Seasonal changes of uptake of nitrogenous nutrients (NH sub(4) sup(+), NO sub(3) sup(-), NO sub(2) sup(-), and urea) and regeneration (NH sub(4) sup(+) and NO sub(2) sup(-) 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 sub(4) sup(+) (16%), NO sub(2) sup(-) (6%), and urea (6%). Changes of nutrient concentrations followed clear seasonal cycles and were mainly regulated by in situ biological processes. The plankton took up NO sub(3) sup(-) and NH sub(4) sup(+) in more or less equal proportions (39 and 44% respectively), followed by urea (11%) and NO sub(2) sup(-) (6%). Seasonal patterns of uptake were distinct, with a dominance of NO sub(3) sup(-) and NO sub(2) sup(-) uptake in the postmonsoon followed by a dominance of NH sub(4) sup(+) and urea uptake in the premonsoon. The high and prolonged use of NO sub(3) sup(-) at the beginning of the productive season was due to a strong allochthonous supply of NO sub(3) sup(-), dominance of microplankton, and low NH sub(4) sup(+) regeneration rates. Heterotrophs may take up all four nutrients and could account for half of the annual total N uptake. Ammonium and NO sub(2) sup(-) regeneration rates were among the highest known from nearshore waters and showed clear seasonal patterns. Production and use of NH sub(4) sup(+) were closely coupled. Nitrite production rates were related to NH sub(4) sup(+) production rates in a rectangular-hyperbolic fit. Nitrogen balance analyses 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.