Food webs of geographically isolated wetlands: A comparison across an agricultural disturbance gradient

Background/Question/Methods

Geographically isolated wetlands are abundant across the southeastern United States and provide habitats for unique flora and fauna, as well as other valuable ecosystem services.  Due to the lack of recognition under the Clean Water Act, isolated wetlands are being altered or removed from the landscape, along with the opportunity to understand the ecological processes that support high levels of biodiversity.  We examined food web structure in isolated wetlands across a gradient comprised of reference (surrounded by longleaf pine forest with native groundwater), restoration (encroachment and subsequent removal of hardwood species), and agriculturally disturbed (row crop and pasture) wetlands.  Macro-invertebrates, amphibians, vascular plants, periphyton, and two fractions of benthic organic matter were collected for stable carbon and nitrogen isotope (δ¹³C and δ¹⁵N) analyses.  SRP (soluble reactive phosphorus), NO₃-N, and NH₄-N, pH, and alkalinity were used to evaluate water quality parameters.

Results/Conclusions

SRP (soluble reactive phosphorus), NO₃-N, and NH₄-N concentrations were greater and more variable with increasing disturbance compared to the reference wetlands.  Elevated nutrient concentrations in the agricultural wetlands indicate chemical fertilizer and animal waste runoff.  High variability is caused by the timing of runoff generated by rainfall and rates of fertilizer applications.  In reference and restoration wetlands, overall, both δ¹³C and δ¹⁵N isotope signatures became progressively enriched (more positive) as canopy cover decreased.  The agricultural wetlands had greater δ¹³C and δ¹⁵N values regardless of canopy cover and were progressively more enriched in both δ¹³C and δ¹⁵N values compared to reference and restoration wetlands.  Our results indicate differences in sources of nitrate and carbon across a disturbance gradient with a greater contribution of fertilizer and animal-derived nitrogen in agriculturally disturbed wetlands.  Preliminary data indicates progressive enrichment in both δ¹³C and δ¹⁵N signatures of basal resources, primary, and secondary consumers regardless of wetland condition; however, the agriculturally disturbed wetlands exhibit greater variability and more enriched signatures as compared to the reference wetlands.  Ongoing studies are addressing trophic linkages between basal resources and consumers across the disturbance gradient.