Autotrophic and heterotrophic activity in Arctic first-year sea ice: seasonal study from Malene Bight, SW Greenland

Abstract

We present a study of autotrophic and heterotrophic activities of Arctic sea ice (Malene Bight, SW Greenland) as measured by 2 different approaches: (1) standard incubation techniques (H 14 CO 3 -and [ 3 H]thymidine incubation) on sea ice cores brought to the laboratory and (2) cores incubated in situ in plastic bags with subsequent melting and measurements of changes in total O 2 concentrations. The standard incubations showed that the annual succession followed a distinctive pattern, with a low, almost balancing heterotrophic and autotrophic activity during February and March. This period was followed by an algal bloom in late March and April, leading to a net autotrophic community. During February and March, the oxygen level in the bag incubations remained constant, validating the low balanced heterotrophic and autotrophic activity. As the autotrophic activity exceeded the heterotrophic activity in late March and April, it resulted in a significant net oxygen accumulation in the bag incubations. Integrated over the entire season, the sea ice of Malene Bight was net autotrophic with an annual net carbon fixation of 220 mg C m -2 , reflecting the net result of a sea ice-related gross primary production of 350 mg C m -2 and concurrent bacterial carbon demand of 130 mg C m -2 . Converting the O 2 net exchange of the bag incubations into carbon turnover estimated an annual net carbon fixation of 1700 ± 760 mg C m -2 (mean ± SD), which was higher than the annual net carbon fixation quantified in the standard incubations.

Nutrient availability limits biological production in Arctic sea ice melt ponds

“…This corresponds with the findings of previous studies linking bacterial production with primary production from both pelagic (e.g. Hoppe et al 2002) and sea ice samples (Søgaard et al 2010). The bacterial production rates in the controls equals to 44-47% of the primary production, while it amounted to 20-25% of the primary production with the addition of both NO 3 − and PO 4 3− .…”

“…Here, the samples were acidified and fumed for 24 h to remove non-bound excess 14 CO 3 and 14 CO 2 prior to adding 10 mL scintillation cocktail (Ultima Gold, Perkin Elmer) to each sample (e.g. Søgaard et al 2010). The fixed amount of 14 C carbon of the respective samples was subsequently quantified with a scintillation counter (Liquid Scintillation Analyzer, Tri-Carb 2800TR, PerkinElmer).…”

“…Bacterial production (mmol C m −3 day −1 ) was calculated by converting the estimated moles of thymidine and leucine incorporated into cell biomass using the coefficients 2.09 × 10 18 and 6.40 × 10 16 cells mol −1 , respectively (e.g. Kirchman and Hoch 1988) and assuming a cell-specific carbon content of 5.7 × 10 −8 µg C cell −1 (Søgaard et al 2010). Values were corrected for the specific activity of the added tracers, the incubation time and the sample volume.…”

Glacial meltwater and primary production are drivers of strong CO<sub>2</sub> uptake in fjord and coastal waters adjacent to the Greenland Ice Sheet

“…The samples were subsequently filtered through 25 mm cellulose ester filters (pore size 0.2 µm, Advantec MSF). The equations from Søgaard et al (2010) were used to calculate bacterial production. For the calculation of the bacterial carbon demand from the bacterial production, a bacterial growth efficiency of 0.5 was used according to Rivkin and Legendre (2001).…”

Oxygen exchange and ice melt measured at the ice-water interface by eddy correlation

“…The 14 C tracer method and 3 H labeled thymidine approach are the most widely applied techniques to determine primary production and bacterial production in sea-ice, respectively. However, both methods are time consuming, making multiple samples and a wide spatial coverage difficult (McMinn and Ashworth, 1998;Søgaard et al, 2010). Both approaches also require melting the ice to evenly distribute the tracers, which grossly changes the natural conditions experienced by the microalgae and bacteria (e.g.…”