Doctor of Philosophy (PhD)
School of Renewable Natural Resources
Subtropical Northern Gulf of Mexico coastal plains watersheds tend to have mild seasonal temperature, low topography, and fine, mostly homogenous substrate sizes; these watersheds may not follow fish and invertebrate community patterns predicted by established lotic paradigms because those paradigms tend to describe communities in either temperate or tropical regions. However, the bases of most generally accepted lotic paradigms involve evaluation of allochthonous and autochthonous contributions to lotic ecosystems via three hydrologic connectivity dimensions including vertical (hyporheic and shallow groundwater), lateral (floodplain), and longitudinal (upstream and downstream). My dissertation goals were to explore these three dimensions within Louisiana coastal plains watersheds to provide further clarity of fish and invertebrate community patterns. In Chapter Two, I explored shallow-groundwater and fish assemblage relationships to the threatened Louisiana pearlshell mussel (Margaritifera hembeli). Results indicated that fish assemblage data was more important at distinguishing non-mussel stream suitability than the abiotic variables measured, with the exception of the temperature difference of stream water to 25 cm hyporheic depth as well as watershed drainage density. In chapters three and four, I explored invertebrate and fish food-web structure in two Louisiana watersheds based on stable isotope analysis (δ13C and δ15N). Chapter Three results indicated some macroinvertebrate functional feeding guilds (FFGs) showed differences among seasons and streams with varying floodplain connectivity (evaluated by data obtained from annual hydrographs). The results of Chapter Four indicated strong differences in fish food webs based on upstream-downstream connectivity differences between the Tickfaw River and Calcasieu River watersheds. In Chapters three and four, a disconnection between the insect and crayfish/fish food webs was apparent in the δ13C and δ15N biplots, and may be a product of unaccounted basal sources, mainly algae. Overall, lack of clearer relationships with shallow groundwater connectivity variables across all chapters signifies a need for the development of long-term groundwater indicators to allow for detection of changes in flow regimes. Conversely, the results in chapters three and four act as a baseline in stable isotope patterns for other subtropical coastal plains lotic food webs to better understand trends future anthropogenic disturbances especially in light of future climate change.
Thayer, Erin Elizabeth, "Fish and Invertebrate Community and Trophic Structure in the Gulf of Mexico Coastal Plains Watersheds across Varying Hydrologic Connectivity." (2022). LSU Doctoral Dissertations. 6018.
Kaller, Michael D.