Netherlands Journal of Sea Research最新文献

The primary production module incorporated in the European Regional Seas Ecosystem Model (ERSEM) is described in detail. It considers two phytoplankton groups, diatoms and autotrophic flagellates, and four different nutrients: nitrate, ammonia, phosphate and silicate (only for diatoms). All the related state variables and fluxes are represented in terms of carbon, nitrogen, phosphorus and silicon.

The potential carbon growth rate process is estimated by considering that nutrient availability acts as a limitation factor on the maximum growth rate which is itself a function of light and water temperature. Respiration, excretion of organic matter, lysis and sinking are the main carbon and nutrient loss processes. Results indicate that the model simulates well the annual phytoplankton dynamics in the central regions of the North Sea, underestimating primary production and chlorophyll in the southern North Sea. The model gave good correlations with the main dissolved nutrients, such as silicate, phosphate or nitrate. The primary production module proved to be especially sensitive to the flagellate/diatom interaction and competitive behaviour for inorganic nutrients as well as with regard to grazing losses. It is suggested that a major improvement could be made by including a third phytoplankton group (e.g., Phaeocystis) in the model structure, and that comparison with other phytoplankton growth schemes based on the Droop formulation is advisable.

The software environment SESAME is a model development and analysis tool designed to facilitate the construction of ecological models using Fortran-77 on UNIX machines. It consists of sets of routines which are called from a menu-driven program to perform tasks such as automatic compiling, linking and loading of a model, simulation runs, graphical and numerical output, analysis of results and comparison with available field data. It provides a choice of integration methods for simulating continuous processes and can also handle discrete processes such as encountered in systems with age- or size-structured populations. One essential feature of the package is a routine which links submodules unambiguously by automatically generating the information which needs to be common to all sections of the model. This feature, together with the concept of the distributed derivative, makes SESAME especially useful to interdisciplinary teams working on the simultaneous development and testing of complex, spatially-resolved ecosystem models comprising many modules. Some examples of models constructed with SESAME are mentioned.

In any ecosystem modelling approach to shallow seas an adequate description of the sediment-water interactions to all the essential nutrients is necessary. With this aim a fairly simple concept has been developed and applied for the modelling of the nutrient cycles of N, P and Si which includes the essential diagenetic processes such as vertical transport, oxic and anoxic mineralization, silicate dissolution, adsorption, nitrification and denitrification. All these processes are explicitly formulated for one or more distinct layers in the sediment. The model contains a low number of variables and is efficient with respect to computer time. The model is part of the European Regional Seas Ecosystem Model (ERSEM) of the North Sea, a joint effort of several institutes around the North Sea. The benthic-pelagic coupling of the ERSEM model is discussed with emphasis on the consequences for the benthic nutrient cycling and the interrelations between the different benthic nutrient submodels. The results show that the model is able to describe the seasonal variation of nutrient fluxes including the sediment-water exchanges and the vertical profiles in the pore water. From the model results is concluded that changes in organic matter deposition directly influence the benthic nitrification and denitrification through changes in the oxygen availability to the nitrifiers and, due to this, the supply of nitrate to the denitrifiers. Short fluctuations in organic deposition (e.g. after the spring bloom) are not reflected in the fluxes due to sorption buffering of phosphate in the oxidized surface layer of the sediment.

The development and analysis of a fish population module within the European Regional Seas Ecosystem Model (ERSEM) is described. Fixed predation rates on ERSEM zooplankton and zoobenthos, which may be used in place of this dynamic module of fish, are described also. The dynamic module is age-structured, with state variables for the number of individuals and their weight, the latter partitioned into structural (skeletal, essentially) and remobilizable ‘reserve’ components. It is capable of prediction of observed patterns of both positive and negative changes in individual weight. The fish are modelled as one population for the whole North Sea, superimposed on the ERSEM box structure. Parameters have been derived from the literature where possible and, for herring, certain parameters have been adjusted to match independent estimates of growth in the field. Preliminary results show that herring growth rate depends inversely on biomass, when changes in the latter are caused by altered fishing mortality. The magnitude of these effects is in agreement with historical data for herring in the North Sea. The model is discussed in relation to previous attempts to model fish population dynamics, particularly in the North Sea.

The submodel describing benthic biology including a bioturbation module as incorporated in the European Regional Seas Ecosystem Model (ERSEM) is discussed. It is linked to a nutrient dynamic model. The structure of the benthic model food web is presented. There are four macrobenthic functional groups, meiobenthos and aerobic and anaerobic bacteria. The modelling uses ‘standard organisms’ as basic building blocks. The choice of parameter values is discussed. The results demonstrate the dependence of the benthic system on the pelagic system. The importance of features such as predation within functional groups for stability of the system is investigated. Detritus input from the pelagic system and detritus recycling is most important in the benthic food web. The web of carbon and nutrient fluxes through the system is analysed. On the basis of the food web analysis, the trophic positions of the functional groups are calculated. Besides the benthic biology, the mathematical formulation of the bioturbation and diffusion enhancement is discussed. Macrobenthic presence and activity enhance diffusion in the sediment and contribute essentially to vertical transport of particulate matter. This is of great importance for the vertical distribution of detritus, and as a consequence, for microbial activity in the sediment layers.

In the framework of the complex dynamical European Regional Seas Ecosystem Model (ERSEM) a module describing the microbial part of the pelagic ecosystem has been developed. The module contains the carbon and nutrient dynamics of pelagic bacteria, heterotrophic flagellates and microzooplankton and interacts with the other parts of the model via phytoplankton, particulate and dissolved organic matter and mesozooplankton. A short description of the module is given and the results are discussed. It is demonstrated that in an application of ERSEM to the North Sea there is a gradual shift in dominance from the continental coast boxes to the offshore deeper areas between the different food webs, from what in the literature is termed the classical food web to the microbial food web, concomitant with a gradual decrease in the efficiency of the microbial loop.