Threshold effects of climate change on benthic diatom communities: Evaluating impacts of salinity and wind disturbance on functional traits and benthic biomass

The responses of biotic communities and ecosystems to climate change may be abrupt and non-linear. Thus, resolving ecological threshold mechanisms is crucial for understanding the consequences of climate change and for improving environmental management. Here, we present a study on the threshold responses of benthic diatom communities that are an important component of all aquatic environments and strongly contribute to global primary production. We reach beyond the taxonomic perspective by focusing on the diversity and functions of diatom communities and benthic biomass along gradients of salinity and wind disturbance, whose climate-change-induced changes have been predicted to strongly affect biotic communities in the marine and brackish systems in the future. To improve the generality of our results, we examine three self-collected datasets from different spatial scales (6-830 km) and ecosystem types. We collected samples from rock pools or from littoral stones and studied taxonomic thresholds using Threshold Indicator Taxa Analysis (TITAN2). We investigated threshold responses of community diversity, community functions, and benthic biomass using t-tests and regression analyses. Our results indicated that decreasing salinity may result in increasing diversity but decreasing biomass of brackish communities, while the effects of increasing wind disturbance were contradictory among spatial scales. Benthic biomass correlated with the taxonomic and functional diversity, as well as with the body size distribution of communities, highlighting the importance of considering community functions and organismal size when predicting ecosystem functions. The most pronounced effects of decreasing salinity and increasing wind disturbance on community functions were changes in the abundance of low-profile diatom species, which, due to the high resilience of low-profile diatoms, may lead to changes in ecosystem functioning and resilience. To conclude, decreasing salinity and increasing wind disturbance may lead to threshold responses of biotic communities, and these changes may have profound effects on ecosystem functioning along marine coastal areas.