Seasonal study of sea-ice exopolymeric substances on the Mackenzie shelf: implications for transport of sea-ice bacteria and algae

Abstract

Bottom sea ice, from under high and low snow cover, and surface water samples were collected in Franklin Bay (Mackenzie shelf) on 21 occasions between 24 February and 20 June 2004 and analyzed for exopolymeric substances (EPS), particulate organic carbon (POC) and chlorophyll a (chl a). Concentrations of EPS were measured using Alcian blue staining of melted ice samples. Chl a and bacterial sinking velocities were also assessed with settling columns, to determine the potential role of EPS in the transport of sea-ice biomass. EPS concentrations in the bottom ice were consistently low in March (avg. 185 µg xanthan equivalents l -1 ), after which they increased to maximum values of 4930 and 10 500 µg Xequiv. l -1 under high and low snow cover, respectively. EPS concentrations in the surface water were consistently 2 orders of magnitude lower than in the sea ice. Sea-ice EPS concentrations were significantly correlated with sea-ice chl a biomass (τ = 0.70, p < 0.01). Sea-ice algae were primarily responsible for EPS production within the sea ice, whereas bacteria produced insignificant amounts of sea-ice EPS. EPS-carbon contributed, on average, 23% of POC concentra- tions within the sea ice, with maximum values reaching 72% during the melt period. Median chl a sinking velocities were 0.11 and 0.44 m d -1 under high and low snow cover, respectively. EPS had little effect on chl a sinking velocities. However, bacterial sinking velocities did appear to be influ- enced by diatom-associated and free EPS within the sea ice. Diatom-associated EPS could facilitate the attachment of bacteria to algae thereby increasing bacterial sinking velocities, whereas the sink- ing velocities of bacteria associated with positively buoyant, free EPS, could be reduced. EPS con- tributed significantly to the sea-ice carbon pool and influenced the sedimentation of sea-ice biomass, which emphasizes the important role of EPS in carbon cycling on Arctic shelves.