Cross-Scale Drivers of the Reciprocal Impacts Among Freshwater Mussels, Hydrology, and Floodplains on River Biogeochemistry
Rivers integrate physical, chemical, and biological processes that occur within the watershed. To characterize the functional role of animal species in shaping biogeochemical cycles and resource patterns, therefore, it is necessary to account for variation in watershed features and hydrologic conditions. Freshwater mussels (Family: Unionidae) provide a valuable model system for examining the influence that species have on biogeochemical cycles and resource use because they are functionally diverse and form dense and species-rich aggregations in rivers. I focus on the Sipsey River (Alabama, USA), an undammed floodplain-river system with extensive mussel aggregations that offers an ideal setting to assess how watershed features and hydrologic conditions in conjunction with diverse and abundant biological communities shape organic matter and nutrient dynamics. In Chapter 1, I measured the biogeochemical metrics of spatial persistence, temporal synchrony, and leverage to infer how a floodplain wetland complex and variation in hydrologic conditions influence material storage and fluxes. Here, I reveal that organic matter and nutrient patterns were variably determined by processes occurring across spatial scales and by the interplay among flow conditions, floodplain-river connectivity, and the wetland complex. In Chapter 2, I combined scanning electron microscopy with stable isotope analysis and found that phylogenetic relatedness and interspecific variation in gill morphology were important factors that influenced food resource partitioning among species, while intraspecific variation in gill morphology likely constrained the range of food items that were ingested by members of a population. In Chapter 3, I performed a series of in situ benthic chamber incubation experiments and demonstrated that four functionally distinct mussel species influenced sediment ammonium and orthophosphate fluxes through their excretion, while their effects on microbial denitrification pathways were comparatively minimal, despite variation in mass-specific excretion and organic matter egestion rates. Light, however, influenced the direction of all nutrient flux pathways, highlighting the importance of accounting for environmental heterogeneity when assessing the functional role of an animal species. Altogether, this research provides valuable insight into how the interplay among watershed characteristics, hydrology, and mussel communities reciprocally influences river biogeochemical and resource patterns from the organismal to landscape scale.
Ecophysiology, Environmental heterogeneity, Functional traits, Organic matter and nutrient cycling, Resource partitioning, Zoogeochemistry
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