Formation and species composition of stormcast beach wrack in the Gulf of Riga, Baltic Sea **The study was supported by the EU Life+project MARMONI ‘Innovative approaches for marine biodiversity monitoring and assessment of conservation status of nature values in the Baltic Sea’, ESF grant No. 8980, Estonian target financed project SF0180104s08 and Institutional research funding IUT2-20 of the Estonian Research Council.

The aim of the study was to investigate hydrodynamic effects on the formation of beach wrack at three locations in the northern Baltic Sea and to quantify the differences between the composition of species found in the beach wrack and in the neighbouring sea. Hydrodynamic measurements and modelling indicated that the beach wrack was mostly of local origin and that it was formed during high sea level and wave events. Comparison of the methods of beach wrack sampling and seabed sampling (diver, underwater video) demonstrated that beach wrack sampling can be considered an alternative tool for describing the species composition of macrovegetation in near-coastal sea areas. Although the hydrodynamic variability is greater in autumn and more biological material is cast ashore, the similarity between the two sampling methods was higher in spring and summer