MSA: A Macrosystems Perspective on River Eutrophication from Chlorophyll Abundance Patterns

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Rivers and streams provide society with water to drink and irrigate, recreational opportunities, and habitat for plants and animals. These valuable services can be threatened by pollution in the form of nitrogen and phosphorus, nutrients that encourage algae to grow in rivers in the same way that weeds grow in a garden. Nutrient-enriched runoff from farms, suburbs, and cities has caused most rivers and streams in North America to have too much algae and aquatic plants. While reducing nutrient inputs to rivers would solve this algal problem, it is difficult to know how much those inputs need to be reduced and in what locations. This is because nutrients and algae from different locations are continually mixed together as rivers converge and flow downstream. River floods can scour patches of algae from the river bed and carry them downstream, making it even more complicated to identify an appropriate level of nutrient reduction for a particular river that can control algae. A recent discovery of how algal concentration changes during river floods may be a key to more accurately predicting how much nutrients need to be reduced to control harmful algal blooms. This research is testing and applying this new method to help guide policies that lead to healthier rivers using effective regulations.This project tests a transformative approach using hysteresis patterns, which are system lags between inputs and outputs, of chlorophyll concentrations during storm events to evaluate river eutrophication in a way that efficiently integrates the heterogeneous distribution of algal biomass and its temporal pattern of growth and transport throughout entire watersheds. The approach helps resolve a long-standing challenge in limnology of separately accounting for growth and transport of benthic and planktonic algae in measurements of river chlorophyll abundance. The project combines two spatial scales, namely regional and continental. The first goal is to monitor high-frequency chlorophyll concentrations in six rivers of the Delaware River Basin with contrasting human influence. These data are enabling linkages between reach-scale controls on benthic and planktonic growth, such as habitat features and hydrologic conditions, to chlorophyll hysteresis responses measured downstream. The output is quantitative relationships between nutrient enrichment, channel morphology, and chlorophyll hysteresis indices. The second goal is to test these relationships with chlorophyll sensor data, algal community characterization, and watershed characteristics from the 28 NEON aquatic field sites. By exploring new metrics of river eutrophication, this project develops analyses of algal growth dynamics over more appropriate spatial and temporal scales and better captures the combined contribution of benthic and water-column habitats to river eutrophication.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.