Macroinvertebrate and Fish Responses to Experimental P Additions in Everglades Sloughs

We used the subsidy–stress model (Odum et al. 1979) as a framework to establish several hypotheses regarding macroinvertebrate and fish assemblage responses to experimental P enrichment:

• 1. Macroinvertebrates and fish are resource limited. Relaxation of P-limitation will result in a positive response in primary production with small doses of P. Macroinvertebrate and fish abundance will mirror responses of primary producers, such that increased in production will increase macroinvertebrate and fish standing stocks.

• 2. Species turnover of primary producers will result in a stress to specialized macroinvertebrate species (e.g., grazers of specific algae). Opportunistic species will proliferate with P additions, while specialists will be disadvantaged by the loss of sensitive algal taxa, resulting in a subsidy–stress dose–response curve for species richness. The opportunistic small fish assemblage, which has few species, will not exhibit a detectable change in species richness.

• 3. Shifts in assemblage structure (e.g., increased dominance by one or a few species) will occur for both macroinvertebrates and fish at relatively low doses of P.

• 4. Succession of macroinvertebrate assemblages will be amplified with P due to greater secondary production and turnover, potentially leading to lesser temporal stability in composition.

• 5. The direct effect of P on periphyton production and nutrient content will be the most important dimension of organization for macroinvertebrate and fish assemblages in Everglades sloughs.

Our goal in this chapter is to emphasize tests of these specific hypotheses. Here, we present a synthesis of the results of approximately 3 years of macroinvertebrate and 1 year of fish studies from the P-dosing experiment.