Nitrogen uptake by size-fractionated phytoplankton in mangrove waters
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Seasonal changes in the uptake of nitrogenous nutrients (nitrate, nitrite, ammonium and urea) in 2 size fractions (netplankton: 20 to 200 Mu m; and nanoplankton: 0.8 to 20 Mu m) were studied in relationship to the phytoplankton species composition in mangrove waters on the west coast of India. Seasonal changes in particulate organic nitrogen in the nano- and netplankton fractions were similar, whereas those of chlorophyll a varied, showing high values in nanoplankton in pre-monsoon and in netplankton in monsoon and early post-monsoon months. Nitrogen uptake as the sum of all 4 nutrients was similar between net- (150 nmol N l sup(-1) h sup(-1)) and nanoplankton (184 nmol N l sup(-1) h sup(-1)), but differed substantially according to the N compound. Netplankton were responsible for greater than 90% of the nitrate taken up, and nanoplankton for greater than 80% of the ammonium uptake. Netplankton also took up ammonium (31 nmol N l sup(-1) h sup(-1), 20% of the total N taken up), whereas nitrate uptake (3 nmol N l sup(-1) h sup(-1)) in the nanoplankton amounted to only about 2% of the total N uptake. The size-dependent differences in the utilisation of nitrate and ammonium appear to be further enhanced by a reduction in nitrate uptake through increased water temperatures and a greater repression of nitrate uptake in the nanoplankton than in the netplankton at ambient ammonium concentrations of greater than 0.5 mu mol N l sup(-1). The proportions of nitrite and urea uptake in the 2 size classes were similar to those of nitrate and ammonium, indicating size-dependent uptake of these 2 compounds as well. Pennate diatoms and flagellates were dominant during high nitrate uptake, and centric diatoms and blue green algae during high ammonium uptake. Species succession, however, is probably related to changes in salinity.
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