Net autotrophy and heterotrophy in the Pontevedra Ria upwelling system (NW Iberian margin)

Abstract

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 (PO₄^3–) 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 PO₄^3– 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 PO₄^3– 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 PO₄^3–. Nutrient limitation was variable throughout the sampling, switching from PO₄^3– to NO₃^– from winter to spring concurrent with a drop in riverine PO₄^3– 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.