Netherlands Journal of Sea Research最新文献

The rationale is given of how the gross physical features of the circulation and the stratification of the North Sea have been aggregated for inclusion in the ecosystem box model ERSEM. As the ecosystem dynamics are to a large extent determined by small-scale physical events, the ecosystem model is forced with the circulation of a specific year rather than using the long-term mean circulation field. Especially the vertical exchange processes have been explicitly included, because the primary production strongly depends on them. Simulations with a general circulation model (GCM), forced by three-hourly meteorological fields, have been utilized to derive daily horizontal transport values driving ERSEM on boxes of sizes of a few 100 km. The daily vertical transports across a fixed 30-m interface provide the necessary short-term event character of the vertical exchange.

For the years 1988 and 1989 the properties of the hydrodynamic flow fields are presented in terms of trajectories of the flow, thermocline depths, of water budgets, flushing times and diffusion rates. The results of the standard simulation with ERSEM show that the daily variability of the circulation, being smoothed by the box integration procedure, is transferred to the chemical and biological state variables to a very limited degree only.

Nutrient dynamics for phosphate, nitrate, ammonium and silicate have been simulated with ERSEM, the European Regional Seas Ecosystem Model. From the model results budgets for the dissolved inorganic nutrients and the corresponding particulate fractions have been calculated. The annual cycles of the nutrients phosphate and silicate compare quite well with the observed ranges of variability. This does not hold for ammonium and nitrate. Biologically mediated transformations such as nutrient uptake and pelagic and benthic mineralization are the dominant processes in changing the nutrient concentrations with the horizontal advective contributions playing a minor role during the productive season.

Vertical advection and vertical diffusion have a clear seasonal signal, with a maximum in February. The decay of the advective nutrient transport in summer is caused by the depletion of the upper layer of dissolved inorganic nutrients by algal uptake. The inflow of nutrients in the northwest is almost balanced by the outflow in the northeast, without causing large nutrient transports into the shallower areas from the north. However, from the coastal areas there is a nutrient flow towards the central North Sea, enhancing primary production in the central area.

A simple biomass-only zooplankton submodel is presented, describing the dynamics of copepods and carnivorous zooplankton in the North Sea. This submodel together with the other process-oriented submodels (viz. phytoplankton dynamics, the microbial food web, benthic processes, fish dynamics and large-scale advective transport) forms a spatially resolved simulation model of the North Sea ecosystem, the European Regional Seas Ecosystem Model (ERSEM). A large set of field measurements of zooplankton abundance has been assembled against which to compare the ERSEM's performance. These data are not only internally consistent, but have also gathered at the large spatial scales appropriate to the ERSEM. In addition to the spatially resolved, monthly estimates of zooplankton abundance, several instantaneous, in situ estimates of the carbon fluxes between different components of the planktonic web in the northern North Sea are presented. Simulated dynamics are in good agreement 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 mesozooplankton biomass during winter may be caused by failing to capture many of the behavioural/physiological changes which zooplankton manifest during winter. It is suggested that the excessive spring growth is a consequence of a. a failure to properly distinguish between somatic 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.