Relative roles of fundamental processes underpinning PEG dynamics in dimictic lakes as revealed by a self-organizing, multi-population plankton model

The longstanding PEG (Phytoplankton Ecology Group) qualitative/conceptual model describes generalized seasonal phytoplankton dynamics in temperate lakes and suggests important mechanisms driving these processes. In the research presented here, we re-created the conceptual PEG dynamics using a novel mechanistic numerical plankton model. The numerical model incorporates only a handful of interacting factors and biological rules. Key model features include seasonally variable temperature, mixing depth, and light, dynamic zooplankton and phytoplankton populations, and multiple dynamic nutrient concentrations. The model design is unique in that it incorporates a population-rich phytoplankton assemblage where life-history traits of members are based on ecological principles (trade-offs between traits), allowing the assemblage to organize over time based on nutrient competition and grazing. Using 50th year solutions to the model solved with fourth-order Runge-Kutta methods, i.e., a period when the dynamics had reached stable non-equilibrium states, sensitivity analyses were run by individually varying model parameters +/- 50% and comparing the effect on model responses of interest. Simulation analyses revealed the important role light and temperature limitation played both early and late in the year controlling plankton biomass. Trade-offs in phytoplankton life-history characteristics determined the composition of the spring bloom under non-nutrient limiting conditions (fast-growing, edible populations dominated) and the post clear-water phase assemblage under low nutrient conditions (slower-growing, less-edible populations dominated). Grazer characteristics, specifically rate of grazing and spring “emergence”, determined the occurrence of the clear-water phase and influenced the post assemblage composition, where a clear-water phase was absent when the grazing rate was low or spring emergence absent, and the highly inedible populations were lost from the assemblage. Hydraulic flushing also played an important role where diminished inflow led to higher richness, evenness and diversity due to greater overwintering of less-edible taxa, which influenced phytoplankton composition during spring bloom, diminishing secondary productivity. These results increase understanding of the relative roles of fundamental mechanisms structuring natural aquatic ecosystems and reinforce the significance of key biotic and abiotic system components of temperate lakes.