Influence of an experimental sheet flow regime on aquatic food webs of the central Everglades

Background/Question/Methods

            Restoring hydrologic connectivity in altered ecosystems can have unknown consequences for the routing of energy and matter in aquatic food webs. We hypothesize that reestablishing freshwater flow to wetlands of the Everglades will affect the habitat use, feeding patterns, and movement of aquatic animals by increasing connectivity of habitats, redistributing particles across the landscape, and reshaping of the biochemistry of algal-detritus linkages. One hypothesized consequence of flow restoration is nutrient loading resulting from an increased rate of supply of phosphorus (P).  To examine the relationship between freshwater flow, P loading, and aquatic consumers in restored marshes, we used enclosures containing experimental food web fragments, within a large-scale flow manipulation, before and during experimental sheetflow treatments. We simulated low-flow with relatively low nutrient loading, and high-flow with elevated nutrient loading conditions. We sampled response variables sensitive to predicted nutrient loading including changes in periphyton total P and species composition, and consumer growth rate and fatty acid composition. We completed a laboratory study prior to the field study to determine the assimilation rate of dietary lipids in consumers to help determine their sensitivity as biomarkers for dietary changes hypothesized to result from nutrient loading effect.

Results/Conclusions

The diet switching laboratory study revealed effects of algal and fish-meal diet treatments on growth rate and lipid turnover in sailfin mollies (Poecilia latipinna), eastern mosquitofish (Gambusia holbrooki), and riverine grass shrimp (Palaemonetes paludosus).  Compared to an algal diet, fish meal decreased days to sexual maturity and increased lipid turnover for juvenile animals rapidly growing.  In the field study, periphyton and epiphytic algal composition and TP content were altered by flow and consumer treatments. Relative abundance of green algae and diatoms decreased in the presence for consumers, while filamentous cyanobacteria increased. Diatoms were the most common algal type in both periphyton and epiphytic algae in all treatments. Increased flow treatments experienced increased periphyton TP when compared to adjacent cages shielded from flow, consistent with loading.  In the presence of consumers, algal species composition was less affected by elevated flow than in their absence, presumably because of selective consumption.  Lipid profiles indicated less heterotrophic bacteria and more diatoms and cyanobacteria in the diets of consumers in the elevated flow treatments. We predict that these results reflect a fundamental shift in energy flow from a microbially based to autotrophic based system caused by the increase in rate of supply of phosphorus.