Grazing and sedimentation of ice algae during and immediately after a bloom at the ice-water interface

Abstract

Sinking of pigmented particulate matter during and immediately after an ice algal bloom was studied in southeastern Hudson Bay at a 45 m deep station, using consecutive short-term deployments of sediment traps 30 m below the undersurface of the ice. Cell sinking and grazing by herbivores were assessed by measuring the vertical fluxes of chlorophyll a and phaeopigments. There were 2 peaks of cell sedimentation, a minor one during the first half of the bloom (mid-April) and a major one at the end of the bloom (mid-May); both were related to atmospheric warming events. The fluxes of both phaeopigments and fecal pellets (mainly from large herbivores) increased at the end of the bloom as ice algae were released from the ice environment. The total export of ice algae to the benthos (sinking cells + fecal pellets) was estimated to be ca 20 % of the ice algal production during the measurement period. A large proportion of ice algal production (ca 30 %) was still suspended in the upper water column at the end of the sampling season. The fate of the remaining 50 '10 is not known, but it is hypothesized that a significant fraction was retained in the pelagic environment.