Legacy Phosphorus and Implications for Lake Restoration Trajectories

Lakes throughout the northern hemisphere are increasingly impacted by cultural eutrophication and subsequent water quality issues such as harmful algal blooms (HABs). During the past several decades significant efforts have been made to reduce nutrient inputs from municipal, agricultural and industrial sources. However, it is now recognized that historical nutrient inputs, in particular phosphorus, have accumulated in watershed soils and lake sediments. These legacy phosphorus sources have the potential to influence ecosystem processes, and affect lake restoration objectives and trajectories, even after external nutrient sources have been addressed. Several recent studies have also highlighted how lake systems are, and will continue to be, impacted by climate change. Warming temperatures, and changing precipitation patterns, are resulting in less ice cover and longer periods of thermal stratification in Northern hemisphere lakes. This can lead to greater deoxygenation of hypolimnetic waters which creates an environment conducive to the release of previously bound phosphorus from lake sediments in a process termed internal loading. Predicting the presence, magnitude and duration of internal phosphorus loading is challenging, as the mechanisms of release from sediments are influenced by a suite of physical, chemical and biological factors. The long term goal of this research program is to develop approaches for predicting internal phosphorus loading in northern lake systems and how this process is influenced by a changing climate, external phosphorus loads, and engineered in-lake restoration techniques. The short term objectives of this proposal are to: (i) identify key factors that influence internal phosphorus loading in lakes recovering from historical organic waste inputs, (ii) build, refine and validate mathematical models to predict internal phosphorus loading, (iii) develop an integrated modeling approach to forecast how climate change will influence internal phosphorus loading, and (iv) identify in-lake restoration techniques to reduce internal phosphorus loading. The research program will focus on two watershed-lake systems that have been historically impacted by organic waste inputs. Field monitoring programs will be implemented in both systems to characterize key variables that could influence internal phosphorus release. Sediment cores will be collected and used to conduct controlled lab-scale experiments to further isolate key mechanisms of release. This information will be used to construct and validate integrated models that simulate lake physical processes and phosphorus mobility from sediments. The validated models will then be used to explore long terms trends in internal phosphorus loading and how climate change and in-lake restoration approaches will influence this phosphorus source.