Dimensions of invertebrate assemblage organization across a phosphorus-limited Everglades landscape.

摘要

Research on the ecology of wetland invertebrate assemblages has been sparse relative to the bounty of published information from stream habitats. Consequently, our understanding of determinants, or dimensions, of assemblage organization is weak. I conducted a series of descriptive and experimental studies to reveal dimensions of wetland invertebrate assemblages across a range of nutrient regimes. A secondary goal was to evaluate assemblage responses in the context of wetland bioassessment, and recommend approaches for detecting environmental impact.; A spatially explicit, hierarchical sampling design was used along a landscape-scale phosphorus (P) gradient to quantify pattern and scale of vegetation, as both were expected to influence invertebrates. Low-P, unimpacted areas exhibited significant heterogeneity at a fine scale but little at a coarse scale—fine-scale pattern was hierarchically nested. At high P-levels, fine-scale pattern was homogeneous, but divergent among widely separated, coarse-scale localities.; Invertebrate biomass and species diversity exhibited a subsidy-stress relationship along the P and vegetation gradient, largely due to an interaction between increased quality (decreased C:N ratios) of periphyton at intermediate-P areas and decreased periphyton quantity at high-P locations. Vegetation had the greatest direct effect on invertebrate assemblage composition; however, invertebrate composition also had numerous other spatial, temporal, abiotic, and biotic dimensions of organization. Spatial pattern in assemblages at a fine scale mirrored pattern in vegetation, but coarse-scale vegetation pattern in P-enriched areas magnified fragmentation of invertebrate species composition because of a loss of landscape connectivity.; A mesocosm experiment revealed that, in the absence of invasive vegetation, assemblage biomass and diversity consistently exhibited log-linear subsidy responses to P dosing. Here, local-scale P additions in sloughs stimulated periphyton production and decreased C:N ratios—multivariate analysis suggested that these factors were primarily responsible for the positive assemblage response to P.; Comparisons among levels of taxonomic resolution indicated that genus- or species-level taxonomy may be necessary to accurately detect excessive nutrient enrichment in wetlands. Subsampling also affected accuracy of assessment—an integrated fixed-count/large-rare-search was recommended.; A synthesis of the P gradient and dosing studies revealed that attributes of invertebrate assemblages exhibited changepoints above 10–20 μg/L of surface-water TP. This result may assist the development of a defensible, numerical TP criterion for the Everglades.