Constraining Net Community Production and Export of Carbon From Daily In Situ Sensor Measurements of Carbon, Nitrate and Oxygen in the Central Labrador Sea

The strength of the biological pump, or export production, is directly related to net community production (NCP) and controls the efficiency of the ocean to sequester and store carbon. Efficiency of the biological carbon pump or a transport efficiency can be defined by e.g. T100 (Buesseller and Boyd, 2009) with large numbers indicating fast transfer of organic matter to below the twilight zone and low numbers indicating rapid remineralization within the euphotic and twilight zones.

Sparse measurements of NCP on the annual scales often come from fixed location sediment trap fluxes or time-series of carbon, nutrients and oxygen from moored sensors or BGC ARGOs. Moored and profiling biogeochemical sensors can provide a wealth of high resolution data throughout the euphotic and twilight zones allowing investigation of ocean productivity and its fate.

NCP in the upper 55m and remineralization in the twilight some (55-150m) in the central Labrador Sea were calculated from daily high-resolution profiles of carbon, oxygen and nitrate as well as bio-optical parameters in the uppermost 150m of the water column. C-,O- and N-inventories above the compensation depth (55m) were corrected for lateral fluxes, entrainment and air-sea exchange. Carbon-based estimates of productivity amounted to NCP of ca 4 mol C m^-2 for the 2016 productive season were in good agreement with N-, and O-based estimates using Redfield stoichiometry. At the same time respiration in the twilight zone removed as much as 1.5 mol C m^-2 and bio-optical sensors suggested about 0.5 mol C m^-2 remained in POC form, and export production constituted 50% of NCP. Calculations were shown sensitive to the contribution of lateral fluxes for carbon and nitrate (ca 15-20% of inventory change) and the choice of air-sea exchange model for oxygen.

Evidence from bio-optical sensors and C-, O-, N- budgets suggest that production in the Labrador Sea may start as early as March before shoaling of mixed layer. Phytoplankton is formed in euphotic zone, but is being transported down to the deeper layers while convective mixing remains in effect. We will discuss how physical transport of POC through vertical mixing increases transport efficiency of carbon and how high-resolution profiles of biogeochemical properties are essential in constraining and describing this phenomenon.