Resumo

HERNANDEZ-AYON, J. Martín et al. Nutrient dynamics in the west arm of San Quintín Bay, Baja California, Mexico, during and after El Niño 1997/1998. Cienc. mar [online]. 2004, vol.30, n.1a, pp.119-132. ISSN 0185-3880.

Time series of 8 h (May, September and November 1997, and March 1998) and 24 h (June and September 1999, and February 2000) were carried out at two (1997-1998) or three sites (1999-2000) of the west arm (Falsa Bay, FB) of San Quintín Bay, in order to assess the nutrient dynamics during and after the 1997/1998 El Niño event. Chlorophyll a concentrations were also deter- mined during each time series. Non-conservative fluxes of dissolved inorganic phosphorus (ΔDIP), dissolved inorganic nitrogen (ADIN), nitrates (ΔNO₃^-) and ammonium (ΔNH₄⁺) were estimated using a nutrient budget model. High NH₄⁺ peaks (18-45 µM) were detected during El Niño 1997/1998, and the mean concentration values were higher than those of 19992000. Ammonium concentration was consistently less than 8 µM during 1999-2000. Significant correlations (r = 0.60 to 0.80) were found between tidal height and NO₃^- concentration in 1999-2000, as well as an inverse relationship between nitrates and salinity, thus reflecting the input of new DIN from the ocean. Chlorophyll a concentration was up to two times higher in 19992000 than during El Niño. There were no significant correlations between tidal height and nutrient concentration during 19971998, suggesting that: (a) the high NH₄⁺ concentrations found in 1997-1998 were due to in situ remineralization of the organic matter and, possibly, the excretion by cultured mollusc bivalves in FB, and (b) the increase of chlorophyll a and NO₃^- during 1999-2000 was mainly due to new inputs from the adjacent ocean. The positive ΔDIP values (0.1 to 0.57) and negative values of the net ecosystem metabolism (NEM = -1 to -60 mmol m^-2 d^-1) observed throughout 1997-2000 indicate that FB permanently exported DIP towards the ocean and behaved as a net heterotrophic system, with or without the presence of El Niño.

Palavras-chave : San Quintín Bay; upwelling; nutrients; budget; El Niño; net metabolism.

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