Effects of organic carbon and phosphorus on lake food web dynamics: A modelling study

Aquatic food webs are increasingly influenced by global environmental change, which alters external nutrient and organic carbon inputs and modifies lake hydrology. Using a chemostat-type model parameterised with values representative of mesotrophic lakes, we examined how inflow concentrations of biodegradable dissolved organic carbon (BDOC; the labile fraction of dissolved organic carbon) and dissolved orthophosphate, together with water retention time, affect the dynamics of a simple pelagic food web comprising algae, bacteria, and small and large zooplankton. We found two qualitative dynamical regimes across the inflow gradients explored: stable equilibria and sustained oscillations in concentrations of the food-web groups. At low-to-moderate orthophosphate inflow, dynamics converged to equilibrium, whereas higher orthophosphate inflow produced oscillations. Increasing BDOC inflow tended to stabilise the system and suppress oscillations. Retention time modulated these outcomes: both short and long retention times promoted stability, while intermediate retention times could sustain oscillations in both biotic and abiotic groups. Overall, the model highlights how coupled variation in external organic carbon and phosphorus inflow and hydrologic forcing can reshape trophic dynamics, offering a mechanistic framework for interpreting food-web responses to eutrophication, brownification (increasing dissolved organic carbon and water colour), and climate-driven shifts in precipitation and flushing.