Dinoflagellate cysts in recent marine sediments from Scapa Flow, Orkney, Scotland

Abstract

To determine the composition, abundance and horizontal distribution of resting cysts in modern coastal sediments from Scapa Flow, Orkney, Scotland, sediment samples were collected from 12 stations. Twenty-six dinoflagellate cyst types representing eight motile-defined genera and one cyst-defined genus were observed. Four species or species groups dominated the assemblage for the Flow as a whole, Scrippsiella trochoidea, unidentified round brown cysts, Polykrikos schwartzii and Alexandrium tamarense. Total cyst abundance ranged from 37–1524 cysts ml−1 wet sediment. The majority of cysts occurred in the central area of the Flow, where higher densities were observed. Of particular importance is the distribution and abundance of cysts of A. tamarense. Since 1991 there have been annual toxic episodes of paralytic shellfish poisoning, caused by the vegetative stage of A. tamarense, in Scapa Flow, leading to widespread bans on all shellfish activities for months at a time. Cysts of A. tamarense were widely distributed within the Flow and ranged from 0–212 cysts ml−1 wet sediment. The overall widespread distribution and higher abundances of cysts of A. tamarense in the central areas of the Flow indicate potential seedbeds for initiation of future vegetative growth and subsequent outbreaks of paralytic shellfish poisoning.