Nutrients in mode waters of the northeast Atlantic

Abstract

Programme Océan Multidisciplinaire Méso Echelle (POMME) seasonal surveys in the northeast Atlantic (39°N-44.5°N; 16.5°W-20.3°W) in 2001 are used to investigate the subduction of nutrients in the subsurface mode waters. Isopycnal subsurface distributions are used to estimate inorganic nutrients at the time of late winter restratification. These nutrient concentrations were close to winter near-surface concentrations, indicating a moderate consumption of nitrate (0.2-0.3 μM kg-1) and dissolved inorganic carbon (2-3 μM kg-1) in the surface layer before the effective subduction. Spring survey nutrient concentrations on isopycnal surfaces are lower north of 41.7°N indicating younger waters than further south. The seasonal increase of subsurface nutrients from spring to late summer diminishes from the shallower isopycnals to the deeper ones of the mode waters. It is also larger north of 41.7°N than south of it with values as large as 2 μM kg-1 for nitrate and 10 μM kg-1 for inorganic carbon. This evolution is mostly attributed to remineralization processes, both from falling particles (at least 15%) and from preformed dissolved organic matter (at most 30%). Ratios of nutrient changes to oxygen changes are often larger than Redfield ratios for nitrate (N:apparent oxygen utilization (AOU) at least 1:7) and phosphate (P:AOU at least 1:150) consistent with favored remineralization of P and N over C, both for dissolved and particulate organic pools. Copyright 2009 by the American Geophysical Union.