Elucidating the Springtime North Atlantic Phytoplankton Bloom and the Biological Pump from a Ship of Opportunity and Satellite Data

Abstract:

The North Atlantic accounts for 25% of the world ocean’s anthropogenic carbon uptake (Sabine et al. 2004). Its propensity as a carbon sink is largely mediated by physical dynamics of the solubility pump in the subarctic North Atlantic (Sarmiento et al. 1997). The biological pump also acts as an important pathway of carbon to the deep ocean; springtime phytoplankton blooms rely on regimes of deep winter convection, spring and summer stratification, and eddy features to flux ~11Gt C yr^-1 of organic carbon via sinking into the deep ocean (Sanders et al 2014; Mahadevan et al 2012; Behrenfeld 2010). To quantify the biological pump in the subarctic North Atlantic, in situ data of chlorophyll fluorescence, nitrate concentration, O₂ saturation and temperature were collected from an underway sensor system during a two-leg, four week ship transect from April to May, 2014. This was compared with satellite observations of ocean color, sea surface temperature (SST), and particulate organic carbon to assess the fidelity of the underway measurements to remotely sensed measurements and to better understand the dynamics of springtime phytoplankton blooms in relation to carbon flux. Underway data was found to agree well with remotely sensed data (3 and 8-day composites), although satellite observations were sparse and had coarser temporal resolution. The in situ data better resolved features of the bloom and physical oceanographic features. Using both data types, we were able to estimate net community productivity and carbon export. SST was negatively correlated with high chlorophyll values and high O₂ saturation, while chlorophyll and O₂ were positively correlated in both the satellite and ship datasets. Nitrate was difficult to quantify as the integrity of the sensor was undermined by biofouling accumulation during the cruise. In situ data from shipboard sensors are at least as competent as satellite measurements of chlorophyll and temperature in the subarctic North Atlantic. This methodology provides new insights into the viability of long term biogeochemical observations from ships of opportunity. It begins to elucidate the mechanisms of springtime phytoplankton blooms and their role in sequestering then fluxing organic carbon into the deep ocean.