RRS Discovery Cruise 369, 09 Aug -15 Sep 2011. Influence of advection and sedimentation on linking microbial phosphorus, carbon and nitrogen cycling in the North Atlantic subtropical gyre (LINK)

Abstract

This physics-led biogeochemical cruise to the North Atlantic subtropical gyre was designed to field test a prevailing hypothesis that open ocean ecosystems are in steady state. We aimed to study microbial community composition, spatial distribution and functioning in the photic and twilight zones, and to put this information into context with physical and chemical characterisation of the sampling region, with additional information on carbon export from the euphotic zone.

Underway sampling followed by flow cytometry was used to assess mesoscale spatial variability of microorganisms. Coarse-scale (CTD) and fine-scale (PumpCast profiler) vertical distribution of microorganisms was determined, with dominant microbial prokaryotic and eukaryotic groups quantified by flow cytometry. Abundance of larger microplankton organisms was assessed using the size-fractionating net (Micronet) and FlowCam microscope. Groupspecific uptake of bicarbonate, phosphate and different nitrogen compounds was determined and group-specific production and grazing assessed using flow sorting. The ambient turnover rates of phosphate, organic phosphorus and labile dissolved organic matter, e.g. amino acids, was bioassayed. Microbial respiration in the twilight zone was studied using radioactive tracing
in a deep-water in situ incubator.

Physical and chemical parameters were measured during the cruise in order to put microbial community data into context. A ship mounted ADCP was used to collect information about physical, mesoscale spatial context; by combining with turbulence profiler data, SeaSoar profiler data, and data collected during intensive CTD sampling for dissolved inorganic nutrients, we aim to estimate mesoscale nutrient fluxes into the photic zone. PELAGRA neutral buoyancy sediment traps were used to estimate biogenic sedimentation. Samples were collected for particulate calcite, opal, and POM measurements, together with the isotopic composition of PON.

The cruise achieved its main goal of collecting data that will allow us to link the functioning of oligotrophic oceanic microbial communities with their fluidic environment, in particular with the propagation of eddies through strongly stratified photic and twilight waters. Close collaboration between physicists, biologists and chemists ensured cohesion in sampling efforts resulting in a rare collection of multidisciplinary scientific evidence. Further analyses of collected samples and data in conjunction with computer modelling have a strong synthesis
potential to qualitatively advance our knowledge of the pelagic ecosystem biogeochemistry in subtropical oceanic gyres – the most extensive regions on Earth (>40% of Earth’s surface) of global climatic importance.

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