The Influence of Herbivory on Submerged Macrophytes and Nitrogen Availability in Created Wetlands.

Title: The Influence of Herbivory on Submerged Macrophytes and Nitrogen Retention in Created Wetlands.

Evan N. Squier, Kimberly A. Lodge, Delanie M. Spangler, Christy Tyler, Carrie McCalley, Nathan Eddingsaas.

Wetlands are frequently created for nutrient removal and improvement of water quality. However, wetlands are complex systems and the abiotic and biotic interactions that determine functionality are not fully understood in natural wetlands, and even less so in created wetlands. This may lead to shortcomings in meeting desired restoration outcomes. Herbivory is an important indirect control on nutrient cycling and other biogeochemical processes in wetlands through top-down controls. Herbivores can significantly decrease plant biomass and community structure, potentially altering nitrogen immobilization by plants, denitrification, nitrogen fixation and regeneration of inorganic nutrients in the sediments. Caged and uncaged plots were established in two created wetlands in Western New York State, and the impact of grazer exclusion on vegetation community structure and nitrogen cycling processes assessed. Herbivores, predominantly waterfowl, selectively removed emergent vegetation, leading to significantly higher submerged macrophyte cover in uncaged plots where light availability was greater. Potential denitrification was enhanced in the absence of grazers where emergent plants dominated, perhaps due to increased organic matter availability. We hypothesize that enhanced nitrogen fixation and benthic nitrogen release in uncaged plots may exacerbate the negative influence of grazers on nutrient removal capacity. Our results suggest that control of large grazing waterfowl in created wetlands will enhance nutrient retention and removal services and improve downstream water quality.