Nutrient budgets for Muthupet lagoon, southeastern India

Budgets of nitrogen and phosphorus from the semi-enclosed Muthupet lagoon were constructed through monsoon observations and modelling. The lagoon is a shallow water body and hence surface water samples associated with the lagoon were collected and measured for hydrochemical properties, inorganic and organic nutrients (dissolved inorganic nitrogen, dissolved organic nitrogen, dissolved inorganic phosphorus and dissolved organic phorphorus). The average inorganic N and P concentrations of the lagoon were almost equal to that of their river concentrations, while organic phosphorus was lower by 30% and organic nitrogen was higher by 26%. Terrestrial inputs through run-off, mixing and residual fluxes were dominant forcing mechanisms in maintaining lagoon nutrient concentrations. Water exchange time of the lagoon was estimated at 1.4 days. However, the nutrient, especially DIP, DIN and DON, exchange time was higher by approximately 50% of water exchange time, whereas the same for DOP was half that of water exchange time. The nonconservative fluxes △DIP, △DOP, △DIN and △DON from the lagoon were 0.03, −0.06, 0.92 and 5.28 mmol m⁻² d⁻¹ respectively, inferring that DIP, DIN and DON were removed from the system when DOP was added to it.