation packages in 1988.