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

Abstract

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) is strongly dependent 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 from Swedish fish survey databases. The data covered nearly five decades (1975–2020), a 1,300 km north-south distance and a 10-fold salinity gradient along sub-basins of the Baltic Sea plus Skagerrak. Focusing on shallow coastal and offshore areas (< 30 m depth), we calculated standardized SR (SRstd) and estimated SR (SRest), and related these to sub-basin annual mean salinity and water temperature. IC was high, 98.5 %–99.9 %, in the 10 sub-basins with sufficient data for analysis. The recorded fish species were of 75 % marine and 25 % freshwater origin. Total fish SRobs was 144 for shallow coastal areas, and 110 for shallow offshore areas. Sub-basin specific SRest for coastal areas varied between 35 ± 7 (SE) and 109 ± 6 fish species, and was ca. three times higher in the most saline (salinity 29-32) compared to the least saline sub-basins (salinity 2.7). Completing information on functional attributes showed that differences along the salinity gradient reflected an increased share of coastal resident fish species in lower salinities, and a higher share of migratory fish at higher salinities. The proportion of benthic and demersal fish species was also lower in the least saline sub-basins, and increased with increasing salinity. If climate change lowers the salinity regime of the Baltic Sea in the future this may hence influence the SR and community composition of fish.