MODELING THE DYNAMICS OF THE NORTH-SEA MESOZOOPLANKTON

A simple biomass-only zooplankton submodel is presented, describing th e dynamics of copepods and carnivorous zooplankton in the North Sea. T his submodel together with the other process-oriented submodels (viz. phytoplankton dynamics, the microbial food web, benthic processes, fis h dynamics and large-scale advective transport) forms a spatially reso lved simulation model of the North Sea ecosystem, the European Regiona l Seas Ecosystem Model (ERSEM). A large set of field measurements of z ooplankton abundance has been assembled against which to compare the E RSEM's performance. These data are not only internally consistent, but have also been gathered at the large spatial scales appropriate to th e ERSEM. In addition to the spatially resolved, monthly estimates of z ooplankton abundance, several instantaneous, in situ estimates of the carbon fluxes between different components of the planktonic web in th e northern North Sea are presented. Simulated dynamics are in good agr eement with the data only during the mid-summer to mid-winter period. During the latter part of the winter and throughout the spring period zooplankton abundance is underpredicted and the simulated zooplankton growth rate is overpredicted during spring. The excessive decline of m esozooplankton biomass during winter may be caused by failing to captu re many of the behavioural/physiological changes which zooplankton man ifest during winter. It is suggested that the excessive spring growth is a consequence of a. a failure to properly distinguish between somat ic and population growth, b. an inadequate representation of the small scale processes which influence feeding success, and c. an excessive spring phytoplankton bloom. The large phytoplankton bloom is, in part at least, a consequence of the excessively low simulated standing crop of omnivorous zooplankton in spring.