Stoichiometry of the net ecosystem metabolism in a coastal inlet affected by upwelling. The Ria de Arousa (NW Spain)

The net uptake of inorganic carbon and nitrogen, phosphate and silicate and the net production of dissolved oxygen and organic carbon, nitrogen and ph osphorus have been examined in the Ria de Arousa, a large coastal embayment in the NW Iberian upwelling system. Fluxes and net budgets were estimated with a non-stationary 2-D box model [Roson, G., Alvarez-Salgado, X.A., Pere z, F.F., 1997. A non-stationary box-model to determine residual flows in a partially mixed estuary, based on both thermoline properties. Application t o the Ria de Arousa (NW Spain). Estuarine, Coastal Shelf Sci., 44, 249-262] and the distributions of the different species acquired twice a week betwe en May and October 1989 [Roson, G., Perez, F.F., Alvarez-Salgado, X.A., Fig ueiras, F.G., 1995. Variation of both thermohaline and chemical properties in an estuarine upwelling ecosystem: Ria de Arousa: 1. Time Evolution. Estu arine, Coastal Shelf Sci., 41, 195-213]. High N/P and N/Si net uptake ratio s of 21 and 3.2 were observed during the upwelling season. The rapid recycl ing of phosphorus compared to nitrogen and the recurrent succession from pi oneer diatoms (Si/N similar to 1) to red-tide forming species (Si/N = 0) fo llowing the periodic upwelling pulses are the reasons behind the observed r atios. The molar ratios of dissolved oxygen production to inorganic carbon (- 1.48) and nitrogen uptake (- 10.2) during the upwelling season agree wit h the Redfield stoichiometry. On the contrary, net nutrient regeneration oc curred with N/P, N/Si and O-2/C ratios of 7.4, 1.0 and - 1.02 during an int ense autumn downwelling event. These low ratios are due to the release of a n excess of phosphate, silicate and CO2 from the sediments. Conversely, the production of inorganic nitrogen is associated to the consumption of disso lved oxygen following a Redfield ratio of - 10.0. Whereas the C/N ratio of the suspended organic matter produced during the upwelling season and consu med during the autumn downwelling event is 6.3-6.7, the N/P ratio changes f rom 11 during the upwelling season to 15 during the autumn downwelling. Abo ut 1/5 of the dissolved oxygen produced during the upwelling season and con sumed during the autumn downwelling is delivered to and came from the atmos phere, respectively. Despite the C/N/P/O-2 ratios differ from the Redfield values, the high correlation between nutrient salts consumption and dissolv ed oxygen production (r(2) = 0.74-0.86) allow to estimate an average net ec osystem production (NEP) from the individual elements. The 3-4 d time-scale variability of the average NEP depends on the 2-week periodicity of upwell ing pulses, the heat exchange across the sea surface and the stability of t he water column. As much as 70% of the total variability can be explained w ith a linear combination of these parameters. (C) 2000 Elsevier Science B.V . All rights reserved.