A versatile model of nutrient retention in relation to ecosystem state in shallow lakes: GPLake-R

Nutrient pollution of surface waters contributes to eutrophication problems and constitutes a loss of valuable resources for human food production. Nutrient retention in lakes prevents part of this loss and downstream pollution, and depends on the ecosystem state (e.g., macrophyte-dominated shallow lakes having higher phosphorus retention than phytoplankton-dominated shallow lakes). We developed a relatively simple and versatile model of nutrient retention in relation to ecosystem state in shallow lakes: GPLake-R. Here the "GP" stands for "generically parameterized" and "R" stands for "retention". We build on the GPLake-M model that describes equilibrium macrophyte and phytoplankton abundance in shallow lakes in response to nutrient loading, by adding the ecologically relevant options for co-limitation of nutrients and light in macrophytes and phytoplankton and coexistence of macrophytes and phytoplankton around critical nutrient loadings. In this approach we combined insights from resource competition theory while adhering to the principle of mass conservation. As an outcome, GPLake-R gives a single equation for phosphorus retention in relation to ecosystem state that captures the hysteretic pattern from the more complex PCLake model. GPLake-R has a strong educational potential and can serve as a building block to illustrate the effect of water quality and nutrient retention management strategies in networks of shallow lakes. Moreover, we found that the co-limitation and coexistence processes in GPLake-R generally lessen nutrient retention, which could lead to higher downstream nutrient pollution than expected based on earlier approaches. Therefore, we conclude that it is important to consider resource co-limitation and species coexistence when developing novel water quality management strategies for interconnected water systems.