Reduced river discharge intensifies phytoplankton bloom in Godavari estuary, India

Changes in river discharge alter material load and the consequent estuarine and coastal biogeochemical process. Evidence for biogeochemical response to variable rainfall over catchment area or changes in river discharges due to dam regulations is sparse. Inter-annual variability in monsoon rainfall and the consequent river discharges, either dam regulated or otherwise, in India are the best suited to test the influence of altered discharges on estuarine biogeochemistry. Our experiments in Godavari river estuary over three years revealed that a decrease in precipitation over the Indian subcontinent from 2007 to 2009 resulted in the lowering of mean annual discharge from 748.63 m3 s-1 in 2007 to 218.40 m3 s-1 in 2009. The reduced water discharge, during the peak discharge period, slowed the flushing of the estuary from 1.2 days to 6.3 days, respectively. The consequent increase in stability of water column and reduced suspended material load gave rise to intense phytoplankton blooms (Chl a 18 μg l-1 in 2007 to 28 μg l-1 in 2009). Resilience towards the unwanted phytoplankton bloom and overall health of the Indian estuaries is thus tuned by the variability in monsoon rainfall and dam regulated discharge.

► Indian estuaries receive huge nutrients from land drainage and agricultural fields
► Light limitation and fast flushing prevents potentially intense phytoplankton bloom
► Reduced discharge promotes intense bloom by enhancing stability and stratification
► Changing climate and more dam construction may potentially reduce dischargeCapsule
► Monsoonal estuaries are highly sensitive towards eutrophication in the context of reduced water availability due to dam construction and ‘monsoon vagaries’.