INFLUENCE OF WATER QUALITY ON PHYTOPLANKTON BIOMASS AT OFF KOCHI, KERALA, INDIA DURING POST MONSOON SEASON

Five yearly analyses (2012-2016) was done for delineating the trend of environmental parameters with special reference to water quality on  phytoplankton biomass at 5 m depth and 10 m depth, off Cochin, during post monsoon (October - January).

The concentration of phosphate and nitrate at 5 m depth at surface and bottom waters were found to increase from 2012-16 in October months. These nutrients showed decrease with more fresh water influence, as they showed lower concentrations during high rainfall. The higher content of nitrate in water in October 2016 can be linked to the positive LTA (Local Temperature Anomaly) of 2.16°C, in this region in October 2016, indicating strong upwelling (Fig. 1). The silicate concentration in water was in accordance with the rainfall variation at 5m depth, but not so at 10 m depth, where fresh water influence was less.

At 10m, there was consistently higher biomass of phytoplankton in November in all years from 2012 - 2016.  In November, over the years, the nitrate values showed an increasing trend in surface waters at 10m depth (Fig. 2.). Except for the bloom of Noctiluca sp in December 2013, January 2015, November 2015 and December 2015, diatoms dominated the phytoplankton biomass in the months of October to January during 2012-16. In January 2016, Cyanophyceae and Coccolithophore were dominant.

The results (Fig. 3 and 4) of correlation analysis done for the months of October - January (2012-2016), separately for 5 m and 10 m depths, gave indication of influence of nitrate on phytoplankton biomass. Influence of rainfall on silicate, nitrate, water temperature and phosphate on chlorophyll are also signals of environmental influence on phytoplankton biomass and community structure.