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The rapid physical changes affecting the Arctic Ocean alter the growth conditions of primary producers. In this context, a crucial question is whether these changes will affect the composition of phytoplankton communities, augment their productivity, and eventually enhance food webs. We combined satellite and model products with in situ datasets collected during fall and provide new insights into the response of phytoplankton biomass and production in the Canadian Arctic by comparing an interior shelf (Beaufort Sea) and an outflow shelf (Baffin Bay). Correlation analysis was used to distinguish between seasonal and interannual variability and revealed that most biological variables are responding to the interannual pressures of climate change. In southeast Beaufort Sea, a change in phytoplankton community composition occurred, with a significant increase in diatoms from 2% (2002) to 37% (2010–2011) of the total protist abundance. In 2011, photosynthetic picoeukaryotes were twice as abundant as in 2002. For these two phytoplankton groups, abundance was correlated with the duration of the open-water period, which also increased and affected vertical stratification and sea-surface temperature. In contrast, there was a sharp decline in centric diatom abundance as well as in phytoplankton biomass and production in northern Baffin Bay over the years considered. These decreases were linked to changes in seasonal progression and sea-ice dynamics through their impacts on vertical stratification and freshwater input. Overall, our results highlight the importance of stratification and the duration of the open-water period in shaping phytoplankton regimes—either oligotrophic or eutrophic— in marine waters of the Canadian Arctic.

Summertime Arctic shipboard observations of oxygenated volatile organic compounds (OVOCs) such as organic acids, key precursors of climatically active secondary organic aerosol (SOA), are consistent with a novel source of OVOCs to the marine boundary layer via chemistry at the sea surface microlayer. Although this source has been studied in a laboratory setting, organic acid emissions from the sea surface microlayer have not previously been observed in ambient marine environments. Correlations between measurements of OVOCs, including high levels of formic acid, in the atmosphere (measured by an online high-resolution time-of-flight mass spectrometer) and dissolved organic matter in the ocean point to a marine source for the measured OVOCs. That this source is photomediated is indicated by correlations between the diurnal cycles of the OVOC measurements and solar radiation. In contrast, the OVOCs do not correlate with levels of isoprene, monoterpenes, or dimethyl sulfide. Results from box model calculations are consistent with heterogeneous chemistry as the source of the measured OVOCs. As sea ice retreats and dissolved organic carbon inputs to the Arctic increase, the impact of this source on the summer Arctic atmosphere is likely to increase. Globally, this source should be assessed in other marine environments to quantify its impact on OVOC and SOA burdens in the atmosphere, and ultimately on climate.

The sea-surface microlayer and bulk seawater can contain ice-nucleating particles (INPs) and these INPs can be emitted into the atmosphere. Our current understanding of the properties, concentrations, and spatial and temporal distributions of INPs in the microlayer and bulk seawater is limited. In this study we investigate the concentrations and properties of INPs in microlayer and bulk seawater samples collected in the Canadian Arctic during the summer of 2014. INPs were ubiquitous in the microlayer and bulk seawater with freezing temperatures in the immersion mode as high as −14 °C. A strong negative correlation (R = −0. 7, p = 0. 02) was observed between salinity and freezing temperatures (after correction for freezing depression by the salts). One possible explanation is that INPs were associated with melting sea ice. Heat and filtration treatments of the samples show that the INPs were likely heat-labile biological materials with sizes between 0.02 and 0.2 µm in diameter, consistent with previous measurements off the coast of North America and near Greenland in the Arctic. The concentrations of INPs in the microlayer and bulk seawater were consistent with previous measurements at several other locations off the coast of North America. However, our average microlayer concentration was lower than previous observations made near Greenland in the Arctic. This difference could not be explained by chlorophyll a concentrations derived from satellite measurements. In addition, previous studies found significant INP enrichment in the microlayer, relative to bulk seawater, which we did not observe in this study. While further studies are needed to understand these differences, we confirm that there is a source of INP in the microlayer and bulk seawater in the Canadian Arctic that may be important for atmospheric INP concentrations.

The Canadian Beaufort Sea has been categorized as an oligotrophic system with the potential for enhanced production due to a nutrient‐rich intermediate layer of Pacific‐origin waters. Using under‐ice hydrographic data collected near the ice‐edge of a shallow Arctic bay, we documented an ice‐edge upwelling event that brought nutrient‐rich waters to the surface during June 2008. The event resulted in a 3‐week long phytoplankton bloom that produced an estimated 31 g C m−2 of new production. This value was approximately twice that of previous estimates for annual production in the region, demonstrating the importance of ice‐edge upwelling to the local marine ecosystem. Under‐ice primary production estimates of up to 0.31 g C m−2 d−1 showed that this production was not negligible, contributing up to 22% of the daily averaged production of the ice‐edge bloom. It is suggested that under‐ice blooms are a widespread yet under‐documented phenomenon in polar regions, which could increase in importance with the Arctic's thinning ice cover and subsequent increase in transmitted irradiance to the under‐ice environment.

Nitrous oxide (N2O) contributes ∼6% of the total radiative forcing from long‐lived greenhouse gases. While tropospheric concentrations have increased by 20% since the beginning of the industrial revolution, sources and sinks of N2O are still poorly quantified. In the Arctic, N2O atmospheric concentrations vary seasonally, due mainly to vertical mixing. The contributions of local natural sources to this cycle are still unknown. Here we report on N2O measurements conducted in the bottom 10 cm of the sea ice and in the underlying surface water (USW) from late March to early May 2008 in the southeastern Beaufort Sea and Amundsen Gulf. Bulk N2O concentrations in ice were low (∼6 nM) and were consistently undersaturated with respect to the USW (∼40% saturation) and the atmosphere (∼30% saturation). Loss of N2O via brine rejection during sea ice formation in fall and winter can explain these low N2O ice concentrations. An unknown fraction of this rejected N2O is likely ventilated to the atmosphere either directly from the ice or through leads during ice formation, while in spring and early summer, melting of the N2O‐depleted sea ice is expected to lower the partial pressure of N2O of newly open waters which could act as a sink for atmospheric N2O. These first measurements indicate that sea ice formation and melt has the potential to generate sea‐air or air‐sea fluxes of N2O, respectively. Key Points These are the first measurements of N2O in sea ice N2O was consistently undersaturated in ice with respect to the atmosphere N2O dynamics in freeze‐melt cycle may contribute to atmospheric N2O variations

In an experimental assessment of the potential impact of Arctic Ocean acidification on seasonal phytoplankton blooms and associated dimethyl sulfide (DMS) dynamics, we incubated water from Baffin Bay under conditions representing an acidified Arctic Ocean. Using two light regimes simulating under-ice or subsurface chlorophyll maxima (low light; low PAR and no UVB) and ice-free (high light; high PAR + UVA + UVB) conditions, water collected at 38 m was exposed over 9 days to 6 levels of decreasing pH from 8.1 to 7.2. A phytoplankton bloom dominated by the centric diatoms Chaetoceros spp. reaching up to 7.5 µg chlorophyll a L−1 took place in all experimental bags. Total dimethylsulfoniopropionate (DMSPT) and DMS concentrations reached 155 and 19 nmol L−1, respectively. The sharp increase in DMSPT and DMS concentrations coincided with the exhaustion of NO3− in most microcosms, suggesting that nutrient stress stimulated DMS(P) synthesis by the diatom community. Under both light regimes, chlorophyll a and DMS concentrations decreased linearly with increasing proton concentration at all pH levels tested. Concentrations of DMSPT also decreased but only under high light and over a smaller pH range (from 8.1 to 7.6). In contrast to nano-phytoplankton (2–20 µm), pico-phytoplankton ( ≤  2 µm) was stimulated by the decreasing pH. We furthermore observed no significant difference between the two light regimes tested in term of chlorophyll a, phytoplankton abundance and taxonomy, and DMSP and DMS net concentrations. These results show that ocean acidification could significantly decrease the algal biomass and inhibit DMS production during the seasonal phytoplankton bloom in the Arctic, with possible consequences for the regional climate.

The decline of sea-ice thickness, area, and volume due to the transition from multi-year to first-year sea ice has improved the under-ice light environment for pelagic Arctic ecosystems. One unexpected and direct consequence of this transition, the proliferation of under-ice phytoplankton blooms (UIBs), challenges the paradigm that waters beneath the ice pack harbor little planktonic life. Little is known about the diversity and spatial distribution of UIBs in the Arctic Ocean, or the environmental drivers behind their timing, magnitude, and taxonomic composition. Here, we compiled a unique and comprehensive dataset from seven major research projects in the Arctic Ocean (11 expeditions, covering the spring sea-ice-covered period to summer ice-free conditions) to identify the environmental drivers responsible for initiating and shaping the magnitude and assemblage structure of UIBs. The temporal dynamics behind UIB formation are related to the ways that snow and sea-ice conditions impact the under-ice light field. In particular, the onset of snowmelt significantly increased under-ice light availability (>0.1–0.2 mol photons m–2 d–1), marking the concomitant termination of the sea-ice algal bloom and initiation of UIBs. At the pan-Arctic scale, bloom magnitude (expressed as maximum chlorophyll a concentration) was predicted best by winter water Si(OH)4 and PO43– concentrations, as well as Si(OH)4:NO3– and PO43–:NO3– drawdown ratios, but not NO3– concentration. Two main phytoplankton assemblages dominated UIBs (diatoms or Phaeocystis), driven primarily by the winter nitrate:silicate (NO3–:Si(OH)4) ratio and the under-ice light climate. Phaeocystis co-dominated in low Si(OH)4 (i.e., NO3:Si(OH)4 molar ratios >1) waters, while diatoms contributed the bulk of UIB biomass when Si(OH)4 was high (i.e., NO3:Si(OH)4 molar ratios <1). The implications of such differences in UIB composition could have important ramifications for Arctic biogeochemical cycles, and ultimately impact carbon flow to higher trophic levels and the deep ocean.

We measured the isotopic composition and accumulation of particulate organic matter (POM) and the uptake of carbon (C) and nitrogen (N) in an early bloom of the most productive recurring polynya of the Arctic Ocean. The estimated compensation irradiance at the onset of the bloom was similar to the average for the North Atlantic Ocean, implying that shallow mixing was of critical importance for the bloom's early initiation. Planktonic POM had a much lower δ13C than ice POM, suggesting that ice-algae contributed little to the pelagic biomass. The overall isotopic fractionation of pelagic N during bloom development was consistent with in situ diatom growth under saturating irradiance and limiting NO3 -. Soon after the ice cleared, rapid physiological changes induced an order of magnitude increase in the C and NO3 -uptake capacity of diatoms, leading to very high f ratios (NO3 -uptake : total N uptake). Most of the NO3 -taken up appeared in the POM, so that little net release of reduced N occurred during the period of active growth. Given the tight coupling between photosynthesis and NO3 -uptake under N limitation, the magnitude of primary production in the Arctic Ocean is expected to respond to changes in N supply.

Phytoplankton light absorption spectra (aϕ(λ)) were measured in the Canadian Arctic (i.e., the Amundsen Gulf, Canadian Arctic Archipelago, northern Baffin Bay and the Hudson Bay system) to improve algorithms used in remote‐sensing models of primary production. The absorption by algae, dominated by picophytoplankton (<5 μm), was not the major light absorption factor in the four provinces; the colored dissolved organic matter (CDOM) contributed up to 70% of total light absorption. During the fall, the low total chlorophyll a‐specific aϕ*(443) (aϕ(443)/TChl a) coefficients of the Canadian High Arctic were associated with photoacclimation processes (i.e., the package effect) occurring in light‐limited environments. Low light availability and high proportion of CDOM (absorbing strongly the ultraviolet) seem to allow the growth of phytoplankton with accessory pigments absorbing light at longer wavelengths. The ratio of photoprotective and photosynthetic carotenoids (PPC:PSC) was inversely proportional with the salinity and the cell size, and mostly decreases throughout the Canadian High Arctic during fall. In return, the highest TChla‐specific phytoplankton light absorption coefficients at the blue peak (aϕ*(443)) were observed in the Hudson Bay system from September to October (i.e., fall) as well as in the Amundsen Gulf from May to July (i.e., spring/summer). These results will ultimately allow the accurate monitoring of phytoplankton biomass and productivity evolution that is likely to take place as a result of the fast‐changing Arctic environment. Key Points Characterize spatial and temporal variations of phytoplankton light absorption Determine the main sources of variability

In several parts of the world, mytilid mussels, Mytilus spp., are infected with pathogenic, single-celled, photosynthetic algae belonging to the genus Coccomyxa . The posterior shell edge of heavily infected mussels becomes considerably thickened with an extra shell material. Also, the external shell surface is usually eroded as a result of the microboring activity of endolithic cyanobacteria. We compared the number of bioeroded shells, the bioerosion degree, and the number of badly eroded shells, in uninfected and Coccomyxa -infected Mytilus spp. from the Lower St. Lawrence Estuary, Québec, Canada. The thickness of prismatic and nacreous layers was measured. The epibionts (pink calcareous algae, crustose brown algae, and barnacles) which encrusted surface of studied shells, were counted. Epibionts did not occur frequently and their possible relationship with the partners of a three-way symbiosis, Coccomyxa sp. – Mytilus spp. – endolithic cyanobacteria, has been neglected. We suggest that the mussel provides the alga Coccomyxa a protected space and metabolic carbon for photosynthesis. The alga stimulates shell thickening, and this protects mussel against ocean acidification and predators. The endolithic cyanobacteria remove black-colored periostracum providing the mussel and alga with an increased ability to survive during sunny days when exposed at low tide. The eroded shells become more translucent which encourages alga photosynthesis. However, shell degradation caused by endolithic cyanobacteria is a possible reason for the death of the Coccomyxa -infected mussels at the studied sites.