Will novel combinations of invasive species alter nutrient retention in a subtropical reservoir?

Background/Question/Methods

Novel species assemblages are being created as non-native organisms disperse northward as a result of warmer winter temperatures.  These assemblages can alter ecosystem processes and result in diminished water quality as well as altering ecosystem services.  Invasive submerged aquatic vegetation (SAV) is known to affect overall productivity and biogeochemical cycling within aquatic ecosystems, thus altering nutrient transformation and storage.  Lake Seminole is a shallow reservoir 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 addition to Hydrilla, large populations of Corbicula fluminea are present within the lake, and invasive apple snails of the genus Pomacea have been recently introduced.  Apple snails are voracious herbivores and could affect SAV distribution.  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.  Combining these surveys with a water-quality monitoring program documented the effects of invasive SAV.  A series of snail surveys tracked the dispersal of Pomacea and examined interactions with the other invasive species.

Results/Conclusions

We found large variation in overall SAV and Hydrilla coverage during the three-year period.  Total SAV coverage decreased from covering 48% of the lake surface area to 25%, while Hydrilla decreased from 47% to 19%.  The largest areas of reduction were not correlated with snail presence but were associated with annual hydrologic variation.  SAV coverage was greatest during an extended drought.  NO₃-N concentration was elevated during years of decreased SAV coverage. Changes in SAV abundance and nutrient cycling and the downstream water quality in the Apalachicola River are dependent on nutrient transformations within the lake.  As conditions change in a warmer climate, new combinations of species will continue to create novel aquatic ecosystems.  Predicting how these ecosystems will function will require understanding the direct and indirect interactions of the new species combinations as well as their effects on biogeochemical cycling.