Integrated Wetland Assessment Program. Part 5: Biogeochemical and Hydrological Investigations of Natural and Mitigation Wetlands
We performed a comprehensive investigation of the biota (structure) and biogeochemical cycles (processes or functions) of a population of natural (n = 9) and mitigation wetlands (n = 10). Intensive data were collected on various wetland ecosystem components including: hydrology, soil and water chemistry, characteristics of the plant, macroinvertebrate and amphibian communities, biomass production, decomposition, and nutrient cycles. The goals of the project were as follows: 1) to demonstrate the efficacy of floral and faunal community-based indicators in order to assess the performance of mitigation wetlands, 2) determine the links between floral and faunal community structural attributes and ecosystem processes in natural and mitigation wetlands, 3) compare the biological and physical characteristics, as well as patterns of biogeochemical cycling in natural and mitigation wetlands in order to assess their relative condition, and 4) identify simple, cost-effective biogeochemical indicators for use in mitigation monitoring and as performance standards. The biological and biogeochemical characteristics of the natural and mitigation wetlands were substantially different. The mitigation wetlands were generally “dryer” than the natural sites based on measures of ground water. Mean depth to ground water averaged -53.8 + 11.1 cm and -25.0 + 6.1 cm in the mitigation and natural sites, respectively in 2001 (p = 0.04) and -44.5 + 9.1 and -25.4 + 4.9 in 2002 (p = 0.09). Concentrations of soil organic carbon (%OC), %N, and plant available P (μg P g-1 soil) were 4.8 times, 4.3 times, and 1.6 times higher in the natural compared to the mitigation sites. Mean values for soil bulk density and percent solids were significantly
higher in the mitigation wetlands (p = 0.001). These measures quantify the extremely heavy soils found in the mitigation sites that may lead to reduced root growth and limit carbon accumulation. The Vegetation Index of Biotic Integrity (IBI) scores for natural sites ranged from 9 to 82, reflecting the fact that the natural wetlands were selected along a gradient of human
disturbance. The range of scores for mitigation wetlands was narrower, ranging from 16 to 50. This compression of scores is due in part to the fact that the community composition of the mitigation sites is similar, with a dominance of ubiquitous, tolerant plant species. Mean VIBI scores were more than twice as high at natural wetlands (p = 0.005). Aboveground biomass production was also significantly higher in the natural sites ( p = 0.04) where production averaged 34.7 g 0.1 m-2 compared to 20.9 g 0.1 m-2. The invertebrate data showed major differences in the numbers of taxa, abundance of tolerant and sensitive
species, and the community metrics in the Wetland Invertebrate Community Index scores between the mitigation sites and natural sites. Taxa richness averaged 46 in natural sites compared to 34 at mitigation sites. Amphibian communities of the mitigation wetlands differed markedly from natural forest and shrub dominated wetlands. However, amphibian communities of natural emergent wetlands and the mitigation sites were similar and factors like permanence of hydrology and presence of predatory fish appeared to be more important in determining amphibian community composition. Despite this, average Amphibian Index of Biotic Integrity scores were 0.3 for the mitigation sites in this study and 6.5 for the natural emergent sites.
Both decomposition rates and litter nutrient concentrations were higher in the natural wetlands. The soil and water data demonstrate the low levels of organic carbon contained in the mitigation sites. Low organic carbon levels can limit the activity of decomposers (heterotrophic microbes and invertebrates), limiting diversity and leading to slower rates of decomposition. Multivariate analyses show that the natural and mitigation sites group as two separate populations, indicating that wetland mitigation is currently creating a new subclass of wetlands on the landscape. Based on the results of this study, several indicators could serve as measures of mitigation performance relative to natural wetlands: 1) soil chemical and physical
characteristics especially soil organic carbon and soil nitrogen content and percent solids in the soil or bulk density; 2) hydrological characteristics including mean depth to ground water and percent time water is found in the root zone; and 3) multimetric indices developed from natural reference wetland data sets.