THE ROLE OF AQUATIC BIRDS IN THE REGULATION OF TROPHIC RELATIONSHIPS OF CONTINENTAL SODA PANS IN HUNGARY

The aim of this study was to estimate the population sizes, food resources, food selection and trophic regulation of aquatic birds in these soda pans. We classified the estimated density of birds into 3 simple nutrient cycling guilds: net-importer, exporter-importer and the net-exporter. The most important aquatic bird guild was the net-importer guild (51-70%), and the second was the exporter-importer guild (41-27%), while the relative densities of the net-exporter guild was the lowest (8-3%) in the investigated 2 pans. The captive foraging experiment demonstrated that the filter-feeder wildfowl (Anas species) could successfully remove the microcrustacean plankton and invertebrate nekton from the water. The biomass of plank-tonic crustaceans was significantly more by an order of magnitude than the biomass of the other invertebrates (benthos, nekton). The relatively simple trophic relationships demonstrate the bottom up function of some keystone herbivore aquatic bird species, while the top down control is determined by several wildfowl and wader species. The external nutrient load of the aquatic birds causes hypertrophic level of inorganic nutrient resources for the algae, while the planktonic primary production varied only between oligotrophy and mesotrophy because of the extreme physical conditions of these waters. The observed net heterotrophy and several trophic relationships seem to be regulated by aquatic birds.