Biogeochemical data from Marine Snow Catchers deployed in the mesopelagic zone of the Atlantic sector of the Southern Ocean (South Georgia) during the decline of a spring bloom November - December 2017 (COMICS 1).

The DY086 cruise was the first of two cruises focused on the functioning of the mesopelagic zone, the region between approximately 100 - 1000 m depth. The general aim of the cruise was to test the hypothesis that variability in surface community structure drives variability in the flux of material through the mesopelagic. In particular, this cruise, and this dataset, focused on evaluating the role of large diatom blooms, which are hypothesised to have very shallow remineralisation length scales. This dataset sampled two regions of contrasting productivity in the Atlantic sector of the Southern Ocean, the less productive site of P2 south of the North Scotia Ridge and the naturally iron fertilised, highly productive site P3 north of the North Scotia Ridge. Further sampling of the P2 site was abandoned due to the inability to find a suitable deployment region for the PELAGRA sediment traps in the presence of several large icebergs, resulting in a less comprehensive dataset at this site compared to P3. At P3, we arrived in the middle of a diatom bloom, allowing us to follow in detail the decline of the phytoplankton bloom over the coming weeks. Each site visit took ~7 days and we conducted 3 full cycles (P3A, P3B and P3C) during 15 Nov – 22 Nov, 29 Nov – 5 Dec, and 9 Dec – 15 Dec 2017. A fleet of Marine Snow Catchers (MSCs) were used to collect water samples in order to estimate the concentrations of different sinking particle pools throughout the mesopelagic zone. This dataset provides concentrations of chlorophyll a, biogenic silica, particulate inorganic carbon, particulate organic carbon and stable carbon isotopes in four factions from the marine snow catcher: tzero, top, bottom and tray.