A non-stationary state mass balance of the type advocated by the International Geosphere-Biosphere Programme/Land-Ocean Interactions in the Coastal Zone (IGBP/LOICZ) has been used to assess net ecosystem metabolism in the Pontevedra Ria (NW Spain), a large embayment subject to transient upwelling of Eastern North Atlantic Central Water (ENACW) under favourable meteorological conditions. Nitrate (NO3–) and phosphate (PO43–) data spanning 12 months were resolved at the oceanic boundary by treating the ria as a single box of constant volume. The nutrient budget was scaled up to an estimate of net community production (NCP) using the compositional relationship of local phytoplankton. The mean annual NCP predicted by the NO3– balance was 24.0 mgC m–2 h–1, and 35.8 mgC m–2 h–1 by the PO43– budget. Nutrient fluxes into the reservoir were strongly driven by the incoming oceanic flow throughout the year, which supplied 88% and 98% of the total NO3– and PO43– load, respectively. A high NCP (NO3–) of 57.2 mgC m–2 h–1 was calculated for the height of the growth season contemporaneous with ENACW intrusion, whereas a slightly higher value of 61.4 mgC m–2 h–1 corresponded to PO43–. Nutrient limitation was variable throughout the sampling, switching from PO43– to NO3– from winter to spring concurrent with a drop in riverine PO43– input. Scaling the inorganic nutrients to carbon units, the Pontevedra Ria may be considered as net autotrophic, although the NO3– budget suggests alternation between autotrophy and heterotrophy over an annual cycle.