Organic-walled dinoflagellate cyst production, composition and flux from 1996 to 1998 in the central Strait of Georgia (BC, Canada): A sediment trap study

Bi-weekly fluxes of dinoflagellate cysts and assemblage composition were recorded from March 1996 to January 1999 in the central part of the Strait of Georgia (BC, Canada). The study period captured the 1997-98 El Niño event, which was characterized locally by increased Fraser River discharge resulting from earlier than usual snowmelt in 1997 and warmer sea-surface temperatures in 1998. Thirty dinoflagellate cyst taxa were identified in the sediment trap samples. The dinoflagellate cyst flux varied from ∼ 600 to 336,200 cysts m^{- 2} day^{- 1}, with an average of 20,000 cysts m^{- 2} day^{- 1}. In general, dinoflagellate cyst flux and species composition reflected seasonal variation of water conditions in the Strait of Georgia. Throughout the study period, assemblages were dominated by cysts produced by heterotrophic dinoflagellates, such as Protoperidineaceae (Brigantedinium spp., Quinquecuspis concreta, and Protoperidinium americanum). The greatest abundance of cysts of the potentially toxic Alexandrium spp. was recorded in the spring of 1996. Our results demonstrate that cysts produced by heterotrophic dinoflagellates peak in June each year, during or following diatom blooms, as indicated by biogenic silica flux. Cysts produced by autotrophic taxa were most abundant during August-September. The total annual dinoflagellate cyst flux was lower in 1997 and 1998 than in 1996, mostly due to the bloom of Alexandrium spp. in 1996. Warmer sea-surface temperature and the early spring of 1998 had a positive effect on the production of both autotrophic and heterotrophic dinoflagellates as reflected in the cyst fluxes. Cyst assemblages from sediment trap samples were consistent with a cyst assemblage recovered from a core sample at the same site.