Leads in Arctic pack ice enable early phytoplankton blooms below snow-covered sea ice

Abstract

The Arctic icescape is rapidly transforming from a thicker multiyear ice cover to a thinner and largely seasonal first-year ice cover with significant consequences for Arctic primary production. One critical challenge is to understand how productivity will change within the next decades. Recent studies have reported extensive phytoplankton blooms beneath ponded sea ice during summer, indicating that satellite-based Arctic annual primary production estimates may be significantly underestimated. Here we present a unique time-series of a phytoplankton spring bloom observed beneath snow-covered Arctic pack ice. The bloom, dominated by the haptophyte algae Phaeocystis pouchetii, caused near depletion of the surface nitrate inventory and a decline in dissolved inorganic carbon by 16 ± 6 g C m^-2. Ocean circulation characteristics in the area indicated that the bloom developed in situ despite the snow-covered sea ice. Leads in the dynamic ice cover provided added sunlight necessary to initiate and sustain the bloom. Phytoplankton blooms beneath snow-covered ice might become more common and widespread in the future Arctic Ocean with frequent lead formation due to thinner and more dynamic sea ice despite projected increases in high-Arctic snowfall. This could alter productivity, marine food webs and carbon sequestration in the Arctic Ocean.

Figure 1. Study location and vertical and spatial extent of the under-ice bloom.

(a) European Arctic with bathymetry. Orange and green lines are the drift trajectories of floes 3 and 4, respectively, with start and end dates. The location when we first drifted into the under-ice bloom on 25 May is indicated with an orange star. The area demarcating the ice-edge positions between April and June 2015 is shaded in grey. The ice-edge position on 25 May is indicated by the broken blue line and is representative for the bloom period. We define the ice edge as the outer perimeter of a polygon where ice concentration is >10%. The white outline demarcates the area shown in panels b and c. Map created by the Norwegian Polar Institute, Max König with permission from IBCAO. Drift trajectories of floes 3 and 4 showing (b) Chlorophyll a, and (c), nitrate concentrations for the upper 100 m of the water column. The dashed line in (b) indicates depth of the pycnocline.

Figure 2. Composition of the under-ice phytoplankton bloom and particulate organic carbon standing stocks.

(a) Integrated stocks of phytoplankton carbon (coloured bars) with contributions of Phaeocystis pouchetii, diatoms and other phytoplankton and particulate organic carbon (black stars) for the upper 50 m surface layer. Micrographs of (b), solitary cells (600x magnification) and (c), a colony of P. pouchetii (100x magnification).

Figure 3. Primary production model and water mass circulation over the Yermak Plateau.

(a) Open water, thin and thick ice concentration and weight-averaged E_PAR right below the sea surface based on the aerial fractions of the three different surface types. The white and coloured areas represent the area fraction of open water and sea ice, respectively, derived from satellite data (Supplementary Fig. S4). E_PAR values are modelled from surface E_PAR measurements and taking into account the diurnal cycle, different fractions of ice and open water and their respective optical properties. (b) Temporal evolution of Chl a concentration and net primary production (NPP) during the bloom period predicted by the model. Map of (c), surface (20 m) and (d), subsurface (80 m) simulated currents from model outputs with currents >2 cm s⁻¹. Current velocity is indicated by the size of the vectors (scale on figure). Black lines show drift trajectories. Colour dots show surface Chl a concentrations as measured along track indicating the bloom locations. Background colours show surface and subsurface water masses where blue is Polar Surface Water (PSW) and red is Atlantic Water (AW). Areas shallower than 20 m (c) and 80 m (d) are white. Topography of the Yermak Plateau is shown as thin black lines (500, 1000, 2000 and 3000 m). The maps in (c) and (d) were generated with the m-map package of Matlab 8.4 (https://www.eoas.ubc.ca/~rich/map.html).