Nutrient Biogeochemistry and Net Ecosystem Metabolism in a Tropical Coastal Mangrove Ecosystem

Abstract

Dissolved nutrients and hydrological variables are employed to evaluate the nutrient biogeochemical processes and net ecosystem metabolism (NEM) of the Pichavaram estuarine–mangrove ecosystem (PEME) of India. Distribution of dissolved nutrients is deviated from the conservative mixing explaining substantial amount of nutrients are derived from allochthonous sources and influences the nutrient dynamics. Nutrient structure in mangrove is explained in terms of nutrient stoichiometric ratios (Si:N and N:P) which are largely deviated from the Redfield Ratio (Si:N = 1:1 and N:P = 16:1) indicating the nutrient balance is altered by eutrophication. Mass balance calculations depicts that high fluxes of DIN and DIP were transported from PEME to the Bay of Bengal in monsoon (896 mol d^-1 DIN and 765 mol d^-1 DIP) than in summer (589 mol d^-1 DIN and 287 mol d^-1 DIP) indicate that PEME supplies substantial amount of nutrients to the adjacent coastal waters. Furthermore, NEM is ranged from –8.24 mmol m^-2 d^-1 (summer) to –4.74 mmol m^-2 d^-1 (winter), with an average of –6.49 mmol m^-2 d^-1, highlighting heterotrophic character of PEME. We hypothesize that anthropogenic nutrients and marine nutrient inputs are transported, retained and remineralized within the ecosystem and may be responsible for the heterotrophic conditions.