Intra-annual variability in nutrients in the Godavari estuary, India

Daily variations in nutrients were monitored for 15 months (September 2007–November 2008) in the Godavari estuary, Andhra Pradesh, India, at two fixed locations. River discharge has significant influence on nutrients loading to the estuary, which peaks during June–August (peak discharge period; monsoon) whereas exchanges at the sediment–water interface, groundwater and rainwater contribute significantly during other period. Despite significant amount of nutrients brought by discharge to the study region, phytoplankton biomass, in terms of chlorophyll-a (Chl a), did not increase significantly due to high suspended load and shallow photic depth. Nutrients showed downward gradient towards downstream of the estuary from upstream due to dilution by nutrient poor seawater and biological uptake. The N:P ratios were higher than Redfield ratio in both upstream and downstream of the estuary during no discharge period suggesting PO4 to be a limiting nutrient for phytoplankton production, at levels <0.10 μmol L−1. On the other hand, Si:N ratios were always more than unity during entire study period at both the stations indicating that Si(OH)4 is not a limiting nutrient. Our results suggest that suspended matter limits phytoplankton biomass during peak discharge period whereas PO4 during no discharge period.