Net ecosystem production of dissolved organic carbon in a coastal upwelling system: the Ria de Vigo, Iberian margin of the North Atlantic

Net ecosystem production (NEP) rates of dissolved organic carbon (DOC) are estimated in a coastal upwelling system. The study site is a large coastal inlet (2.76 km(3)) in the northern boundary (42-43 degreesN) of the eastern North Atlantic upwelling system. The two-dimensional circulation pattern i n the system is governed by an offshore Ekman transport quite variable in m agnitude and direction. A mass balance of the short-timescale (2-4 d) chang es in measured DOC profiles is performed to obtain the NEP rates. Microbial oxidation of imported labile DOC (8% of total DOG, recycling time tau < 5 d) at a maximum net rate of -37 mmol C m(-2) d(-1) occurred during a downwe lling episode in the middle of the highly productive spring period. On the contrary, extensive export of labile DOC (<15% of total DOG, tau < 7 days) produced at net rates >42 mmol C m(-2) d(-1) took place during an upwelling episode in July, the middle of the upwelling season. This rate represents similar to 20% of the net primary production, demonstrating in the field th e relative importance of horizontal offshore transport of labile DOC to the export of new production in upwelling systems. An autumn wind relaxation p eriod results in dramatic changes in DOC standing stocks (+/-9 mu mol C L-1 ) caused by a conspicuous time segregation between sustained net phytoplank ton production of labile DOC (+15 mmol C m(-2) d(-1), 11 d) and subsequent rapid bacterial degradation (-63 mmol C m(-2) d(-1), 3 d). Net horizontal e xport during this period was prevented by reduced offshore Ekman transport values, indicating that net DOC production is not always synonymous with ne t export. Finally, during the winter period, the large wind-driven net DOC horizontal exchange rates affected mainly the DOC standing stocks of no bio reactive materials in the system, whereas bacterial oxidation rates during this period reduced to less than -0.14 mmol C m(-2) d(-1).