A numerical simulation of the role of zooplankton in C, N and P cycling in Lake Kinneret, Israel

We quantify the role of zooplankton in nutrient cycles in Lake Kinneret, Israel, using field data and a numerical model. A coupled ecological and hydrodynamic model (Dynamic Reservoir Model (DYRESM)–Computational Aquatic Ecosystem Dynamics Model (CAEDYM)) was validated with an extensive field data set to simulate the seasonal dynamics of nutrients, three phytoplankton groups and three zooplankton groups. Parameterization of the model was conducted using field, experimental and literature studies. Sensitivity of simulated output was tested over the full parameter space and established that the most sensitive parameters were related to zooplankton grazing rates, temperature responses and food limitation. The simulated results predict that, on average, 51% of the carbon from phytoplankton photosynthesis is consumed by zooplankton. Excretion of dissolved nutrients by zooplankton accounts for 3–46 and 5–58% of phytoplankton uptake of phosphorus and nitrogen, respectively. Comparison of nutrient fluxes attributable to zooplankton with nutrient loads from inflows and release from bottom sediments shows that the relative contribution by zooplankton to inorganic nutrients in the photic zone varies seasonally in response to the annual hydrodynamic cycle of stratification and mixing. As a percent of total dissolved organic sources relative contributions by zooplankton excretion are highest (62%) during periods of stratification and when inflow nutrient loads are low, and lowest (2%) during the breakdown of stratification and when inflow loads are high. The results illustrate the potential of a lake ecosystem model to extract useful process information to complement field data collection and address questions related to the role of zooplankton in nutrient cycles.