Role of environmental factors on phytoplankton bloom initiation under landfast sea ice in Resolute Passage, Canada

Abstract

It has been common practice in scientific studies to assume negligible phytoplankton production when the ocean is ice-covered, due to the strong light attenuation properties of snow, sea ice, and ice algae. Recent observations of massive under-ice blooms in the Arctic challenge this concept and call for a re-evaluation of light conditions prevailing under ice during the melt period. Using hydrographic data collected under landfast ice cover in Resolute Passage, Nunavut, Canada between 9 May and 21 June 2010, we documented the exponential growth phase of a substantial under-ice phytoplankton bloom. Numerous factors appeared to influence bloom initiation: (1) transmitted light increased with the onset of snowmelt and termination of the ice algal bloom; (2) initial phytoplankton growth resulted in the accumulation of biomass below the developing surface melt layer where nutrient concentrations were high and turbulent mixing was relatively low; and (3) melt pond formation rapidly increased light transmission, while spring-tidal energy helped form a surface mixed layer influenced by ice melt -both were believed to influence the final rapid increase in phytoplankton growth. By the end of the study, nitrate+nitrite was depleted in the upper 10 m of the water column and the under-ice bloom had accumulated 508 mg chl a m −2 with a new production estimate of 17.5 g C m −2 over the upper 50 m of the water column. The timing of bloom initiation with melt onset suggests a strong link to climate change where sea ice is both thinning and melting earlier, the implication being an earlier and more ubiquitous phytoplankton bloom in Arctic ice-covered regions.

Late winter biogeochemical conditions under sea ice in the Canadian High Arctic

“…We found that the biogeochemical conditions in the water column under the sea ice during late winter in the outer regions of the Canadian Arctic Archipelago were more similar to surface conditions of the offshore Arctic Ocean than what has been observed in the past in the centre of the archipelago (e.g., Mundy et al 2014). Specifically, the conditions at our ice base off Ellef Ringnes Island were similar to the low-nutrient waters of the multi-year ice-covered Canada Basin (Else et al 2013).…”

“…Indeed, we found considerably low nutrient concentrations, as well as low C T and A T resulting in low seawater pCO 2 conditions, in the surface mixed layer throughout the study period. Furthermore, although there was a significant detectable increase in chlorophyll in the surface waters, which correlated with an increase in day length, we caution that there was no corresponding removal of nitrate'nitrite or carbon (even when normalized to salinity) and that the increase is relatively small compared to other observed ice algae and underice blooms (e.g., chlorophyll !10 mg m (2 ; Mundy et al 2014). The under-ice conditions we observed therefore suggest that there may not be sufficient nutrient levels in the northern reaches of the archipelago to instigate or sustain a large bloom as the light continues to increase.…”

“…Cota et al (1987) measured larger tidal flows in April through June in the central archipelago, than we did at our study site, which was likely influenced by the winter ice dampening effect (Prinsenberg & Ingram 1991 However, our findings temper the idea that large underice phytoplankton blooms are a widespread, yet underdocumented, phenomenon in polar regions (Mundy et al 2009) because a larger bloom could not be sustained with these low-nutrient conditions. Although both Arrigo et al (2012) and Mundy et al (2014) have found substantial under-ice phytoplankton blooms in the Arctic Ocean (phytoplankton biomass !1000 mg C m (3 in shallow shelf in the Chukchi Sea and 508 mg Chl m (2 in the inner Canadian Arctic Archipelago, respectively), their observations were later in the season (MayÁJuly) and in areas with higher initial nutrient concentrations than those found in our study area. Measured C T and A T were low at the surface (1Á10 m) compared to other oceanic regions (Fig.…”

Summer and fall distribution of phytoplankton in relation to environmental variables in Labrador fjords, with special emphasis on Phaeocystis pouchetii

“…Whether our observations in Labrador fjords can be extrapolated to other fjords along Canada's east coast is yet to be confirmed by future studies. To determine a more precise annual succession in protist community, a sampling expedition in Labrador fjords should be conducted in late spring to assess whether phytoplanktom blooms occur under the sea ice as observed elsewhere in the Arctic (see Arrigo et al 2012, Mundy et al 2014, Assmy et al 2017 and in the first half of July, immediately after the sea ice break-up. Moreover, in future investigations, it will be interesting to deter-mine how protist taxonomic composition is affected by proto-and metazooplankton grazing throughout the seasons.…”

Status and trends in the structure of Arctic benthic food webs

“…In the summer, limited nutrients support a low phytoplankton biomass (Wassmann & Reigstad 2011), with episodic bloom conditions facilitated by occasional nutrient availability . These phytoplankton blooms, although responsible for the bulk of annual production, are usually restricted to open waters (but see Arrigo et al 2012;Mundy et al 2014). Boetius et al (2013) also showed that sinking ice algae aggregates constitute an important food input to benthos in the Arctic basins.…”