ALBERT

Description: Surface current drifters were deployed east of Brava island (Cape Verde archipelago, Atlantic ocean) from the RV Meteor during the cruise M160, which took place between 22. November and 20. December 2019. The drifters were deployed in 3 clusters with 3 drifters per cluster. The utilized drifters are the MD03i from Albatros Marine Technologies, Spain. They have a cylinder shape with a 10 cm diameter and 32 cm length. About 8 cm protrude from the water surface and a drogue of both 50 cm length and diameter is attached to each drifter 50 cm below the sea surface so that difter represent currents in the upper 1 m surface layer. The drifter obtains the GPS position and transmits it via the satellite communication system Iridium to the vessel. The overall ratio of drag area inside to drag area outside the water is 33.2.

Description: Redfield stoichiometry has proved a robust paradigm for the understanding of biological production and export in the ocean on a long-term and a large-scale basis. However, deviations of carbon and nitrogen uptake ratios from the Redfield ratio have been reported. A comprehensive data set including all carbon and nitrogen pools relevant to biological production in the surface ocean (DIC, DIN, DOC, DON, POC, PON) was used to calculate seasonal new production based on carbon and nitrogen uptake in summer along 20°W in the northeast Atlantic Ocean. The 20°W transect between 30 and 60°N covers different trophic states and seasonal stages of the productive surface layer, including early bloom, bloom, post-bloom and non-bloom situations. The spatial pattern has elements of a seasonal progression. We also calculated exported production, i.e., that part of seasonal new production not accumulated in particulate and dissolved pools, again separately for carbon and nitrogen. The pairs of estimates of 'seasonal new production' and 'exported production' allowed us to calculate the C : N ratios of these quantities. While suspended particulate matter in the mixed layer largely conforms to Redfield stoichiometry, marked deviations were observed in carbon and nitrogen uptake and export with progressing season or nutrient depletion. The spring system was characterized by nitrogen overconsumption and the oligotrophic summer system by a marked carbon overconsumption. The C : N ratios of seasonal new as well as exported production increase from early bloom values of 5-6 to values of 10-16 in the post-bloom/oligotrophic system. The summertime accumulation of nitrogen-poor dissolved organic matter can explain only part of this shift.