Stoichiometry of dissolved organic matter and the kinetics of its microbial degradation in a coastal upwelling system.

The degradation of dissolved organic carbon (DOC), nitrogen (DON) and phosphorus (DOP) collected in the coastal upwelling system of the Ria de Vigo (NW Iberian Peninsula) was assessed following the time-course of DOC, DON and DOP concentrations in laboratory incubations. Initial concentrations varied from 73 to 94 mu M for DOC, 4.5 to 7.2 mu M for DON and 0.12 to 0.32 mu M for DOP. The bioavailable fraction (BDOM) represented 17 +/- 6% (average +/- SE) of DOC, 38 +/- 6% of DON and 65 +/- 9% of DOP. BDOM was significantly correlated with temperature (R-2 > 0.3, p <0.05) and chlorophyll a (R-2 > 0.5, p <0.05), indicating that the differences in DOM bioavailability were associated with the seasonal variations in plankton biomass and activity. The C:N:P stoichiometry of BDOM, 111 ( 38):18 ( 6):l, was not significantly different from the Redfield ratio (106:16:1), pointing to a phytoplankton origin of BDOM. Accordingly, exponential rate constants of BDOM suggest that this pool is very labile. Despite the reduced flushing times of the Ria de Vigo, from 3 to 8 d, as much as 70 +/- 13 % of BDOC (average +/- SD), 81 +/- 8 % of BDON and 88 +/- 5 % of BDOP are mineralized within the embayment. The remaining C-rich BDOM is exported to the adjacent oligotrophic waters, suggesting that the offshore transport of labile DOM must be accounted for when considering the carbon balance of ocean surface waters adjacent to productive coastal areas.