Modelling benthic nitrogen cycling in temperate coastal ecosystems

Field: Environmental Science (Physical Sciences)

This chapter describes the ecological processes of the inshore regions of the continental shelf in temperate latitudes. Inmany of these areas, particularly those of the North Atlantic shelf, the high turbulence due to tides and winds ensures good vertical homogeneity of a rather shallow water column. Under such circumstances, sediments playa prominent part in the overall working of the ecosystems, and are in close interaction with the planktonic phase (Paasche; Blackburn, this volume). Besides being the starting point of the benthic trophic chain, of considerable economical importance in coastal regions, sediments play two key roles with respect to the plankton system: 1. They act as regenerators of mineral nutrients for the water column. Direct comparison between benthic nitrogen release and planktonic primary production shows that sediments often supply 30-70% of nitrogen requirements by phytoplankton (Table 14.1). 2. Sediments are often the only site in coastal marine ecosystems where anaerobic conditions exist. They are therefore the site of denitrification, a process which can result in the elimination of up to 25% of nitrogen mineralized in the benthos (Table 14.1). In shallow systems, sediments constitute by far the most important reservoir of nitrogen. Owing to the buffering effect of this large stock, benthic nutrient regeneration is more steady than most water column remineralization processes, and is also more constant than exchange through thermoclines, which are often of intermittent nature. We will see that the seasonal variations of planktonic processesin temperate coastal ecosystemsare largely dependent on the behaviour of the large pools of organic material deposited in the sediments. For understanding the role of benthos in nutrient cycling at the ecosystem level, and for relating this role to the basic microbiological and physicochemical processes occurring in the sedimentary environment, mathematical modelling can be a very powerful tool. A model can be defined as a simplified representation of reality, relating processes at a certain level of complexity to processes at a lower level.Inthis sense models are very close to what is generally considered to be a scientific explanation, and do not necessarily involve mathematics. Mathematical formulation of models is, however, often the simplest way to achieve quantitative predictions. Mathematical models can be used at different stages of an ecological investigation. The current practice is to consider modelling as the final step of the study, as a way of summarizing all observational and experimental data collected, and checking their coherence. Models can also constitute the final aim of the study, because models are often urgently required as a management tool, allowing optimization of human action for altering or restoring the working of ecosystems in a desired direction. But modelling can also be practised at an earlier stage of ecological investigations, and in a more interactive way with respect to experimental and observational work. In this way it can help in planning field work and in identifying the processes which are worth a more detailed study. Result. of mathematical simulation, by aid of roughly idealized models, suggest experiments and observations, which in turn allow us to reconsider and refine the model. It is an implicit ambition ofthe present review to illustrate these different ways of practicing mathematical modelling in ecological investigations. Our aim is not to present the final model of nitrogen recycling in sediments, which would be in any case quite presumptuous! Instead, we shall begin with deliberately oversimplified models of nitrogen transfer in coastal marine ecosystems which will help in understanding the basic roles of sediments in the dynamics of planktonic processes. In the light of these models we shall then examine, in much more detail, the data available in the literature concerning the control mechanisms of those sedimentary processes, identified as important by the preceding discussion. This will allow us to develop a more realistic model of benthic nitrogen cycling, the results of which will be compared to observations reported in the literature.