Seasonal variations in uptake and in situ regeneration of nitrogen in mangrove waters

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