Ecosystems are spatially linked by the flux of materials and energy. Over the past decade there has been growing appreciation that the movement of resources across the landscape can act to subsidize populations in local habitats. In addition, theory suggests that resource subsidies may act to stabilize food webs and ecosystems, but these predictions remain largely untested. This study will examine the effects of terrestrial-derived dissolved organic carbon (DOC) on the stability of aquatic ecosystem metabolism. Traditionally been viewed as a low quality resource, it is now recognized that terrestrial DOC inputs are a resource subsidy for bacteria that can determine whether recipient aquatic ecosystems function as sources or sinks of atmospheric carbon dioxide. The first goal of this project is to describe the shape, magnitude, and direction of subsidy-stability relationships using a set of whole-pond experiments that manipulate the supply rate of terrestrial DOC. The second goal of this project is to identify the mechanisms by which terrestrial DOC influences aquatic ecosystem stability. Specifically, the project will assess whether DOC affects ecosystem stability by modifying nutrient cycling, altering temperature dynamics, or by altering interaction strengths among different groups of microorganisms. The objectives of this project will be assessed using a combination of autonomous sensor technology to quantify ecosystem metabolism and temperature variability, radioisotope assays to measure nutrient cycling, and molecular techniques to evaluate changes in the metabolic activity of microbial communities.
At a global scale, the export of terrestrial DOC to aquatic ecosystems is increasing due to a combination of factors, including atmospheric deposition, climate variability, and shifting land use. The tools and approaches developed in this proposal will address this phenomenon and help scientists and managers predict how changes in DOC loading affect ecosystem functioning and water quality under existing and future climate scenarios. This project has a significant education and outreach component that will foster interactions among the Kellogg Biological Station (KBS), Michigan State University (MSU), and rural Michigan K-12 school districts. Specifically, the project will continue an ongoing collaboration that will introduce concepts of ecological stability, microbial diversity, and biogeochemical cycling into the high-school classroom. The project will also provide interdisciplinary research training in the fields of ecosystem science, quantitative ecology, and molecular biology for undergraduate students, a graduate student, a high school teacher, and postdoctoral researcher.
Intellectual Merit: Global change is altering the movement of materials and energy across landscapes in ways that are modifying the structure and function of ecosystems. For example, the export of dissolved organic carbon (DOC) from terrestrial to aquatic ecosystems is increasing on a global scale. It is generally believed that this "browning" phenomenon will alter the stability of recipient aquatic ecosystems, but different bodies of theory make contrasting predictions about the form and direction of this response. In this project, we used a set of whole-ecosystem experiments that were designed to test the subsidy-stability relationship. This involved adding various amounts of humic substances to a set of experimental ponds at the W.K. Kellogg Biological Station, Michigan State University. We deployed a variety of automated instruments that measured light, temperature, and dissolved oxygen concentrations. These measurements allowed us to measure gross primary productivity and respirations – the two major components of ecosystem metabolism. The DOC inputs reduced metabolism of the ponds, but aquatic food webs were still subsidized by the humic substances. After establishing these baseline conditions that simulated different browning regimes, we perturbed the systems to quantify the stability of the pond ecosystems. We measured how resistant and resilience the ponds were to a nutrient pulse. We found that browner ponds were less stable than clear ponds. This means that the browning of inland water bodies may create tipping points in the stability and reliability of freshwater resources. Broader Impacts: This project trained nine undergraduates, along with three junior technicians who are now enrolled in Masters or Ph.D. programs at Michigan State University, Indiana University Biology, and University of Georgia. We have mentored two students from historically black colleges and universities (Taiquitha Robins, Jackson State University and Ebony Rogers, Bennett College). Through an NSF-sponsored GK-12 program, a middle school teacher (Sandy Erwin) and a high school (Marty Green) teacher were trained in a range of topics, which were integrated into their classroom teaching modules. Stuart Jones (Co-PI), who was a postdoc on this project, was hired as an Assistant Professor in the Department of Biology at the University of Notre Dame. We have given numerous seminars and talks on research that has resulted from this funding, including invited tutorials related to aquatic browning at the Ecological Society of America and the American Society of Limnology and Oceanography.
