DO SEDIMENT CONDITIONS AFFECT THE INCIDENCE OF ALEXANDRIUM CATENELLA AND PARALYTIC SHELLFISH POISONING? A STUDY OF SITES FROM PUGET SOUND

Paralytic shellfish poisoning (PSP) is common in the Puget Sound, resulting in restrictions on shellfish harvesting and health concerns for humans. PSP is caused by Alexandrium catenella, a toxic dinoflagellate with cyst and motile phases. The cysts reside in bottom sediment, the motile phase is released after excystment and blooms of the motile phase cause outbreaks of PSP. Although such outbreaks are monitored in Puget Sound by the Washington Department of Health, the factors that control their distribution and intensity are poorly understood. Sedimentological, biological and hydrological factors that might affect the incidence of PSP in Puget Sound were studied. The sedimentological component of the work is considered here. Sediment may affect the incidence of PSP in a number of ways. Alexandrium cysts may be transported and deposited with sediment and the location of cysts may be related to sediment properties such as grain size and organic content. Considering cyst abundance and distribution in relation to sediment may help predict areas likely to experience future PSP outbreaks. Surface sediment was collected and box cores 10-40 cm in length were extracted from thirty-two sites throughout Puget Sound. Cysts of Alexandrium catenella were found at more than half of the sites. Grain size of bottom sediments at sites varied from sand to clay-rich silt although most sites were predominantly silty. Total organic carbon (TOC) content averaged 6% by weight varying from 1-11%. No good correlations were observed between either grain size or TOC content with cyst abundance. Sequim Bay, in the north of Puget Sound and Quartermaster Harbor (QMH) in the south had the highest cyst abundances and cysts were present to the bottom of the cores from both of these sites. Quartermaster Harbor and Sequim Bay are in areas protected from strong bottom currents and their sediments are predominantly silty sand. Sequim Bay sediment has above average levels of TOC but QMH sediment has average carbon content. Neither site has unusual sediment characteristics that might account for the high levels of Alexandrium cysts observed in their sediments. The lack of correlation between sediment properties and Alexandrium cyst abundance indicates that sediment probably does not play an important role in determining the intensity and distribution of Alexandrium blooms.