Species richness and functional attributes of fish assemblages across a large-scale salinity gradient in shallow coastal areas

Abstract

Coastal ecosystems are biologically productive, and their diversity underlies various ecosystem services to humans. However, large-scale species richness (SR) and its regulating factors remain uncertain for many organism groups, owing not least to the fact that observed SR (SRobs) depends on sample size and inventory completeness (IC). We estimated changes in SR across a natural geographical gradient using statistical rarefaction and extrapolation methods, based on a large fish species incidence dataset compiled for shallow coastal areas (<30 m depth) from Swedish fish survey databases. The data covered a ca. 1300 km north–south distance and a 12-fold salinity gradient along sub-basins of the Baltic Sea plus the Skagerrak and, depending on the sub-basin, 4 to 47 years of samplings during 1975–2021. Total fish SRobs was 144, and the observed fish species were of 74 % marine and 26 % freshwater origin. In the 10 sub-basins with sufficient data for further analysis, IC ranged from 77 % to 98 %, implying that ca. 2 %–23 % of likely existing fish species had remained undetected. Sample coverage exceeded 98.5 %, suggesting that undetected species represented <1.5 % of incidences across the sub-basins, i.e. highly rare species. To compare sub-basins, we calculated standardized SR (SRstd) and estimated SR (SRest). Sub-basin-specific SRest varied between 35 ± 7 (SE) and 109 ± 6 fish species, being ca. 3 times higher in the most saline (salinity 29–32) compared to the least saline sub-basins (salinity < 3). Analysis of functional attributes showed that differences with decreasing salinity particularly reflected a decreasing SR of benthic and demersal fish, of piscivores and invertivores, and of marine migratory species. We conclude that, if climate change continues causing an upper-layer freshening of the Baltic Sea, this may influence the SR, community composition and functional characteristics of fish, which in turn may affect ecosystem processes such as benthic–pelagic coupling and connectivity between coastal and open-sea areas.