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Seasonal changes of uptake of nitrogenous nutrients (NH , NO , NO , and urea) and regeneration (NH and 122 1 432 4 NO production) in the plankton fraction of a mangrove ecosystem on the west coast of India were investigated. 2 2 Nitrate was the major fraction of assimilable N (72%), followed by NH (16%), NO (6%), and urea (6%). Changes 12 42 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 by 21 34 urea (11%) and NO (6%). Seasonal patterns of uptake were distinct, with a dominance of NO and NO uptake 2 22 2 32 in the postmonsoon, followed by a dominance of NH and urea uptake in the premonsoon. The high and prolonged 1 4 use of NO at the beginning of the productive season was due to a strong allochthonous supply of NO , dominance 2 2 3 3 of microplankton, and low NH regeneration rates. Heterotrophs may take up all four nutrients and could account 1 4 for half of the annual total N uptake. Ammonium and NO regeneration rates were among the highest known from 2 2 nearshore waters and showed clear seasonal patterns. Production and use of NH were closely coupled. Nitrite 1 4 production rates were related to NH production rates in a rectangular-hyperbolic fit. Nitrogen balance analyses 1 4 showed 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. |
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