Seasonal changes of uptake of nitrogenous nutrients (NH , NO , NO , and urea) NO production) in the plankton fraction of a mangrove ecosystem on the west coast of India were investigated.Nitrate was the major fraction of assimilable N (72%), followed by NH (16%), NO (6%), and urea (6%). Changes ϩ Ϫ 4 2 of nutrient concentrations followed clear seasonal cycles and were mainly regulated by in situ biological processes. The plankton took up NO and NH in more or less equal proportions (39 and 44% respectively), followed byurea (11%) and NO (6%). Seasonal patterns of uptake were distinct, with a dominance of NO and NO uptakein the postmonsoon, followed by a dominance of NH and urea uptake in the premonsoon. The high and prolonged ϩ 4use of NO at the beginning of the productive season was due to a strong allochthonous supply of NO , dominanceof microplankton, and low NH regeneration rates. Heterotrophs may take up all four nutrients and could account ϩ 4for half of the annual total N uptake. Ammonium and NO regeneration rates were among the highest known from production rates were related to NH production rates in a rectangular-hyperbolic fit. Nitrogen balance analysesshowed that proximity to mangrove vegetation enhanced the flux rates, noninclusion of nitrification may lead to an overestimation of new production by 30%, and regeneration in the plankton fraction provided about 40% more N than was assimilated.Since the demonstration that 15 N is a sensitive tracer for measuring rates of nitrogen transformations in biological systems (Neess et al. 1962) and the postulation of the concepts of new and regenerated production (Dugdale and Goering 1967), there has been an increased interest in quantifying N fluxes in the marine environment. In the last three decades, the uptake of NO and NH , representing the ma-

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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 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 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 -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 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 > 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 during high ammonium uptake. Species succession, however, is probably related to changes in salinity.KEY WORDS: Nitrogen uptake · Size fractions · Phytoplankton · Mangrove · Tropical waters · West coast of India Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 41: [281][282][283][284][285][286][287][288][289][290][291] 2005 were temporally separated, with uptake of the former becoming dominant in the post-monsoon period (October to January), and of the latter in the pre-monsoon period (February to May). At that time, we also measured the uptake of 4 N compounds (nitrate, ammonium, nitrite and urea) in 2 size fractions (netplankton: 20 to 200 µm; and nanoplankton: 0.8 to 20 µm). In the present study, we report on these data and examine the relationships between phytoplankton size classes and the pattern of N nutrient utilisation. MATERIALS AND METHODSThe Achara mangrove (16°12' to 16°14' N, 73°25' to 73°30' E), where the present study was carried out, is the least disturbed mangrove ecosystem on the west coast of India (Fig. 1). The seasonal climatic pattern in the study area can be broken down into the monsoon (June to September -heavy rainfall), post-monsoon (October to January -dry and cool) and pre-monsoon (February to May -dry and hot) periods.Environmental parameters, the nutrients and rates of N uptake and the remineral...