Lakes provide myriad ecosystem services that support human well-being. At the same time, human activities compromise water quality, threatening the crucial ecosystem services upon which we rely. Understanding the drivers behind water quality degradation is essential to designing sound policy to protect valuable freshwaters. However, this is especially challenging in lake systems, where water quality depends on complex interactions and feedbacks between humans and nature. We approach this challenge using coupled natural-human systems modeling as a tool to understand how feedbacks between people and lakes influence water quality. We trace the role of human decision-making in altering nutrient fluxes through lake ecosystems, examining how changes in nutrient loads affect water quality. The resulting changes in water quality subsequently alter the provision of ecosystem services, affecting human well-being and motivating changes in human behavior that can affect freshwater resources long into the future.

To capture the complex, reciprocal linkages between human and lake systems, we develop an empirical modeling approach that leverages the disciplines of economics, agronomy, hydrology, ecology, and social psychology to examine key system components, including agricultural land-management decisions, terrestrial nutrient cycling, hydrologic-solute transport, aquatic nutrient cycling, residential property values, and civic engagement. We develop a coupling methodology to link system components via key variables, applying the model to the detailed study of two lake systems—one oligotrophic and one eutrophic—and generalizing to lake systems spanning the Midwest and northeast United States.

Results from our coupled models indicate that policy seeking to improve water quality by limiting nutrient loading drives changes in agricultural land-management practices that generate water quality benefits locally, but that also generate significant reductions in greenhouse gas emissions with global benefits. However, nutrient loads from the landscape alone do not fully explain changes in lake water quality; rather the interaction between nutrient loads, air temperature, and trophic state drives water quality dynamics. In particular, we find that warming is likely to intensify the effects of nutrient loading, but the consequences of that interaction play out at a much lower temperature threshold in our oligotrophic study system than in our eutrophic study system. Our results suggest a need to pursue more aggressive nutrient reduction targets to prevent algal growth in oligotrophic lakes, particularly in the presence of ongoing warming.

Changes in water quality and corresponding ecosystem services are capitalized into housing values along the lake shore, but we find that the response of property values to water quality is asymmetric and depends on trophic state. For a eutrophic lake in particular, large reductions in nutrient loads are required to drive significant changes in property values. However, the effects of water quality on human welfare extend beyond those captured by property values. Declining water quality often motivates civic engagement in the form of lake associations, which are groups of citizens who work together to maintain and improve lake water quality. We examine active lake associations to document systematic differences between in organizational mission, focus, motivation, goals, actions, and outcomes. We find that lake associations seek to influence environmental outcomes via several pathways, including direct actions, education and guidance, advocacy and lobbying, and monitoring, and that those activities differ across oligotrophic and eutrophic systems. 

Scaling up and extrapolating from our in-depth study systems to lake catchments at the regional to continental scale, we find that nutrient concentrations in lakes vary significantly with near-stream agricultural production. However, total nitrogen concentrations are more strongly related to the presence of agriculture than are total phosphorus concentrations, which are more closely related to lake and watershed characteristics. These results point to a substantial lag between agricultural production activities and lake degradation, which implies that reversing trophic state likely requires sustained behavioral change for years to decades.

To support broader impacts, our project engaged and partnered with lake associations to use research and modeling to support science-based monitoring, advocacy, and volunteerism. We co-sponsored workshops and other events to bring together researchers, water managers, and citizens to understand the state of the science concerning algal blooms in lakes. We worked with lake association partners to survey landowners to understand benefits and obstacles to the adoption of best management practices, and to develop a data visualization tool to display how the use of best management practices affects lake water quality. We also developed human capital for coupled natural-human systems research by training graduate students in qualitative and quantitative analytical techniques grounded in diverse social and natural science disciplines. Graduate students were mentored to work in a multidisciplinary team environment, co-author manuscripts, and lead workshops targeted to academic and non-academic audiences. Additional broader impacts activities included the development of undergraduate teaching modules focused on systems thinking in coupled natural-human systems and on modeling techniques to generate insight into complex systems.   

Last Modified: 03/31/2021
Modified by: Kelly Cobourn