A Modular Approach for Modelling of the North Sea Ecosystem

Abstract

A modular approach for generating an ecosystem model for the North Sea is presented. The model structure consists of modules describing physical, chemical and biological processes. The modular approach is selected to facilitate stepwise improvements in the total ecosystem model by replacing existing modules with improved modules being developed. The modules constituting the pelagic ecosystem describe the biological and chemical dynamics of particle production and dissolution simulating the flux of carbon and nutrients (N, P, Si) through the food web. This might be done as a suite of submodules based on functional groups of organisms or as modules describing the different trophic levels based on size distributions. Algal growth is dependent on cellular content of the limiting nutrient, or on net photosynthesis, whichever is the most restricting, allowing for “luxury uptake” of nutrients which may be stored for subsequent periods of shortage. The role of bacteria and other microorganisms is emphasized in recognition of the importance of the “microbial loop”. The Zooplankton module describes prey ingestion in terms of feeding behaviour and the partitioning of ingested carbon, nitrogen and phosphorus into growth and reproduction and the losses through respiration, excretion and defecation. The benthic modules concentrate on describing the small food web, since benthic biological activity in terms of carbon flow, as well as mineralization, are often dominated by micro- and meiofauna in the generally soft sediments of the North Sea. Macrobenthos are described as being a major link between the benthic small food web and higher trophic levels in the ecosystem. Higher trophic levels are described in separate modules taking into account in principle the role of fish, mammals and seabirds. Nutrient cycling is described, focusing on the identification and conceptual modelling of the chemical processes and mechanisms in order to describe the relationships between the biology of the North Sea and the nutrient chemistry. This represents a first step towards forecasting the response of the system to long-term changes due to e.g. eutrophication. The modelling of the nutrient sediment-water interaction emphasizes the role of sedimentation of particulates and the regeneration of inorganic components to the water column. This totally modular concept of the North Sea ecosystem model reflects the authors' view on the present state of the North Sea, the basic knowledge about ecosystem behaviour and a way of creating models as a tool for better understanding of the ecosystem and how man affects the North Sea environment.