Macroinvertebrate and Fish Responses to Experimental P Additions in Everglades Sloughs

A variety of anthropogenic influences threaten the Everglades. One of the most publicized perturbations to the system has been excessive inputs of phosphorus (P). The Everglades is a P-limited ecosystem, and its biota are adapted for survival under highly oligotrophic conditions (e.g., Browder 1982; Steward and Ornes 1975a,b; Swift and Nicholas 1987; Davis 1991). Indeed, numerous experiments have demonstrated the sensitivity of Everglades biota to P enrichment. In one of the first fertilization experiments, Steward and Ornes (1983) showed that small additions of P resulted in significant increases in the productivity of sawgrass (Cladium jamaicense Crantz.) seedlings, the most abundant macrophyte in the Everglades (Loveless 1959). However, high levels of P actually inhibited sawgrass production (Steward and Ornes 1983). Ensuing field fertilization studies have shown that P enrichment can have profound effects on open-water slough communities, a unique and potentially critical habitat in the Everglades ecosystem (e.g., Loveless 1959). For example, Walker et al. (1989) demonstrated that continuous dosing of high concentrations of inorganic P in sloughs of Everglades National Park nearly eliminated the common submergent macrophyte Utricularia purpurea Walt. and attached floating mats of calcareous periphyton, but stimulated growth of several emergent macrophyte species. In the same study, Flora et al. (1988) and Hall and Rice (1990) found that periphyton accumulation on artificial substrates showed as much as a tenfold increase in biomass as a direct result of P. This biomass increase was concomitant with significant shifts in algal species composition and changes in tissue nutrient content (Hall and Rice 1990). Subsequently, other P-fertilization experiments have been conducted in slough habitats, producing similar responses for both periphyton (Vymazal et al. 1994; Craft et al. 1995; McCormick and O’Dell 1996) and macrophytes (Craft et al. 1995). Despite this clear connection between P enrichment and changes in productivity and species composition of primary producers in the Everglades, remarkably little has been done to assess how these effects manifest themselves at higher trophic levels. To date, only papers resulting from the P-dosing experiment that is presented in this chapter (King 2001; King and Richardson 2003; Qian et al. 2003) have examined responses of invertebrates or fish to P enrichment using experimental