Abundance and distribution of vernal bloom dinoflagellate cysts in the Gulf of Finland and Gulf of Riga (the Baltic Sea)

In the northern Baltic Sea, brackish water dinoflagellates Peridiniella catenata, Woloszynskia spp. and Scrippsiella hangoei form a major part of the spring bloom biomass, comparable to, or even exceeding the biomass of diatoms. The life cycle of these dinoflagellates involves a relatively short period of vegetative growth in early spring (2–3 months), followed by encystment during the first part of May, and a resting period in the form of benthic cysts. The bloom intensity of the Woloszynskia/Scrippsiella complex in the Baltic Sea is spatially variable, with peak abundances in the central and eastern Gulf of Finland, while the species are rare in the Gulf of Riga. During a field survey in late May 2004, we investigated the abundance and distribution of benthic cysts of P. catenata and Woloszynskia spp. in surface (5 cm) sediments around the Estonian coast. The broad distribution patterns of benthic cysts reflected the overall knowledge of basin-wide planktonic phase distribution of the species. On a finer scale, sediment properties (percent of clay, organic matter content) demarked the accumulation regions of the cysts. Woloszynskia cyst abundances in the surface 5 cm sediment were up to 3.2×106 cysts cm−2 and Peridiniella cyst abundances were approximately an order of magnitude less, up to 0.3×106 cysts cm−2.