Dissolved and particulate organic carbon and nitrogen in the Northwestern Mediterranean

Abstract

The distribution of dissolved organic carbon (DOC) and nitrogen (DON) and particulate organic carbon (POC) and nitrogen (PON) was studied on a transect perpendicular to the Catalan coast in the NW Mediterranean in June 1995. The transect covered a hydrographically diverse zone, including coastal waters and two frontal structures (the Catalan and the Balear fronts). The cruise was conducted during the stratiÞed period, characterized by inorganic nutrient depletion in the photic zone and a well established deep chlorophyll a maximum. DOC concentrations were measured using a high-temperature catalytic oxidation method, and DON was determined directly, with an update of the Kjeldahl method, after removal of inorganic nitrogen. The ranges of DOC and DON concentrations were 44Ð95 lM-C and 2.8Ð6.2 lM-N. The particulate organic matter ranged between 0.9 and 14.9 lM-C and from 0.1 to 1.7 lM-N. The DOC:DON molar ratio averaged 15.5$0.4, and the mean POC:PON ratio was 8.6$0.6. The distribution of dissolved organic matter (DOM) was inverse to that of the salinity. The highest concentrations of DOMwere found in coastal waters and in the stations a¤ected by the Catalan front, located at the continental shelf break. It was estimated that recalcitrant DOM constituted 67% of the DOM pool in the upper 50 m. The data suggest that accumulation of DOC due to the decoupling of production and consumption may occur in the NW Mediterranean during stratiÞcation and that the organic matter exported from the photic layer is dominated by C-rich material.