Adaptations in phytoplankton to changing conditions in tropical estuaries

S.Z. Qasim


Experimental studies on phytoplankton organisms from Cochin backwaters (a tropical estuary) have shown that many algal forms are adapted to changing conditions of salinity, light and nutrients. Photosynthesis in 12 species of algae was found to be maximum in the salinity range 10-20 ppt. This feature is an adaptation to enable the organisms to utilize the enrichment of water for growth to a maximum degree during the monsoon months when large dilutions in the estuary occur. Reduction in light penetration due to high turbidity is another important factor in the estuary. Most of the organisms were found to have high values for light saturation intensity (Ik) and they appear to have no wavelength dependence in saturating light. Such a chromatic adaptation in the algae is to counteract the changing light conditions with depth to which they are exposed during their floatation. The green alga Tetraselmis gracilis was found to have a high requirement for phosphorus and this organism occurs in the estuary practically throughout the year but not in profusion. The diatom Biddulphia sinensis occurs very abundantly in the estuary when both phosphorus and nitrogen are maximum (monsoon months), while Ceratium furca forms blooms during the premonsoon season when these two nutrients are low. These deductions were made from the values of the half saturation constant (Ks) obtained for the three organisms


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