Description: Dinoflagellate cyst concentrations (cysts/g) from the top 0,5 cm surface sediment from the Belcher Islands Archipelago, south-eastern Hudson Bay, Canada on April 15th, 2019, using a Kajak-Brinkhurst gravity corer. Dinoflagellate cyst quantifications were done using a light microscope (Zeiss Axio Imager.A2 upright research microscope, University of Helsinki) with bright-field optics at a x600 magnification.

Description: Data includes under ice temperature (C) and salinity (psu) measurements from each sampling day, measured by using with RBR Concerto® and CastAway-CTD® sensors.

Description: Highly branched isoprenoid (HBI concentrations (ng/ml) from the bottom 5 cm of modern sea ice samples from the Belcher Islands Archipelago, south-eastern Hudson Bay, Canada between April and May 2019 using a Kovacs Mark II core barrel with an internal diameter of 9 cm.

Description: Relative abundances (%) of dinoflagellate cysts from the top 0,5 cm surface sediment from the Belcher Islands Archipelago, south-eastern Hudson Bay, Canada on April 15th, 2019, using a Kajak-Brinkhurst gravity corer. Dinoflagellate cyst quantifications were done using a light microscope (Zeiss Axio Imager.A2 upright research microscope, University of Helsinki) with bright-field optics at a x600 magnification.

Description: Relative abundances (%) of diatoms from the top 0,5 cm surface sediment from the Belcher Islands Archipelago, south-eastern Hudson Bay, Canada on April 15th, 2019, using a Kajak-Brinkhurst gravity corer. The surface sediment data includes relative diatom abundances (%). Diatom quantifications were done using a light microscope (Zeiss Axio Imager.A2 upright research microscope, University of Helsinki) with phase contrast optics at a x1000 magnification.

Description: Melt pond covered sea ice is a ubiquitous feature of the summertime Arctic Ocean when meltwater collects in lower-lying areas of ice surfaces. Horizontal transects were conducted during June 2008 above and below landfast sea ice with melt ponds to characterize surface and bottom topography together with variations in transmitted spectral irradiance. We captured a rapid progression from a highly flooded sea ice surface with lateral drainage toward flaws and seal breathing holes to the formation of distinct melt ponds with steep edges. As the mass of the ice cover decreased due to meltwater drainage and rose upward with respect to the seawater level, the high-scattering properties of ice above the water level (i.e., white ice) were continuously regenerated, while pond waters remained transparent compared to underlying ice. The relatively stable albedos observed throughout the study, even as ice thickness decreased, were directly related to these surface processes. Transmission through the ice cover of incident irradiance in the 400-700 nm wave band ranged from 38% to 67% and from 5% to 16% beneath ponded and white ice, respectively. Our results show that this transmission varied not only as a function of surface type (melt ponds or white ice) areal coverage but also in relation to ice thickness and proximity to other surface types through the influence of horizontal spreading of light. Thus, in contrast to albedo, this implies that regional transmittance estimates need to consider melt pond size and shape distributions and variations in optical properties and thickness of the ice cover.

Description: During summer 2008, as part of the Circumpolar Flaw Lead system study, we measured phytoplankton photosynthetic parameters to understand regional patterns in primary productivity, including the degree and timescale of photoacclimation and how variability in environmental conditions influences this response. Photosynthesis-irradiance measurements were taken at 15 sites primarily from the depth of the subsurface chlorophyll a (Chl a) maximum (SCM) within the Beaufort Sea flaw lead polynya. The physiological response of phytoplankton to a range of light levels was used to assess maximum rates of carbon (C) fixation (P*m), photosynthetic efficiency (alpha*), photoacclimation (Ek), and photoinhibition (beta*). SCM samples taken along a transect from under ice into open water exhibited a 〉3-fold increase in alpha* and P*m, showing these parameters can vary substantially over relatively small spatial scales, primarily in response to changes in the ambient light field. Algae were able to maintain relatively high rates of C fixation despite low light at the SCM, particularly in the large (〉5 µm) size fraction at open water sites. This may substantially impact biogenic C drawdown if species composition shifts in response to future climate change. Our results suggest that phytoplankton in this region are well acclimated to existing environmental conditions, including sea ice cover, low light, and nutrient pulses. Furthermore, this photoacclimatory response can be rapid and keep pace with a developing SCM, as phytoplankton maintain photosynthetic rates and efficiencies in a narrow ''shade-acclimated'' range.

Description: The environmental factors influencing the microalgal bloom during sea-ice breakup in Hudson Bay were investigated during June 2018, producing the first results ever on the seasonal development of the marine ecosystem in the offshore waters of this vast inland sea. As is typical in the Arctic, primary production was found to commence at the onset of ice melt, with surface nutrient depletion leading to the formation of a subsurface chlorophyll maximum in the open waters of western Hudson Bay. Simultaneously, the melting mobile ice cover in central Hudson Bay created favorable conditions for a diatom-dominated under-ice bloom, with the results of irradiance-photosynthesis curves confirming that phytoplankton cells were acclimated to increasing light levels in the surface water. The high production rates measured in ice-covered and ice-free waters highlight the considerable plasticity of phytoplankton in terms of photosynthetic performance in this highly variable environment. Interestingly, the maximum values of primary production and phytoplankton biomass observed under the sea ice (343 mg C m-2 d-1 and 35.10 mg TChl a m-2) were lower than those observed in open waters during the late-bloom stage in the western region (486 mg C m-2 d-1 and 57.12 mg TChl a m-2), which is attributed to a confined euphotic zone (reduced light availability? Since the euphotic zone in clear waters under the ice can be as thick as elsewhere, but simply receive less irradiance overall) under the ice and low surface concentrations of inorganic nitrogen (〈2 mmol L-1) in central Hudson Bay. However, the highly abundant sub-ice diatom Melosira arctica contributed an estimated additional 287 mg C m-2 d-1 to under-ice production in this region, which implies that this filamentous diatom has a similar role in the seasonally ice-covered sub-Arctic as in the central Arctic Ocean where it significantly contributes to local production. Refining the historical total production estimates of Hudson Bay with our spring observations, we recalculated annual production to be ca. 69 g C m-2, which equates to mean value for interior Arctic shelves.