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1 September 2007 Subsidy–stress response of macroinvertebrate community biomass to a phosphorus gradient in an oligotrophic wetland ecosystem
Ryan S. King, Curtis J. Richardson
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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.

Eutrophication of aquatic ecosystems caused by excessive inputs of N or P is a problem throughout the world (Carpenter et al. 1998). Nutrient enrichment can particularly affect the structure and function of aquatic food webs through an array of indirect and direct pathways (Chase 2003, King et al. 2004). In wetlands, the effect of nutrient enrichment on aquatic consumers is likely to be tightly coupled with its effect on macrophytic vegetation. Macrophytes form the physical habitat template for most other wetland biota and play a major role in modulating consumer dynamics (Batzer and Wissinger 1996). Living macrophytes are not thought to be a significant food resource for wetland consumers, but these plants contribute a significant amount of detrital material to the ecosystem. Much of this accumulated energy and nutrients is thought to fuel higher trophic levels through decomposition and the action of detritivores (Murkin 1989, Batzer and Wissinger 1996). For this reason, many ecologists have regarded wetlands as detritus-based ecosystems (Mitsch and Gosselink 2000).

Despite the ostensible importance of macrophyte detritus in wetland food webs, an increasing body of evidence indicates that periphyton—attached and floating communities of algae, bacteria, fungi, and other microbes—also can be a significant food resource (e.g., Browder 1982, Campeau et al. 1994, Keough et al. 1996, Hart and Lavvorn 2003, Liston and Trexler 2005). Periphyton is an important food source to many invertebrates that occur in wetland habitats (e.g., Lamberti and Moore 1984). Similar to macrophytes, periphyton production and tissue nutrient concentrations often benefit from nutrient additions. However, macrophyte density and shading caused by emergent macrophytes can limit periphyton abundance (e.g., Goldsborough and Robinson 1996, Grimshaw et al. 1997). Thus, taxa that rely heavily on periphyton as a food resource might benefit from nutrient inputs up to the level of enrichment at which tall emergent macrophytes begin to shade periphyton. Beyond this point, detritivores might begin to replace periphyton-feeding taxa in the food web. However, the ultimate effect of a shift toward heterotrophy on overall community biomass might depend on the relative importance of periphyton vs macrophyte detritus in a wetland food web.

The Florida Everglades, USA, is a P-limited, oligotrophic wetland ecosystem that has been the focus of significant research and restoration efforts in relation to anthropogenic P enrichment. Large inputs of agricultural runoff rich in P have induced steep eutrophication gradients in some areas of this subtropical wetland. P enrichment in the Everglades has profound effects on the productivity, biomass, and species composition of both macrophyte (e.g., Urban et al. 1993,