Stoichiometry of the degradation of dissolved and particulate biogenic organic matter in the NW Iberian upwelling

The average composition of the dissolved and particulate products of early degradation of marine phytoplankton has been established for the first time in a coastal upwelling system using a mixing analysis along isopycnal surfaces combined with a stoichiometric model. About 17–18% of the mineralized organic matter is derived from the decomposition of organic particulates, and 16–35% is from the dissolved organic matter. The remaining 50–70% is derived probably from large fast sinking particles. On average, the mineralized material on large particles has the closest composition to the Redfield formula. The ratio of dissolved saccharides to dissolved organic matter respiration is >40% higher than expected from a material of Redfield composition. Finally, the ratio of lipid to particulate organic matter respiration is >80% larger than expected from a material of Redfield composition. Regarding the decomposition of hard structures, biogenic silica dissolves predominantly in the inner shelf, where organic carbon oxidation is more intense, and diatom deposition occurs preferentially

Formation of the Alboran oxygen minimum zone

8 páginas, 6 figuras, 1 tabla.The enhanced oxygen minimum in the western Alboran Sea is the result of a chain of processes starting with nutrient injection into the inflowing Atlantic water at the Strait of Gibraltar. These nutrients originate in the outflowing Levantine Intermediate Water, outflowing Mediterranean deep water, and inflowing North Atlantic Central Water (from 200 m). They are injected into the inflowing Atlantic surface water by strong mixing at the eastern end of the Strait. They move with Atlantic surface waters along the Spanish coast, mix with nutrients upwelling in the northwestern Alboran Sea and stimulate phytoplankton productivity. The organic matter produced by this mechanism is transported both with the anticyclonically flowing waters of the AIboran gyre and with the waters that converge at the center of the gyre. Sedimentation in this convergence zone helps to deliver this organic matter to the Levantine Intermediate Water where bacteria metabolize it to CO2 at the expense of the existing oxygen. This mechanism develops the most intense oxygen minimum zone in the Mediterranean Sea.This work was supported by ONR contract N00014-85-C-0230, NSF grants OCE 8316610 and INT 8212505 (to TYP), CNRS GRECO 24 (to HJM), and a fellowship from the U.S.-Spain Joint Committee for Scientific Cooperation (to RM). This is Bigelow Laboratory for Ocean Science Contribution No. 86034.Peer reviewe