Nitrogen uptake by size-fractionated phytoplankton in mangrove waters

Abstract

Seasonal changes in the uptake of nitrogenous nutrients (nitrate, nitrite, ammonium and urea) in 2 size fractions (netplankton: 20 to 200 µm; and nanoplankton: 0.8 to 20 µm) 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 simi- lar, 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 -1 h -1 ) and nanoplankton (184 nmol N l -1 h -1 ), but differed substantially according to the N compound. Netplankton were responsible for >90% of the nitrate taken up, and nanoplankton for >80% of the ammonium uptake. Netplankton also took up ammonium (31 nmol N l -1 h -1 , 20% of the total N taken up), whereas nitrate uptake (3 nmol N l -1 h -1 ) in the nanoplankton amounted to only about 2% of the total N uptake. The size-dependent differ- ences 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 >0.5 µmol N l -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 dur- ing high ammonium uptake. Species succession, however, is probably related to changes in salinity.