Temporal variation of diatom benthic propagules in a monsoon influenced tropical estuary

Temporal variations in the diatom benthic propagule (DBP) community and its role in the phytoplankton community in a monsoon-affected tropical estuary, Zuari estuary, Goa (India) is presented. The DBP from the sediments was enumerated using an extinction dilution method (Most Probable Number method), which allows estimation of resting stages through examination of germinated vegetative cells in culture. The DBP community was dominated by planktonic species belonging to the genera Skeletonema, Fragilariopsis, Thalassiosira and Chaetoceros. Benthic propagules (BP) of S. costatum and Fragilariopsis sp. were dominant throughout the year. Between these two species, only S. costatum showed a linear relationship between the BP and planktonic cells indicating that this species is particularly important in coupling of pelagic and benthic ecosystems. During the onset and restart of monsoon after an intermittent break, water column was stratified, with a low salinity layer arising from riverine discharge and precipitation at the surface and relatively cold, saline, low oxygen waters at the bottom. Stratification favored blooming of S. costatum and Fragilariopsis sp. in nutrient rich surface and bottom waters respectively. The decline in these blooms ensuing nitrate depletion and salinity change resulted in an increased abundance of BP. Chaetoceros bloom was observed during the monsoon break as well as during nonmonsoon period and on both the occasions the decline in bloom was coupled with freshwater discharge. During the non-monsoon season, Thalassiosira blooms were encountered subsequent to high nitrate inputs. These findings suggest that in such shallow tropical regions, physical processes during monsoon (freshwater discharge) and non-monsoon seasons (currents, waves and tides) cause resuspension of diatom BP. Since light is not a limiting factor for germination in such regions, the blooming of resuspended BP depends upon nutrient availability.