Nitrogen cycling in an estuarine upwelling system, the Ria de Arousa (NW Spain). II. Spatial differences in the short-time-scale evolution of fluxes and net budgets

Abstract

Coupling between residual estuarine circulation and nitrogen biogeochemistry in the Ria de Arousa (NW Spain) was studied in 4 characteristic hydrographic regimes during the upwelling season. The characteristics of each regime are as follows: (1) In upwelling conditions after a pronounced stratification 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 an efficient nutrient- salt trap. High net community production (194 mg N m-2 d-1) occurs in the whole water column. Nitrogen partitioning favours the particulate organic nitrogen pool in the upper layer and the dissolved organic nitrogen pool in the lower layer, while 1/3 of the net community production settles to the bottom. (2) When stratification is low prior to upwelling, vertical mixing 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 biomass 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 mg N m-2 d-1) 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 ria and meet the ammonium-rich waters flowing out from the Ulla estuary. This advected ammonium and the ammonium diffused from the sediments is 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 layer.