Habitat-related patterns of soft-bottom macrofaunal assemblages in a brackish, low-diversity system (southern Baltic Sea)

Coastal areas provide a high variety of sedimentary habitats that influence the structure of resident fauna even on small geographical scales. Therefore, examinations of spatial variations in benthic assemblages require background knowledge of the environmental and biotic heterogeneity of habitats in order to understand ecological processes in such areas. The effect of habitat-related abiotic and biological variables on macrofaunal benthic assemblages was studied seasonally in 2010 and 2011 in the brackish, semi-enclosed Puck Lagoon (Gulf of Gdańsk, southern Baltic Sea). Based on macrophytal biomass, two discrete benthic regions were identified in the lagoon: a region of large biomass and a region of few macrophytes. The quality of the surface sediment organic matter (measured as C/Nsed ratio), depth, and benthic macrophyte composition accounted for within-region variation, which led to the identification of four habitats. Shallow sandy sediments with low C/Nsed ratios provide high quality sedimentary food for animals that, together with species-rich, dense macrophyte vegetation, support diverse assemblages. High C/Nsed ratios and peat outcrops in shallow sands exert a negative effect on macrofaunal diversity. Two deeper sandy habitats with less massive, species-poor vegetation tend to host distinct faunal assemblages of higher abundance and biomass. The importance of benthic vegetation for macrofaunal assemblages in the southern Baltic Sea is suggested to stem from its complex spatial structure that offers a number of microniches for infaunal and epifaunal species. The effect of macrophytes on benthic faunal assemblages was consistent throughout most of the year with the strongest influence in summer when macrophytes reached the highest biomass.