Diatoms as indicators of isolated herbaceous wetland condition in Florida,USA

Benthic, epiphytic, and phytoplanktonic diatoms, as well as soil and water physical–chemical parameters, were sampled from 70 small (average 0.86 ha) isolated depressional herbaceous wetlands located along a gradient of human disturbance in peninsular Florida to (1) compare diatom assemblage structure between algal types; (2) develop biological indicators of wetland condition; (3) examine synecological relationships between diatom structure and environmental variables, with the ultimate goal of developing an index of biological integrity using a single assemblage. Collected diatom samples were enumerated to 250 valves and identified to species or subspecies. An assessment of wetland condition was made using a landscape-scale human disturbance score (Landscape Development Intensity index, LDI), calculated for each site using land use maps and GIS.Assemblages from both impaired and reference sites were compared using blocked multi-response permutation procedures, the percent similarity index, and visually examined using non-metric multidimensional scaling (NMDS). No ecologically significant compositional differences were found within sites. Mantel's test (Mantel's r = 0.29, p < 0.0001) and NMDS (stress: 14.52, variance: 78.5%) identified epiphytic diatoms as the most responsive to human disturbance. Strong significant correlations (|r_s| > 0.50, p < 0.05) were found between epiphytic NMDS site scores and soil pH, specific conductivity, water total phosphorous, and LDI, while soil pH, water color, soil TP, and turbidity were also significantly correlated (p < 0.05).Metrics to assess wetland condition were developed using epiphytic abundance data. Epiphytic taxa sensitive or tolerant to human landscape modification were identified using Indicator Species Analysis, and autecological indices relating diatom sensitivity to nutrients, pH, dissolved oxygen levels, saprobity, salinity, and trophic status were calculated. Fourteen final metrics were identified, scored on an ordinal scale, and combined into the Diatom Index of Wetland Condition (DIWC). The DIWC was highly correlated with the disturbance score (Spearman's r_s = −0.71, p < 0.0001), although the results need to be validated.