β-diversity as a measure of species turnover along the salinity gradient in the Baltic Sea, and its ­consistency with the Venice System

Abstract

Transition zones between marine and freshwater environments are characterized by a pronounced salinity gradient and concomitant variation in invertebrate species richness. Here we use the β-diversity concept to depict the species turnover of macrobenthic species along the salinity gradient of the Baltic Sea with salinities ranging from 34 in the transition zone to the North Sea to less than 5 in the Bothnian Sea. Based on 250 data sets from 72 locations that were grouped into 2 habitats defined according to their depths and sediment types (shallower: 15 to 19 m, fine to medium sand; deeper: 20 to 35 m, silt to silty sand), we calculated the Jaccard dissimilarity index (β 1-J ) as a measure of species turnover. To keep the focus on the salinity gradient, sediment characteristics and the time period covered by the data sets (spring and summer 1995 to 2005) were predefined. The mean hydrographic parameters, including temperatures, salinities and dissolved oxygen (DO) concentrations of the sample locations were derived from model calculations based on data gathered over a 3 yr period before sampling. At the deeper stations, the total number of macrofaunal species was 255, while at the shallower ones, 172 taxa were found. Statistical analyses revealed salinity to be the main structuring factor for macrobenthic species turnover. High correlations between the β 1-J index and mean salinities in both habitats (Spearman's rank correlation coefficient ρ = 0.88 in shallower and ρ = 0.86 in deeper areas) confirmed these findings. β-diversity values with median β 1-J varied between 51 and 65% within the salinity classes eu-, poly-, α-meso-and β-mesohaline, while values of 75 to 100% characterized between-group comparisons. Furthermore, these high β-diversity values depict a discontinuous change in the communities and are found at salinities of around 10, 18, and 30, which ties in fairly well with the existing salinity boundaries postulated by the Venice System. KEY WORDS: Species turnover · β-diversity · Salinity gradient · Macrofauna · Venice System · Baltic Sea Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 436: 101-118, 2011 102 agreed on the Venice System (Caspers 1959), in which the following generalized salinity class boundaries were defined: euhaline zone > 34 to 30, polyhaline zone 30 to 18, α-mesohaline zone 18 to 10, β-mesohaline zone 10 to 5, α-oligohaline zone 5 to 3, β-oligohaline zone 3 to 0.5, and limnetic zone 0.5 to 0. The System is widely used, e.g. in the implementation of the European Union's (EU) Water Framework Directive in the coastal waters of the Baltic Sea (von Weber 2004). The Venice System's boundaries are also consistent with distribution limits determined by coenosis (Hiltermann 1963). Nevertheless, other studies have claimed that no objective criteria exist for these boundaries (e.g. Bulger et al. 1993). Except for the minimum species richness at salinities from ~5 to 8, the boundaries of the Venice System cannot be derived from Rema...

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