Chlorophyll a biomass and growth of sea-ice microalgae along a salinity gradient (southeastern Hudson Bay, Canadian Arctic)

Abstract

The biomass of microalgae at the bottom of first-year sea ice, in southeastern Hudson Bay (Canadian Arctic), parallels an inshore-offshore salinity gradient caused by the under-ice plume of the Great Whale River. The present study was designed to test the hypothesis that the variation of ice-algal biomass (chlorophyll a) along the salinity gradient was mainly controlled by nutrient availability, with the alternative hypothesis of a direct control by ambient salinity. The approach was that of differential in situ bioassays, conducted at the ice-water interface of two stations, located in the plume of the Great Whale River (lower salinity) and in the offshore waters of Hudson Bay (higher salinity). The inoculum (collected at the higher salinity station) was diluted with three types of seawater, i.e. (1) from the higher salinity station, (2) from the lower salinity station, and (3) from the latter but with salinity artificially increased to the level of the higher salinity station. The three sets of cultures were differentially enriched. In situ incubations for the first set were at the higher salinity station and, for the other two, at the lower salinity station. Results indicate possible Si limitation of the algal biomass at the higher salinity station. First, concentrations of Si(OH)4 observed at this station were lower than in the plume of the Great Whale River; in addition, the Si∶P molar ratios were lower than ca. 15; also, Si was the only nutrient whose addition (alone or combined with others) yielded biomasses higher than in the reference enrichment; finally, the highest growth rate for a singly added nutrient was with Si and subtraction of Si (single nutrient) was more detrimental to growth rate than that of N or P. In contrast, there was no strong indication of nutrient effects at the lower salinity station, so that nutrient limitation could not explain the lower ice-algal biomasses in lower salinity waters. At this same station, on the other hand, growth rates in water with artificially increased salinity were 2–3 times higher than those in unaltered water. These results are consistent with the hypothesis that salinity, and not nutrients, is the main factor that limits the development of ice algae in the lower salinity waters of southeastern Hudson Bay.