Monsoon Effects on Biomass and Composition of Microphytobenthic Diatoms in the Cochin Estuary, Southwest India

Microphytobenthos are important primary producers in shallow aquatic ecosystems, but processes regulating this group are poorly understood in comparison to phytoplankton. We examined microphytobenthos at a station in Cochin Estuary for 13 months (February 2005–February 2006) to evaluate effects of the annual southwest monsoon on total biomass, depth distribution and species composition of the microphytobenthos. Significant declines in microphytobenthos and phytoplankton biomass occurred with onset of the southwest monsoon in June 2005. Phytoplankton biomass began to recover before the monsoon rains ended in September 2005, but microphytobenthos biomass recovered only after the rains ended. Microphytobenthos biomass declined throughout the top 5 cm of the sediment column concurrently with declines in abundance of most pennate diatom taxa, particularly larger forms, as well as some macrobenthos taxa. Changes in the microphytobenthos were not consistent with direct effects of the monsoon (changes in salinity, temperature, pH, physical disturbance, and turbidity), but were consistent with the effects of mud deposition by land runoff caused by heavy monsoon rains. These results suggest the hypothesis that sediment loading into estuaries from storm water runoff may have significant negative effects on microphytobenthos and the communities they support.