Studies on dinoflagellate cyst assemblages in two estuarine Mediterranean bays: A useful tool for the discovery and mapping of harmful algal species

The composition and assemblages of living dinoflagellate cysts from two estuarine bays (Alfacs and Fangar bays) in the northwestern Mediterranean Sea were investigated, focusing on the presence and distribution of harmful species. Sediment cores were taken from 10 stations in Alfacs Bay and from 6 stations in Fangar Bay. Sediment samples from the surface (the top 1 cm) and the subsurface profile (from 2 to 5 cm depth) in selected stations, were analyzed. Sixty-two morphotypes were recovered, some of which are new reports for the northwestern Mediterranean area. Few morphotypes dominated in terms of abundance and relative percentage (e.g. the Scrippsiella trochoidea complex was the dominant and most widely distributed morphotype in each bay, reaching maxima of 163 cysts cm−3 wet sediment (ws) and 102 cysts cm−3 ws in Alfacs and Fangar bays, respectively). The assemblage in Alfacs Bay was also characterized by the presence of Biecheleria cincta (maximum 116 cysts cm−3 ws), whereas the occurrence of Pentapharsodinium tyrrhenicum (maximum 37 cysts cm−3 ws) was greater in Fangar Bay. Twelve morphotypes belonging to potentially toxic or noxious species were detected, with the genus Alexandrium dominating. Among the harmful species, Gymnodinium litoralis and Vulcanodinium rugosum are reported for the first time from the study areas. Furthermore, cysts of the high biomass bloom-forming species Kryptoperidinium foliaceum are reported for the first time in the Mediterranean Sea. All the harmful species, with the exception of Alexandrium minutum, showed greatest abundances in subsurface samples. Profile analysis led to the description of a new cyst morphotype belonging to the Alexandrium genus (presumably A. insuetum). Our results provide information on the presence of harmful species in the studied bays, confirming the usefulness of cyst analysis in assessment of the potential risk of harmful blooms in aquaculture areas.

► Sixty-two cyst morphotypes were found in the study area, including twelve harmful species.
► Five cysts represented new detections in natural sediments of the Mediterranean Sea.
► Cysts of three species are described for the first time in natural sediments.
► Two harmful species not previously reported in the bays were detected.
► Cyst studies contribute to knowledge of cyst-forming species biodiversity.