Mounting evidence suggests that aquatic invertebrate activity can enhance wetland sediment greenhouse gas (GHG) flux. However, waterbirds have been shown to reduce densities of aquatic invertebrates, which could potentially moderate GHG flux. Alternatively, carbon (C), nitrogen (N), and phosphorus (P) subsidies in waterbird guano may stimulate microbial activity. These bottom-up forces may stimulate GHG flux and dampen the top-down effects of predation. For the proposed study, a network of long-term waterbird exclosure plots will be established within the Doñana Natural Space in southern Spain. 144 experimental plots will manipulate the presence of waterbirds in vegetated and unvegetated wetlands, and will create a gradient in C, N, and P (guano) inputs, with the following six treatments: X = no birds present, W = waterfowl present, and F = flamingos + waterfowl present. Controlled laboratory and outdoor mesocosm studies will further examine the interacting stimulatory effects of waterbird guano and benthic invertebrates on GHG flux. Responses compared among treatments will include benthic and water-air fluxes of nitrous oxide, methane, and carbon dioxide, invertebrate densities and biomass, and abundances of microbial genes that are functionally linked to GHG flux (e.g. nirK, nirS, nosZ). Environmental data will be incorporated into predictive GHG flux models, including benthic invertebrate density and biomass; macrophyte density and biomass; water and sediment C, N and P content; and temperature and dissolved oxygen. By manipulating waterbird and invertebrate densities, this will be the first manipulative field study to simultaneously test the hypotheses that benthic invertebrates stimulate GHG emissions, and that higher trophic levels (waterbirds) exert both top-down and bottom-up influence on this process. This represents a new and potentially transformative line of inquiry into the roles of multiple trophic levels in regulating global wetland GHG flux.
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