Surface sediment dinoflagellate cysts from the Hudson Bay system and their relation to freshwater and nutrient cycling

Abstract

Abstract Surface sediment samples from the Hudson Bay system were analysed in order to examine the role of key regulators of arctic marine productivity — light and nutrients as affected by freshwater stratification and sea-ice cover — on the spatial distribution and production of dinoflagellate cysts. Total cyst fluxes vary from 0.2 × 10 6 to 30.6 × 10 6 cysts m − 2 a − 1 , with the highest values observed in eastern Hudson Bay. A total of 24 cyst taxa, representing 11 genera of five orders, were identified and distribution maps of the most common taxa have been produced. This is the first record of Echinidinium aculeatum , Echinidinium karaense , cf. Echinidinium delicatum , Islandinium brevispinosum , Selenopemphix quanta , cysts of Protoperidinium americanum , cysts of cf. Biecheleria sp. and Polarella glacialis in the Hudson Bay system. Dinoflagellate cyst assemblages show distinct spatial patterns revealing three compositional domains: eastern Hudson Bay, western-central Hudson Bay and Hudson Strait. The eastern domain is characterised by a dominance of autotrophic cysts of Pentapharsodinium dalei whereas the western-central domain is characterised by autotrophic Operculodinium centrocarpum with some contribution by heterotrophic Polykrikos sp. var. arctic morphotype and Polykrikos spp. Sites from Hudson Strait are distinguished by an overwhelming prevalence of heterotrophic Protoperidiniaceae cysts, mainly Islandinium minutum , and have the highest values of sedimentary biogenic silica, used as a proxy for diatom productivity. Sediment geochemical tracers are used as proxies for freshwater inputs (lignin and its biomarkers) and nitrate availability (nitrogen isotopes), and sea-ice concentrations derived from passive microwave data as a proxy for light availability. Sea-ice regulated length of the dark season has a negligible influence on the proportion and production of heterotrophic (dark-adapted) versus autotrophic (light-dependent) dinoflagellate cysts, perhaps due to the location of our study area on the southern fringe of the Arctic. Instead, cyst populations in Hudson Bay are primarily regulated by vertical stratification and nitrate availability, while in Hudson Strait the pivotal mechanism constitutes food availability.