From September 1994 to September 1995 a time-series station in the Ria d e 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 -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, -70% of total DOC in surface water, was carried by upwelled ENACW (6Oor>, 10%) of which was semi-refractory) and continental water (10%). Net production of semi-labile DOC occurred in the bottom layer (-10 pM C) The average DOC excess in surface waters coinparcd to bottom waters was 21 pM C ,with a C/N molar ratio of 12 The excess was a mixture of labile and semi-labile material wlth a recycling tlme > S d, which represented -23 and -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 -0.4 and -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 downwelllng season DOM was balanced by the external inputs and the DOM excess in surface waters was due to the freshwater contribut~on.
Coupling between residual estuarine circulation and nitrogen biogeochemistry in the Ria d e Arousa (NW Spain) was studied in 4 characteristic hydrographic regimes during the upwelling season. The characteristics of each regime are a s follows: (1) In upwelling conditions after a pronounced stratificat~on of the water column, vertical transport of nitrate to the photic layer occurs mainly by advection. Nitrate is instantaneously taken up by autotrophs. making the ria a n efficient nutrient-salt trap. High net community production (194 mg ! V n1rZ d.') occurs In the whole water column. Nitrogen partitioning favours the part~culate organic nitrogen pool in the upper layer and the dissolved organic n~trogcn pool in the lower layer, while !;j of the net community production settles to the bottom.( 2 ) When stratification is low pnor to upwelling, vertical mlxlng prevails, upwelled nutrients are retained only by hydrodynamic accumulation, and the efficiency of the ria as a nutrient-salt trap clearly diminishes. In this situation net uptake by phytoplankton IS delayed due to the low b~omass in newly upwelled waters, and to the lag time for adaptation to the nutrient and light conditions. (3) During prolonged periods of upwelling relaxation, high water column stability develops and vertical transport is limited. The low net community production in the upper layer (80 m g N m -? d-') is supported entirely by nutrients regenerated in the lower layer and at the sediment-water interface from sinking organic matter (only 3% of the particulate organic matter produced is exported to the shelf). (4) Finally, during strong autumn downwelling, nutrient-poor shelf surface waters enter the Inner n a and meet the ammonium-rich waters flow~ng out from the Ulla estuary. This advected ammonium and the ammonium diffused from the sediments 1s transported to the bottom layer over the shelf by reversed residual circulation. High sedimentation rates result from intense transport of particulate matter to the lower ldyer KEY WORDS: Nitrogen speclatlon . Rias Baixas (NW Spain) . Hydrodynamic-biogeochemical coupling r?itrnn~n X --te the p h~t i c Z C T~~, !he i r n p c~t 3 z t~ cf hydrcdynamic accumulation versus net uptake by the community of organisms and the short-time-scale coupling between input and net uptake of external nitrogen are O Inter-Research 1996R~s d l e of full arllcle not permitted Mar
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