Award: PLR-1142065

The goal of the TRACERS (TRacing Algal Carbon Export in the Ross Sea) expedition to the western Ross Sea in Antarctica was to measure algal carbon production, release as dissolved organic carbon (DOC) and export as particulate organic carbon (POC) to deep waters of the Ross Sea. In addition, our group also focused on measurements of the climatically-active biogenic trace gas, dimethylsulfide (DMS) and its non-volatile precursor dimethylsulfoniopropionate (DMSP). DMSP is produced by certain algae (e.g. diatoms and haptophytes) that thrive in the Ross Sea. These algae are prolific producers of DMSP and cause the Ross Sea to have one of the worldÆs highest oceanic concentrations of DMS and DMSP. Once DMS reaches the atmosphere, it is oxidized to sulfate particles that function as cloud condensation nuclei. Due to clouds reflection of sunlight, DMS can significantly impact the radiation balance reaching the Earth. These carbon and sulfur measurements in the Ross Sea have been made extensively during the austral spring and summer bloom periods (November to February), but very few data have been collected during the austral autumn season. We observed a 2-fold increase in the biological removal of CO2 above 200 m depth in Terra Nova Bay (TNB) between mid-February and mid-March, despite increased CO2 recharge from the atmosphere at that time. Primary production rates were measured using C14 techniques and averaged 682 + 150 mg C m-2 d-1. Such late season primary production is not well studied, but is likely fueled by the wind-driven convective resupply of iron and nitrate. In addition, we concluded that frazil ice formation is an important mechanism that can maintain diatom productivity in the sunlit surface waters during the autumn period. We regularly observed frazil ice in TNB with pronounced green-to-gold coloration, suggesting high algal biomass. This ice is initially set up in surface streaks that are aligned with the wind field (i.e.windrows), which we attribute to a Langmuir circulation pattern. Langmuir helical cells begin to form at wind speeds above 8 m s-1, and form consistently at wind speeds above 13 m s-1. With declining air temperatures and during low wind intervals, this frazil ice aggregated to form pancake ice overlying frazil ice layers 10-30 cm thick. We sampled the green frazil ice, associated pancake ice, and open water, both between windrows and beneath the pancake/frazil ice complex on two occasions. On average, POC was enriched by 6-fold in the frazil and pancake ice relative to open water samples with maximum enrichments on the order of 20-fold. Chlorophyll-a, a proxy for algal biomass, was similarly enriched by 8 to 10-fold in the ice. POC concentrations in a 40 cm thick layer of frazil and pancake ice were as high as 1 g C m-2. Daily primary production was not measured directly, but may well exceed 1 g C m-2 d-1. Interestingly, the DMSP concentrations measured in the golden pancakes of the frazil ice displayed some of the highest levels that have been recorded from the worldÆs oceans, reaching levels of > 100 micromoles per liter. These DMSP values are orders of magnitude higher than the values typically measured in sea ice diatom assemblages that have been sampled in the Ross Sea pack ice. Larger aggregations of new and algae-enriched frazil ice are visible in satellite images. A cursory review of past AQUA/MODIS imagery suggests that these features occur regularly in the Ross Sea during the autumn, particularly in the TNB region. Hence, the autumn season in the western Ross Sea and TNB in particular may be significantly more important to the regionsÆ carbon and sulfur cycles than previously recognized. Further studies to investigate this late season production are warranted. Last Modified: 08/26/2015 Submitted by: Giacomo R Ditullio