Freshwater ecosystems comprise a small proportion of the surface of the Earth but are important for the biogeochemical cycling of nitrogen, phosphorus, and carbon. The establishment of invasive species creates novel ecosystems, which can alter nutrient cycling within large reservoirs. Climate change and human dispersal facilitate the spread of non-native species in reservoirs. If successful in establishing, invasive submerged aquatic vegetation (SAV) can cover large spatial areas, potentially altering nutrient transformations and storage. Lake Seminole is a shallow reservoir (mean depth = 3 m, maximum depth = 10.7 m) formed by the confluence of the Flint and Chattahoochee rivers in southwestern Georgia. The SAV community of the lake is dominated by an invasive macrophyte, Hydrilla verticillata. In order to investigate the effects of SAV on a reservoir scale, we used a visual survey by boat followed by post-processing in ArcMap to create vegetation maps and determine the spatial coverage of SAV on Lake Seminole. Field measurements of physicochemical parameters were collected at 10 sites extending to all three inflows and the outflow. Combining the vegetation surveys with a water-quality monitoring program documented the effects of invasive SAV under contrasting hydrological conditions, including abnormally low and high flows.
Results/Conclusions
Combined average annual discharge of the three inflows was 62% lower in 2012 than 2013 and 19% lower than 2014. A similar, but stronger, pattern was observed during the early season for SAV growth. Total SAV coverage decreased each year over the three-year period from 3560 hectares (48% of lake surface area) to 1825 hectares (25% of lake surface area), while Hydrilla decreased from 2037 hectares to 258 hectares. NO3-N concentrations in the outflow of the lake were reduced when SAV coverage was high and were elevated when SAV coverage decreased, suggesting significant in-reservoir nitrogen cycling. Elevated downstream flux of NO3-N could contribute to eutrophication of downstream systems including the Apalachicola Bay. Recent introductions of invasive apple snails of the genus Pomacea, which are voracious herbivores, could potentially reinforce these hydrologically driven changes in SAV coverage resulting in further alteration of nitrogen processing. As the quality of water, not only the quantity, becomes more important for management and regulation of freshwater resources, it is important to understand how novel ecosystems composed of invasive species will alter biogeochemical cycling.