Seasonal variations and decadal trends in concentrations of dissolved inorganic nutrients in Pichavaram mangrove waters, southeast India

A survey was conducted from July 2003 to June 2004 to measure changes in dissolved inorganic nutrient concentrations in relation to salinity in a river-dominated mangrove lagoon (the Pichavaram mangroves, southeast India). Monthly measurements of surface water temperature, salinity, dissolved oxygen, and dissolved inorganic nutrients (NH₄ ⁺, NO₃ ⁻ + NO₂ ⁻, PO₄ ³⁻, and Si(OH)₄ ⁺) were made at several sites at daytime low tides. With few exceptions, the overall pattern was for higher concentrations of nutrients in the monsoon and post-monsoonal periods. Conversely, during summer and pre-monsoon months, ocean water inputs were greater relative to river flow, leading to lower dissolved nutrient concentrations. Most site differences in dissolved nutrient concentrations were related to differences in salinity. Stoichiometric ratios deviated greatly from the Redfield ratio, reflecting nutrient behavior that was often non-conservative due to long residence times and high rates of productivity and organic matter decomposition. Comparison to historical data indicates a significant increase in dissolved inorganic nutrient concentrations since the 1980s, reflecting increased land use, discharges from agriculture, and the growth of human settlements.