Dr. Mike Vanni's Lab, Ecology of Lakes and Watersheds logo
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My lab studies the ecology of aquatic ecosystems and their surrounding watersheds. Our research spans multiple scales and encompasses several approaches, including whole ecosystem studies, experiments at various scales, and simulation modeling.

Current Projects

Linkages between watersheds and food webs

This research examines how landscapes (watersheds) and food webs interact to regulate ecosystems, focusing on reservoirs of the Midwest USA. This research is collaborative with two other Miami faculty, María González and Bill Renwick, and many students. We are examining how watersheds and omnivorous fish influence lakes along a gradient of watershed land use, ranging from mostly forested watersheds to those that are nearly entirely agricultural. Agricultural activities increase the amounts of nutrients and sediments entering these lakes, with profound implications for nutrient cycling, lake productivity, biodiversity, and food web dynamics. In particular, lakes in agricultural landscapes are very productive, and fish biomass is dominated by omnivorous gizzard shad (Dorosoma cepedianum). Gizzard shad larvae are zooplanktivorous and can strongly affect zooplankton communities. Once individuals reach a size of about 25-35 mm (about 2-3 months of age), they become detritivorous and feed mostly on lake sediments. Gizzard shad allocate some of the nutrients they assimilate from detritus into growth, but they also excrete into the water column a substantial proportion of these sediment-derived nutrients. Within this framework of watershed-reservoir linkages, my lab is addressing several questions:

  • How important are gizzard shad in sustaining primary production, by cycling nutrients into the water column, and how does this vary with lake productivity and watershed land use?
  • How does the relative importance of gizzard shad and watersheds vary at different temporal scales and along a productivity (land use) gradient?
  • How do watershed land use and watershed size influence lake productivity and algal biomass?
  • What is the long term response of a eutrophic reservoir (Acton Lake) to improvements in agricultural practices within its watershed?
  • Are populations of detritivorous fish subsidized by inputs of allochthonous detritus from watersheds? Or do gizzard shad rely primarily on autochthonous detritus (i.e., phytodetritus)?
  • How do inputs of sediments and nutrients mediate trophic efficiency in the pelagic food chains of reservoirs by altering stoichiometric relationships between trophic levels?

Nutrient cycling by animals in freshwater ecosystems

The conventional view of nutrient cycling is that animals are relatively unimportant relative to microbes. Yet, many examples exist showing that animals are important, especially at local scales. Our research strives to understand when and where animals are important in nutrient cycling in freshwater ecosystems, including both temperate and tropical ecosystems. Much of our work focuses on nutrient cycling by fish, but other taxa under investigation include amphibians and various invertebrate species. We approach this question from several directions. Using ecological stoichiometry as a framework, we seek to understand how variation in the nutrient contents (and ratios) of animals and their food resources mediate nutrient cycling rates and ratios. We also study how animals are important at the scale of whole ecosystems, in terms of nutrient cycling as well as sequestration and transport of nutrients. Collaborators on this research include many students as well as Alex Flecker (Cornell University) and Pete McIntyre (Wright State University).

Carbon cycling in linked watershed-reservoir systems

Until recently it was believed that freshwater ecosystems (lakes, reservoirs, rivers, wetlands) played a minor role in regional or global carbon cycles. However, recent work suggests that large quantities of organic carbon are buried in the sediments of freshwater ecosystems, perhaps more than is buried in all the world’s oceans. At the same time, many lakes are supersaturated with CO2, meaning that they release CO2 to the atmosphere. Along with my Miami University colleagues Bill Renwick and Craig Williamson, my lab has recently started developing carbon budgets in reservoirs, focusing on inputs from watersheds, downstream outputs, and especially the burial of carbon in sediments. To extend the temporal scale of study, we also take sediment cores to quantify long-term burial rates.

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