Abstract: Aims: Benthic microalgae are one of the primary producers in coastal wetlands, providing basic energy and material to the food web of coastal wetlands and playing an important role in maintaining the stability of coastal wetland ecosystems. Previous studies have focused on the effects of environmental factors, vascular plants and herbivores on benthic microalgae, neglecting the trophic cascade effects of predators. Methods: We investigated the trophic cascade effects of waterbirds on benthic microalgae in a manipulative field experiment in a coastal wetland in the Yangtze estuary, where we measured the biomass of benthic microalgae monthly and their species composition quarterly for a year. Important findings: Results showed that: (1) benthic microalgal biomass was significantly decreased in waterbird exclusion treatments. In waterbird exclusion treatments, although the lack of direct consumption by waterbirds facilitated benthic microalgae, increases in the abundance of crabs feeding on microalgae suppressed benthic microalgae more strongly. (2) Crab exclusion treatments (simulating intense predation by waterbirds) also significantly reduced benthic microalgal biomass. In crab exclusion treatments, the abundance of snail feeding on microalgae increased substantially. (3) waterbird and crab exclusion treatments both significantly reduced the abundance of the dominant microalgal taxa, diatoms, and increased the diversity of benthic microalgal communities. This study demonstrates that waterbirds in coastal wetlands can exert strong top-down effects on benthic microalgal communities through multiple direct or indirect processes, and enriches our understanding of the mechanisms controlling microalgal communities in coastal wetlands.

Key words: coastal wetlands, trophic cascade, animal consumers, benthic microalgae, community diversity