No 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.

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.

[1] Extensive spatial and temporal observations of sea ice algae remain limited due in part to current destructive and time intensive sampling techniques. In this paper we examine the influence of snow cover and ice algal biomass on the spectral dependence of photosynthetically available radiation transmitted through the snow-ice matrix using a data set collected in Resolute Passage, Canada, from 3 to 21 May 2003. The relationships between a normalized difference index (NDI) of transmitted irradiance with ice algal biomass and with snow cover provided a means to examine and compare observational and modeled data. In contrast to the dominant scattering properties of snow, absorption largely controls the spectral diffuse attenuation coefficient of algae. Our results show that snow has little effect on the distribution of transmitted spectral irradiance at wavelengths between 400 and 550 nm, whereas algae have a strong absorption peak near 440 nm that dominates changes in spectral transmission across this wavelength range. Up to 89% of the total variation in algae biomass was accounted for with a single NDI wavelength combination. Therefore the blue wavelength peak in algal spectral absorption lends particularly well to the remote estimation of algae biomass using transmitted irradiance. Deviations between observed and modeled data highlight the need for improvements to model inputs and therefore more detailed observations of processes controlling snow, ice, and algae in situ optical properties.Citation: Mundy, C. J., J. K. Ehn, D. G. Barber, and C. Michel (2007), Influence of snow cover and algae on the spectral dependence of transmitted irradiance through Arctic landfast first-year sea ice,

Analysis of the organic geochemical biomarker IP 25 in marine sediments is an established method for carrying out palaeo sea ice reconstructions for the Arctic. Such reconstructions cover timescales from decades back to the early Pleistocene, and are critical for understanding past climate conditions on Earth and for informing climate prediction models. Key attributes of IP 25 include its strict association with Arctic sea ice together with its ubiquity and stability in underlying marine sediments; however, the sources of IP 25 have remained undetermined. Here we report the identification of IP 25 in three (or four) relatively minor (o5%) sea ice diatoms isolated from mixed assemblages collected from the Canadian Arctic. In contrast, IP 25 was absent in the dominant taxa. Chemical and taxonomical investigations suggest that the IP 25 -containing taxa represent the majority of producers and are distributed pan-Arctic, thus establishing the widespread applicability of the IP 25 proxy for palaeo Arctic sea ice reconstruction.