Aim: Global biodiversity loss has raised interest in understanding variation in diversity at different scales. Especially studies conducted across large spatial gradients are crucial, because they can increase perspectives on how ecological patterns change relative to environmental factors, and facilitate predictions of possible responses to environmental change. We explored the full extent of a brackish sea to test the hypotheses that (i) benthic communities are defined by species’ limited ranges, controlled by varying drivers along a large environmental gradient, (ii) the responses of taxonomic and functional community composition and turnover to the environmental gradient are different, thus highlighting the need to include both measures in ecological studies, and (iii) diversity reaches the minimum at intermediate salinities (Remane curve) due to the low adaptation of freshwater and marine species.

Location: A large environmental and spatial gradient spanning the entire Swedish coastline (ca. 2300 km; salinity 1.2-27.6), the Baltic Sea

Time period: August 2018

Major taxa studied: Benthic diatoms

Methods: We assessed environmental drivers for the communities and calculated the taxonomic and functional alpha and beta diversity along the gradient. We also compared the taxonomic and functional composition and diversity of communities between areas with different salinity.

Results: We found support for the hypothesis of limited species ranges, as taxonomic beta diversity, mainly induced by changes in salinity and climate, was high, whereas functional beta diversity remained considerably lower, and the composition and diversity of communities, as well as environmental drivers controlling the communities, differed between regions with different salinity. The lowest taxonomic diversity was found at intermediate salinities of 5-6.

Main conclusions: These findings advance understanding of large-scale patterns of benthic diversity, emphasize the importance of large gradient studies for a better understanding of general ecological patterns, and highlight the vulnerability of brackish water ecosystems as ecologically important tipping point realms.

The dataset was collected from the freshwater-marine gradient along the Swedish coastline in northern Europe. The dataset covers ca. 2300 km of coastline. Benthic diatoms were collected from littoral stones.