Shelf-sea gross and net production estimates from oxygen-to-argon ratios and triple oxygen isotopes.

Isabel Seguro1,2, Jan Kaiser1,3, Alina D Marca1, Suzanne Painting4, Jamie D Shutler5 and David J Suggett6, (1)University of East Anglia, Norwich, United Kingdom, (2)Centre for Ocean and Atmospheric Sciences, (3)University of East Anglia, Centre for Ocean and Atmospheric Sciences, Norwich, United Kingdom, (4)Centre for Environment, Fisheries and Aquaculture Science, Lowestoft laboratory, Lowestoft, (5)University of Exeter, Centre for Geography, Environment and Society, Penryn, United Kingdom, (6)University of Technology Sydney, Climate Change Cluster, Sydney, Australia
Although only covering around 10% of the oceans, shelf sea areas are highly productive regions representing up to 30% of total oceanic primary production. However, spatial and temporal variation of the primary production in shelf seas has not been completely understood. Subsequently, quantifying primary production is essential to understand the shelf sea carbon pump and the anthropogenic contribution. The marine oxygen cycle is strongly associated to the carbon cycle and in this study we are using O2/Ar ratios and the triple oxygen isotopic method of the sea water dissolved gases for quantifying primary production.

Dissolved oxygen (O2) concentrations and their variations over time can be used to estimate biological net community production (NCP). However, physical process such us variations in temperature and pressure, mixing and bubble injection also influence dissolved O2 in seawater. To correct for these processes, I measured O2/Ar ratios using a shipboard membrane inlet mass spectrometer (MIMS) on board of RRS Discovery during four SSB cruises in 2014-2015 (summer-winter-spring-summer) in the Celtic Sea.

The data, together with wind speed-based gas exchange parameterisations, give biological oxygen fluxes, which, at steady-state and disregarding advection, eddy diffusion and entrainment, equal NCP.In order to calculate gross production (GP), I took discrete samples of triple oxygen isotopes.

The resulting data show variations in shelf-sea net and gross biological production with unprecedented temporal and spatial resolution. We estimate annual exchanges of carbon between the shelf sea, the atmosphere and the open ocean, as well as production rates.