RRS Discovery Cruise DY086, 12 November – 19 December 2017. Controls over Ocean Mesopelagic Carbon Storage (COMICS)

Abstract

This cruise was the first of two cruises focussed on the functioning of the mesopelagic
zone, the region between approximately 100 and 500 m depth. Both cruises are funded by the NERC large grant “Controls over Ocean Mesopelagic Carbon Storage” (COMICS). The general objective of this cruise was to test the hypothesis that variability in surface community structure drives variability in the flux of material through the mesopelagic. A particular focus was to evaluate the role of large diatom blooms which are hypothesised to have very shallow mineralisation length scales. The highly seasonal nature and rapid changes in the community structure of such events requires multiple observations across the bloom progression.

This first cruise, COMICS1, sampled the highly productive region downstream of South Georgia in the vicinity of British Antarctic Survey station P3 (52.40 S, 40.06 W). Our sampling strategy was to fully characterize the ecosystems in the epipelagic and upper mesopelagic zone during repeated visits of the same site. We had originally planned to sample a second station further South (P2) but abandoned this plan owing to the inability to find a suitable deployment region for the PELAGRA sediment traps and the presence of several large icebergs. Instead, we focussed our efforts on station P3, which was considered biologically more interesting.

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 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. In total, 345 events were completed in 32 days of science.

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