MEPS 276:3-17 (2004)  -  doi:10.3354/meps276003

Inferring nitrification rates with an inverse method in a coastal upwelling system, Ría de Vigo (NW Spain)

X. A. Álvarez-Salgado*, M. Gilcoto

Consejo Superior de Investigaciones Cientificas, Insituto de Investigacións Mariñas, Eduardo Cabello 6, 36208 Vigo, Spain

ABSTRACT: Net ecosystem ammonification and nitrification rates were estimated in the inner and outer segments of the coastal upwelling system of the Ría de Vigo during September 1990 and 1991. A multiparameter inverse method was used, combining short-time-scale changes (2 to 4 d) in the distributions of salinity, temperature, ammonium, nitrite, nitrate and dissolved oxygen with freshwater and heat budgets for the embayment. The robustness of the calculations has been assessed by random perturbation of the measured variables within the limits of their oceanographic errors. During September 1990, when a complete coastal upwelling/downwelling cycle of ~2 wk was sampled (average surface outgoing velocity in the middle ría, v = +0.3 km d-1), nitrogen regeneration was limited in the lower layer of the ría and the only relevant process was ammonification: average 0.1 mmol m-3 d-1, increasing from 0.0 mmol m-3 d-1 from 14 to 18 September to 0.3 mmol m-3 d-1 from 24 to 27 September. Concomitantly, nitrogen assimilation rates in the upper layer were low, average -0.2 and -0.1 mmol m-3 d-1 in the inner and outer segments, respectively. Conversely, during September 1991, when coastal upwelling was dominant (v = +1.8 km d-1), nitrogen regeneration rates in the lower layer were much larger: average ammonification was 0.2 mmol m-3 d-1 in the outer and 0.6 mmol m-3 d-1 in the inner ría. Nitrification was also remarkably intense: ~70% of the regenerated ammonium was oxidized to nitrite and ~90% of the regenerated nitrite was oxidized to nitrate in the inner segment, whereas 100% of the regenerated ammonium was converted to nitrate in the outer segment. These nitrogen regeneration rates were accompanied by intense nitrogen assimilation in the upper layer of the inner (average -1.3 mmol m-3 d-1) and outer (average -0.6 mmol m-3 d-1 ) segments, ~60% of which was based on nitrate and the remaining ~40% on ammonium assimilation. Consequently, regeneration processes in the lower layer provided ~50% of the nitrate and ~30% of the ammonium assimilated in the upper layer.

KEY WORDS: Ammonification · Nitrification · Upwelling · Downwelling · NW Spain

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