Modelling How Bottom-Up and Top-Down Processes Control the Major Functional Groups of Biota in a Large Temperate Shallow Lake

Ecosystem models that measure the impact of quantitative interactions between trophic levels are widely used tools in ecosystem studies and fishery management. We constructed a mass-balance trophic model using an Ecopath with Ecosim (EwE) modelling suite for large shallow Lake Vortsjarv, Estonia. The model was calibrated for 36 years (1983-2018) and included 23 functional groups. We examined trophic relationships, functional group interactions, energy fluxes, and keystone groups having a high impact on the ecosystem relative to their biomass. We tested 6 hypothetical scenarios based on future biomass changes for the major functional groups (phytoplankton, zooplankton, macrozoobenthos, piscivorous fish, and bream) for 20 years. The output of the predictive scenarios showed that the biomass changes of planktonic groups would affect the whole food web. Among consumers, macrozoobenthos was crucial for the food web balance because a reduction of their biomass would also reduce the biomass of the fish community. Changes in fish catches would cause minimal biomass difference in other groups. While increased fishing pressure on large piscivores would have a marked effect on the rest of the food web, the reduction of nonpiscivorous fish like bream would have little effect. The results suggested a positive relationship between the biomass of small phytoplankton and fish, alluding to the prevalence of bottom-up trophic processes. These outcomes could be helpful for assessing trophic dynamics in shallow lakes and important aspects for fisheries and ecosystem management.