Expansion of the model ‘Rostherne’ for fish and zooplankton: role of top-down effects in modifying the prevailing pattern of ecosystem functioning

This paper presents a new version of the well-established aquatic ecosystem model ‘Rostherne’, incorporating lake fish and zooplankton. The model currently comprises differential and algebraic equations describing processes and forcing functions most important for a freshwater ecosystem. These include seasonal changes of solar radiation and water temperature, processes of algal and cyanobacterial population dynamics and nutrient uptake, water and chemical budgeting, stratification of the water column and sedimentation of suspended particles, and dynamics of detritus and its chemical constituents. Although it was shown previously for some years that a reasonable simulation of changes in most state variables could be achieved without accounting for fish and zooplankton, in other years, consideration of these compartments improved the fit between observations and model simulations dramatically. This was particularly true for the year 1998, when the high zooplankton numbers in spring appeared to prevent any significant development of the diatom population. The latter, however, bloomed during the first part of the summer (following a decrease in zooplankton counts) causing an unusually delayed major drop in ambient Si levels. As biogoechemical cycles of Si, P and N are interconnected through the dynamics of the primary producers, such a delay has serious implications for the functioning of an aquatic ecosystem model. Since changes in zooplankton dynamics are heavily dependent on the variability in fish recruitment, simultaneous consideration of the two components is helpful both for modelling overall dynamics of the lake ecosystem and for implementation of the biogeochemical regulation proposed previously.