Subsidy–stress response of macroinvertebrate community biomass to a phosphorus gradient in an oligotrophic wetland ecosystem

We used a subsidy–stress model as a basis for predicting macroinvertebrate community response to a steep gradient of P enrichment in the Florida Everglades, a P-limited wetland ecosystem. We tested the hypothesis that consumers were resource limited and their biomass would show a subsidy response (increase) to low-to-moderate levels of P enrichment, but a stress response (decrease) at high levels of P enrichment because dense emergent macrophytes, particularly Typha, might significantly reduce periphyton food resources. We used a spatially extensive sampling design (14 clusters of 9 sites, 126 total) that incorporated vegetation pattern to evaluate consumer responses along the P gradient. We then conducted a 1-y temporal study at 3 of the 14 clusters to evaluate how seasonal hydrological variation interacted with nutrients to influence consumer biomass. Macroinvertebrate community biomass showed a significant unimodal response to increasing P enrichment consistent with a subsidy–stress relationship. Eight of 12 major taxonomic groups (Amphipoda, Decapoda, Diptera, Empheroptera, Gastropoda, Hirudinea, Odonata, Oligochaeta) had this unimodal response, whereas 3 (Coleoptera, Hemiptera, Isopoda) increased monotonically and 1 (Trichoptera) decreased monotonically in response to P. Periphyton C:N and C:P ratios declined with increasing P, but periphyton cover was minimal at high levels of P enrichment where tall invasive macrophytes limited its growth. The temporal study revealed a subsidy–stress response except after marsh reflooding following the dry season when the most P-enriched clusters of sites had the highest consumer biomass, presumably because drought-induced senescence reduced macrophyte cover, which enabled heavy growth of periphyton. Our results suggest that an interaction between increased quality and decreased quantity of periphyton caused the subsidy–stress patterns observed. We suggest our findings could be generalized to other wetland ecosystems where nutrient enrichment leads to invasion of weedy emergent macrophytes, such as Typha, and elimination of open-canopy habitats rich in periphyton.