Aquatic faunal responses to an induced regime shift in the phosphorus‐impacted Everglades

Abstract

Summary Wetland eutrophication increases the susceptibility of native communities to invasion by opportunistic plant species, which then commonly form monotypic stands. Wetland aquatic fauna are negatively affected by associated changes of eutrophication in the physiochemical environment, habitat structural complexity and ecological interactions. In addition to reducing external nutrient loads, it is predicted that efforts that reduce the resilience of the altered ecosystem are required to accelerate ecosystem recovery. We assessed small fish and decapod crustacean responses to a 3‐year ecosystem scale experiment that tested the effectiveness of chemical treatment and controlled burning as a management strategy to accelerate the recovery of eutrophic Everglades marsh. The management strategy resulted in the removal, on average, of 91% and 81% of the emergent macrophyte cover in two regions differing in enrichment effects. The resulting open‐water habitats were characterised by higher dissolved oxygen concentrations, greater periphyton and submersed aquatic vegetation and increased structural complexity. The average combined density and biomass of small fish and decapod crustaceans did not differ significantly between dense emergent vegetation and created openings but regional and water year differences occurred. More importantly, community composition did differ between treatments, with the crayfish Procambarus fallax dominating in dense vegetation and small fish Gambusia holbrooki, Jordanella floridae and Poecilia latipinna and grass shrimp Palaemonetes paludosus dominating in open water. We compared the whole body carbon (C), nitrogen (N) and phosphorus (P) contents (%) and C:N:P stoichiometry among small fish and decapod crustaceans. Fish had greater whole body C and P contents (low C:P) compared with the two crustaceans. While the management action did not alter the quantity (mg carbon m−2) or energy (kcal m−2) of fauna, the resultant shift in composition was coincident with an increase in the nutrient quality (mg P m−2 and low C:P) of fauna. Faunal community composition of the created open‐water habitats in the eutrophic Everglades was similar to the oligotrophic Everglades; however, density and biomass were significantly lower for the oligotrophic region. Moreover, intraspecific comparisons showed that small fish and decapod crustaceans from the oligotrophic region were smaller and had lower whole body phosphorus contents than organisms from eutrophic regions. This suggests that phosphorus limitation in the Everglades extends beyond autotrophs to include consumers. The high density of emergent macrophytes typically formed in eutrophic wetlands creates a physical barrier to predators, limiting the trophic transfer of energy. The combination of burning and chemical treatment not only maintained the abundance and biomass of aquatic fauna, but produced a compositional shift towards higher quality prey for wading birds and predatory fishes. We recognise that the contrasting nature between the created openings in the eutrophic and oligotrophic Everglades highlights major structural and functional differences in natural sloughs versus enriched created ‘sloughs’, but we suggest that active management efforts to accelerate the recovery of P‐impacted regions could provide temporary benefits to Everglades wildlife by improving access to abundant, high‐quality prey.