Surface water nutrients (N, P) and ecological (vegetation, litter, and soil) structure and function were measured in 30 freshwater wetlands, ten each, from three different Nutrient Ecoregions (NE) in Indiana, USA, to develop predictive relationships between wetland nutrient condition and ecological indicators of impairment. The sampled wetlands were located in Nutrient Ecoregion VII (mostly Glaciated Dairy Region), Nutrient Ecoregion VI (Corn Belt and Northern Great Plains), and Nutrient Ecoregion IX (Southeastern Temperate Forested Plains and Hills) that varied in land use, geology, and presumably, nutrient loadings. Wetland nutrient condition was characterized by seasonal measurements of inorganic nitrogen (NH4-N, NO3-N) and phosphorus (PO4-P) in surface waters collected over one year. Vegetation structure (species richness, biomass of invasive species) and function (aboveground biomass, stem height, and live/senesced leaf N, P, C:N:P) were measured by end-of-season harvest and analysis of biomass from five to ten 0.25 m2 quadrats in each wetland. Leaf litter and soil cores were collected from the same quadrats and analyzed for N and P. Regression analysis was used to identify relationships between wetland nutrient condition and measures of ecological impairment.
Wetland surface waters exhibited a broad range of inorganic N and P concentrations. Concentrations of NH4-N ranged from 0.07 mg/L (NE IX) to 0.25 mg/L (NE VI) and surface water P (PO4-P) ranged from 0.06 mg/L (NE VII) to 0.11 mg/L (NE VI). Nutrient Ecoregion VI (Corn Belt) contained the highest concentrations of NH4-N and PO4-P (Figure 1). Stem height, an index of plant productivity, was significantly greater in Nutrient Ecoregion VI, where higher concentrations of surface water NH4-N and PO4-P were measured. Species richness also was significantly lower in this Nutrient Ecoregion (5.2 / site) compared to NE VII (10.1 / site) and NE IX (8.8 / site). Leaf P also was significantly greater in Nutrient Ecoregion VI (1480 ug/g), as compared to NE IX (1136 ug/g) and NE VII (1209 ug/g). There was no significant difference in aboveground biomass, leaf N, or litter mass among the three Nutrient Ecoregions. In contrast to vegetation, there was no significant difference in litter and soil indicators that were linked to wetland nutrient condition.
Figure 1. Mean (± SE) surface water NH4-N (a) and PO4-P (b) among Nutrient Ecoregions VI, VII, IX. Different letters indicate significantly different means at α = 0.05 according to the Ryan-Einot-Gabriel-Welsch multiple range test.
Regression analysis revealed that attributes of vegetation function and structure were strongly related to wetland nutrient condition, especially NH4-N. Stem height and aboveground biomass increased with surface water NH4-N across all Nutrient Ecoregions (r2=0.31, p<0.002). Leaf P (r2=0.21, p<0.02) and senesced leaf N (r2=0.24, p<0.01) and P (r2=0.33, p<0.002) also increased with surface water NH4-N.
Across all Nutrient Ecoregions, species richness was inversely but weakly related (r2=0.16, p<0.03) to surface water NH4-N. Aboveground biomass of aggressive, fast growing species, Phalaris plus Typha, however, was strongly related to NH4-N (r2=0.38, p<0.0005) and this relationship was even stronger (r2=0.46, p<0.0001) when the data were expressed as percentage of total community biomass (Figure 2). Soil indicators were not as strongly related to wetland nutrient condition and were related to surface water PO4-P rather than NH4-N.
Figure 2. Invasive species aboveground biomass (g/m2) (a) versus surface water NH4-N for 28 freshwater wetlands located in three different Nutrient Ecoregions in Indiana. Invasive species aboveground biomass (%) (b) versus surface water NH4-N for 28 freshwater wetlands located in three different Nutrient Ecoregions in Indiana.
Vegetation-based indicators of impairment, especially biomass of invasive species, are better predictors than litter or soil-based indicators of wetland nutrient condition. Whereas our indicators are robust, that is, they have predictive power across all three Nutrient Ecoregions, their ability to predict wetland nutrient condition is improved when separate relationships are developed for each Nutrient Ecoregion. Vegetation-based indicators are a promising tool for addressing wetland nutrient condition, but first they should be tested across a variety of Nutrient Ecoregions, from low to high anthropogenic disturbance, and wetland vegetation types, from emergent to forested vegetation.
This research is supported by USEPA headquarters and Region 5.
Trinity Fen,
a high quality, groundwater-fed Cattail in cornfield,
a nutrient rich wetland in
Recent Publications
Craft, C.B. 2003. Vegetation-based indicators of nutrient condition of freshwater wetlands, Northeastern Indiana. EPA Final Report. Link to the PDF
Craft, C.B. 2005. Vegetation- and soil-based indicators of nutrient condition of freshwater wetlands in Nutrient Ecoregions VI, VII, and IX, Indiana. EPA Final Report.