Legacy Phosphorus and Ecosystem Memory Control Future Water Quality in a Eutrophic Lake

Lake water clarity, phytoplankton biomass, and hypolimnetic oxygen concentration are metrics of water quality that are highly degraded in eutrophic systems. Eutrophication is linked to legacy nutrients stored in catchment soils and in lake sediments. Long lags in water quality improvement under scenarios of nutrient load reduction to lakes indicate an apparent ecosystem memory tied to the interactions between water biogeochemistry and lake sediment nutrients. To investigate how nutrient legacies and ecosystem memory control lake water quality dynamics, we coupled nutrient cycling and lake metabolism in a model to recreate long-term water quality of a eutrophic lake (Lake Mendota, Wisconsin, USA). We modeled long-term recovery of water quality under scenarios of nutrient load reduction and found that the rates and patterns of water quality improvement depended on changes in phosphorus (P) and organic carbon storage in the water column and sediments. Through scenarios of water quality improvement, we showed that water quality variables have distinct phases of change determined by the turnover rates of storage pools-an initial and rapid water quality improvement due to water column flushing, followed by a much longer and slower improvement as sediment P pools were slowly reduced. Water clarity, phytoplankton biomass, and hypolimnetic dissolved oxygen differed in their time responses. Water clarity and algal biomass improved within years of nutrient reductions, but hypolimnetic oxygen took decades to improve. Even with reduced catchment loading, recovery of Lake Mendota to a mesotrophic state may require decades due to nutrient legacies and long ecosystem memory. Lake water quality, as measured by algae concentration near the lake surface, the clarity of the water, and the availability of dissolved oxygen, is greatly reduced in lakes with nutrient pollution from phosphorus. In Lake Mendota, Wisconsin, phosphorus applied to the surrounding landscape for more than a century has accumulated in catchment soils and in the lake water column and sediments (i.e., legacy phosphorus), leading to poor water quality. To investigate how water quality in Lake Mendota might respond to nutrient reduction, we used computer models to simulate the elimination of phosphorus inputs from the catchment and track water quality change. Phosphorus in the lake water column initially decreased quickly, due to water column flushing, but then decreased very slowly due to release of legacy phosphorus from lake sediments. Water quality recovery lagged that of phosphorus, indicating an inherent ecosystem memory for past phosphorus levels. Ecosystem memory was due to biological activity that remained elevated, even when phosphorus was declining in the water column. When nutrient inputs to the lake were eliminated in the model, recovery of algae concentrations and water clarity to pristine conditions required decades, and a return to a fully oxygenated condition required a century. Legacy phosphorus in lake sediments controls long term lake water quality response to nutrient remediationCoupled cycles of nutrients, physics, and metabolism explain ecosystem memory of lake phosphorus, water clarity, and oxygen habitatImprovement in lake water quality to pristine levels will require decades of commitment to nutrient load reductions

Automated summary

Lake water quality is greatly degraded in eutrophic systems due to legacy nutrients. Water quality improvement depends on changes in phosphorus and organic carbon storage, with different variables improving at different rates.