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Measurements of the concentrations of nitrogenous nutrients and primary production were made at 10 stations along 8°N and 10°N in the tropical oceanic Lakshadweep waters. Inorganic nitrogen (NO3, NO2 and NH4) accounted for less than 10%, and
dissolved organic nitrogen (DON) for more than 90%, of the total dissolved N in the euphotic zone. Urea formed a nearly constant fraction of total dissolved N (17–18%) and of DON (19–20%). Addition of NO3, NH4 and urea as N source stimulated carbon fixation
at all stations and at all depths. This, together with the low ambient concentrations of inorganic nitrogen compounds, demonstrates that the phytoplankton in these waters are nitrogen limited. Nitrate flux into the euphotic zone, computed from the vertical eddy diffusion coefficient and NO3
gradient data, showed that the production associated with NO3 assimilation varied from 11 to 60% (average 37%) of the observed primary production. Zooplankton regenerated NH4 at an average rate of 0.59 μg-at N·(mg dry wt)−1·d−1.
Assimilation of this quantity of NH4 would account for 9–50% (average 23%) of the measured primary production. Stimulation of carbon fixation on addition of urea was, on an average, as much as with either NO3 or NH4 addition, demonstrating a potential
for urea uptake by phytoplankton in these waters. Zooplankton regenerated urea at an average rate of 0.32 μg-at N·(mg dry wt)−1·d−1, providing nitrogen for 5–27% (average 13%) of the primary production. Excluding urea, the rest
of the DON, though abundant, does not appear to be directly available for phytoplankton assimilation.
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