Northeast monsoon distribution of biogeochemical properties and phytoplankton pigments from Hooghly–Sundarbans estuarine region, India

Abstract

Phytoplankton marker pigments and their relationships with water column properties were investigated for the first time in the Hooghly–Sundarbans region. The upper estuary (Diamond Harbour) was characterized by low salinity with high silicate and nitrate within the surface waters. This region showed a strong presence of marker pigment fucoxanthin, an indicator of diatom populations. In contrast, marker pigment zeaxanthin and chlorophyll b became equally dominant with the increasing salinity in the lower estuary Silicate and nitrate showed positive relationships with diatom indices suggesting strong dependency. Contrary to that, picoplankton indices increased the high saline oligotrophic waters of the lower estuary (Sundarbans). Divinyl Chlorophyll a and b were not detected in any of the sample sets suggesting the absence of Prochlorococcus populations around the sampling locations. Oxygen distribution was strongly correlated with temperature and salinity, suggesting significant physical control. Further silicate, also showed a positive relationship with turbidity inferring that the river discharge is an important source. Our study suggests 60 % of the chlorophyll a biomass is contributed by microplankton communities with the rest 40 % by picoplankton during the northeast monsoon. As sinking rates and carbon to chlorophyll a ratio varies among different communities, the relative efficiency of primary productivity and carbon sequestration may be re-looked here in light of this new revelation.