ALBERT

Description: We present the first year-round estimates of benthic primary production at four contrasting shallow (3–22 m depth) benthic habitats in a southwest Greenland fjord. In situ measurements were performed using the noninvasive aquatic eddy-correlation (EC) oxygen (O2) flux method. A series of high-quality multiple-day EC data sets document the presence of a year-round productive benthic phototrophic community. The shallow-water sites were on average autotrophic during the spring and summer months, up to 43.6 mmol O2 m22 d21, and heterotrophic or close to metabolic balance during the autumn and winter. Substantial benthic gross primary production (GPP) was measured year-round. The highest GPP rates were measured during the spring, up to 5.7 mmol O2 m22 h21 (136.8 mmol O2 m22 d21), and even at low light levels (, 80 mmol quanta m22 s21) during late autumn and winter we measured rates of up to 1.8 mmol O2 m22 h21 (43.2 mmol O2 m22 d21) during peak irradiance. The benthic phototrophic communities responded seasonally to ambient light levels and exhibited year-round high photosynthetic efficiency. In situ downwelling irradiances as low as , 2 mmol quanta m22 s21 induced an autotrophic response and light saturation indices (Ik) were as low as 11 mmol quanta m22 s21 in the winter. On an annual timescale, the average areal rate of benthic GPP was 11.5 mol O2 m22 yr21, which is , 1.4 times higher than the integrated gross pelagic primary production of the , 30–50 m deep photic zone of the fjord. These results document the importance of benthic photosynthesis on an ecosystem level and indicate that the benthic phototrophic compartment should be accounted for when assessing carbon and nutrient budgets as well as responses of coastal Arctic ecosystems to climate change.

Description: The community respiration of 2 tidally dominated cold-water coral (CWC) sites was estimated using the non-invasive eddy correlation (EC) technique. The first site, Mingulay Reef Complex, was a rock ridge located in the Sea of Hebrides off Scotland at a depth of 128 m and the second site, Stjernsund, was a channel-like sound in Northern Norway at a depth of 220 m. Both sites were characterized by the presence of live mounds of the reef framework-forming scleractinian Lophelia pertusa and reef-associated fauna such as sponges, crustaceans and other corals. The measured O2 uptake at the 2 sites varied between 5 and 46 mmol m–2 d–1, mainly depending on the ambient flow characteristics. The average uptake rate estimated from the ~24 h long deployments amounted to 27.8 ± 2.3 mmol m–2 d–1 at Mingulay and 24.8 ± 2.6 mmol m–2 d–1 at Stjernsund (mean ± SE). These rates are 4 to 5 times higher than the global mean for soft sediment communities at comparable depths. The measurements document the importance of CWC communities for local and regional carbon cycling and demonstrate that the EC technique is a valuable tool for assessing rates of benthic O2 uptake in such complex and dynamic settings.

Description: Sea-ice ecosystems are among the most extensive of Earth’s habitats; yet its autotrophic and heterotrophic activities remain poorly constrained. We employed the in situ aquatic eddy-covariance (AEC) O2 flux method and laboratory incubation techniques (H14CO3−, [3H] thymidine and [3H] leucine) to assess productivity in Arctic sea-ice using different methods, in conditions ranging from land-fast ice during winter, to pack ice within the central Arctic Ocean during summer. Laboratory tracer measurements resolved rates of bacterial C demand of 0.003–0.166 mmol C m−2 day−1 and primary productivity rates of 0.008–0.125 mmol C m−2 day−1 for the different ice floes. Pack ice in the central Arctic Ocean was overall net autotrophic (0.002–0.063 mmol C m−2 day−1), whereas winter land-fast ice was net heterotrophic (− 0.155 mmol C m−2 day−1). AEC measurements resolved an uptake of O2 by the bottom-ice environment, from ~ − 2 mmol O2 m−2 day−1 under winter land-fast ice to~ − 6 mmol O2 m−2 day−1 under summer pack ice. Flux of O2-deplete meltwater and changes in water flow velocity masked potential biological-mediated activity. AEC estimates of primary productivity were only possible at one study location. Here, productivity rates of 1.3 ± 0.9 mmol O2 m−2 day−1, much larger than concurrent laboratory tracer estimates (0.03 mmol C m−2 day−1), indicate that ice algal production and its importance within the marine Arctic could be underestimated using traditional approaches. Given careful flux interpretation and with further development, the AEC technique represents a promising new tool for assessing oxygen dynamics and sea-ice productivity in ice-covered regions.