Relationship between planktonic dinoflagellate abundance, cysts recovered in sediment traps and environmental factors in the Gullmar Fjord, Sweden

Abstract

In order to study the relationship between planktonic dinoflagellates, cyst production and environmental factors, a sediment trap study was conducted in the Gullmar Fjord, Swedish west coast, during 21 days in May–June 1998. Five locations for sediment traps were randomly selected every third day. The traps were moored at the five locations and moved to new locations after 3 days. At every location, a CTD depth profile was obtained and water samples were collected for plankton, chlorophyll a and nutrient analysis. Meteorological and hydrographic data for the period were obtained from continuous monitoring. Three dinoflagellate species, which have not previously been recorded from the Kattegat or the Skagerrak (Scrippsiella crystallina, Scrippsiella lachrymosa and Scrippsiella trifida), were encountered during the analysis of cysts from the sediment traps. The abundance of the different species in the motile form encountered in the water column and cyst form encountered in the sediment traps varied greatly. The discrepancy between the number and species encountered in traps and water samples is discussed. No density-dependent relationship between the abundance of planktonic cyst-forming dinoflagellates and the number of cysts recovered could be observed. A multiple regression showed that the variation in cyst yield from the traps for the most abundant species was correlated with water surface temperature, ambient light radiation and the depth of the halocline. The nutrient concentrations (NH4+, NO2–, NO3– and PO43–), which are known to play a crucial role in induction of sexuality and cyst formation under laboratory conditions, correlated poorly with the number of dinoflagellate cysts encountered in the traps.