MESOSCALE VARIABILITY IN NITROGEN UPTAKE RATES AND THE F-RATIO DURINGA COASTAL PHYTOPLANKTON BLOOM

The proportion of the biological nitrogen demand met by nitrate (f-rat io) is a strong candidate for a biogeochemical parameter to relate pri mary production to the oceanic organic carbon flux, the latter being a potential global carbon sink. There is a controversy over whether f c an be related to spatial and temporal variations in NO3- concentration for the purposes of estimating carbon fluxes. I report measurements o f NO3- and NH4+ uptake rates and of the f-ratio taken during a coastal phytoplankton bloom and address the problem of mesoscale f-ratio vari ability. The results reveal two distinct behaviors of the f-ratio depe nding on light level: at the 50% level, f is nonlinearly related to am bient NO3- concentration; at the 10 and 1% light levels, f is not rela ted to NO3- concentration but is instead related to variations in the stratification in the NO3- profile. There seems to be a ''two-track'' biological system in the euphotic zone: a shallow food web that quickl y incorporates new NO3- supplies and a deep food web that responds mor e slowly to a varying supply. New production tends to be higher near t he surface than deeper, which is opposite the vertical structure gener ally proposed for NO3--limited systems. The implication is that NO3--b ased algorithms to predict f may not work over the full three-dimensio nal NO3- field.