Strong seasonal effects of waterbirds on benthic communities in shallow lakes

Little is known about the effects of waterbirds on benthic macroinvertebrates in nontidal habitats. We conducted 4 waterbird-exclusion experiments in a transformed marshland in Donana Natural Park (southwestern Spain) and compared benthic macroinvertebrates in 3 x 3-m control (no exclusion) and test plots from which Greater Flamingos Phoenicopterus ruber or all waterbirds were excluded. The 4 experimental periods coincided with the waterbird breeding season (summer), postbreeding period (autumn), wintering period (winter), and pre- and early breeding season (spring). Densities of waterbirds were relatively low compared to densities reported in other studies, but we found strong effects of excluding flamingos and waterfowl on the structure of the benthic community in autumn, winter, and spring. In winter, numbers of polychaetes and chironomids were nearly 2x higher in exclusion than in control plots. The difference in macroinvertebrate biomass among treatments was strongest in winter when benthic dry mass was similar to 2x higher in flamingo exclosures than in control plots and similar to 2x higher in all-bird exclosures than in flamingo exclosures. Macroinvertebrate richness was 15% lower in control than in all-bird exclosures in winter and 25% lower in control than in all-bird exclosures in spring. Our results suggest that benthic macroinvertebrate communities in this wetland are affected more by predation, bioturbation, and waterbird herbivory than by competition. These findings suggest that waterfowl and the increasing population of flamingos are of great importance in structuring shallow lakes in the Mediterranean region.