Wetlands are globally important biogeochemical hotspots, and their roles as either sinks or sources of greenhouse gas (GHG) are strongly controlled by environmental drivers such as carbon (C), nitrogen (N), and phosphorus (P) availability, which can stimulate emission of three main GHGs: nitrous oxide (N2O), methane (CH4), and carbon dioxide (CO2). Mounting evidence suggests that aquatic invertebrate activity can enhance wetland GHG flux. However, waterbirds have been shown to reduce densities of aquatic invertebrates, which could potentially moderate GHG flux. Alternatively C, N, and 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. Our FLAMMINGGOS (Functional Links in Avian, Microbial, Macrophyte, and INvertebrate Greenhouse Gas Output Stimulation) project was created in order to test the relative strength of top-down and bottom-up effects of predatory waterbirds on wetland GHG flux. A network of long-term waterbird exclosure plots has been established within the Doñana Natural Space and Marismas de Odiel wetlands in southern Spain. 108 experimental plots manipulate the presence of waterbirds in wetlands, creating a gradient in predation and C, N, and P (guano) inputs. Controlled laboratory studies further examined the interacting stimulatory effects of waterbird guano and benthic invertebrates on GHG flux. Here we present our preliminary results.
Results/Conclusions
In controlled laboratory studies, the addition of guano from predatory and herbivorous waterbirds enhanced greenhouse gas flux, but the relative enhancement was less than that caused by the presence and density of benthic invertebrates. This suggests that flamingos and other benthivorous Mediterranean waterbirds may reduce the emissions of GHG. However, this role may differ considerably between wetlands that are seasonally and permanently flooded. In the latter, resident waterbirds cycle nutrients already present within the system, and limit the densities of invertebrates that would have otherwise strongly enhance microbial activity. Conversely in seasonal wetlands, visitation by waterbirds introduces subsidies of external nutrients that, while crucial to the functioning of the system, may also serve to enhance GHG emissions. Both GHG flux and bird densities are also dependent on wetland salinity, further adding to the complexity and context-dependence of the mechanisms affecting wetland GHG emissions. However, the elucidation of these mechanisms – a central goal of our project – could ultimately improve our ability to understand and predict wetland GHG emissions at larger scales, while simultaneously enhancing our knowledge of the functional roles of waterbirds in aquatic ecosystems.