Reply on RC2

Birgit Koehler

doi:10.5194/bg-2021-302-ac3

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.

Highlights

Biodiversity is essential for ecosystem processes, and for the humans depending on these (IPBES, 2019)
Since fish Species richness (SR) and a number of functional attributes changed along the salinity gradient, respective changes in the coastal fish communities may be foreseen if climate change further alters salinity conditions in the Baltic Sea
While the confidence in future salinity projections remains low (HELCOM, 2021), recent ensemble simulations estimate that the two main drivers of climate-related changes in salinity in the Baltic Sea region, increasing river runoff and sea level rise 470, approximately compensate each other, and may result in no net salinity changes (Meier et al, 2021)

Summary

Introduction

Biodiversity is essential for ecosystem processes, and for the humans depending on these (IPBES, 2019). Occurring coastal species numbers often remain uncertain (Appeltans et al, 2012). This makes improved understanding of their biodiversity especially important to support conservation and management measures (Pan et al, 2013; Rooney & McCann, 2012). Species censuses, are required e.g. for the analysis of biodiversity patterns, delineation of species 35 ranges, and prioritization of conservation efforts (Mora et al, 2008). Species richness (SR), i.e. the number of species in an ecosystem, is a classical indicator of biodiversity, referred to as “alpha diversity” (Gotelli & Colwell, 2001; Hill, 1973). Since achieving complete species inventories is often impracticable with realistic sample efforts, most censuses remain incomplete and many rare species remain unknown. It is important to consider the effect of sample size and inventory completeness (IC) on observed SR (SRobs) to avoid biased or misleading comparisons or interpretations (Chao 40 & Chiu, 2016; Chao et al, 2020; Colwell & Coddington, 1994; Mora et al, 2008)

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