Dissolved organic matter in a temperate embayment affected by coastal upwelling

From September 1994 to September 1995 a time-series station in the Ria de Vigo (NW Spain) was monitored fortnightly. Dissolved organic carbon (DOC) was analysed by high temperature catalytic oxidation. Dissolved organic nitrogen (DON) was determined by the Kjeldahl method, after removal of inorganic nitrogen from the sample. The time courses of DOC and DON changes were parallel. The average C/N molar ratio of dissolved organic matter (DOM) was similar to 15. DOM was strongly influenced by physical and biological processes. During the upwelling season, the entry of DOM-poor Eastern North Atlantic Central Water (ENACW) controlled DOM levels in subsurface waters. Biologically produced DOM excess in surface waters was uncoupled with chlorophyll a on a daily time-scale. A tentative partitioning of DOM during the upwelling season has been inferred from mixing of oceanic and freshwater endmembers. The refractory pool, similar to 70% of total DOC in surface water, was carried by upwelled ENACW (60%, 10% of which was semi-refractory) and continental water (10%). Net production of semi-labile DOC occurred in the bottom layer (similar to 10 mu M C). The average DOC excess in surface waters compared to bottom waters was 21 mu M C, with a C/N molar ratio of 12. The excess was a mixture of labile and semi-labile material with a recycling time >5 d, which represented similar to 23 and similar to 13 % of the net primary production for C and N respectively. The average DOM excess/[POM (particulate organic matter) + DOM excess] ratio in surface waters was similar to 0.4 and similar to 0.3 for C and N respectively, indicating that POM was the most important pool of organic matter net produced in the inner ria. During the downwelling season DOM was balanced by the external inputs and the DOM excess in surface waters was due to the freshwater contribution.