Variable response of two tropical phytoplankton species at different salinity and nutrient condition

The responses of two tropical phytoplankton species (one diatom and one cyanobacterium) at various nutrient concentrations were studied to determine their optimum growth conditions. Optimum salinities for these two species were also determined prior to the nutrient limitation study. As a part of ecosystem monitoring at Asia's largest brackish water lagoon (Chilika), axenic cultures of Chaetoceros simplex and Anabaena sp. were maintained and nutrient concentrations were regulated according to the in situ conditions. At given pre-defined environmental conditions (light, nutrient, temperature, salinity etc.) the maximum growth rate of 0.043 h− 1 and 0.040 h− 1 was recorded for C. simplex and Anabaena sp. at N/P ratios of 12 and 8, respectively. Addition of ammonia significantly stimulated the growth of Anabaena sp., whereas C. simplex did not show such significant increase. Anabaena sp. showed larger positive response towards the enrichment of phosphate–phosphorous (PO4–P) in terms of specific growth rate compared to that of C. simplex. For both the species, relative decrease in available dissolved inorganic nitrogen (DIN) was indicated by an apparent decrease in chlorophyll-b/a ratios. In in vitro conditions chlorophyll to carbon ratio increases with increasing N concentration, and was found to be higher for C. simplex relative to Anabaena sp. The results suggested that Anabaena sp. is the better competitor at low nutrient availability and has higher adaptation ability than C. simplex.

► Responses of two tropical phytoplankton species were studied under different nutrient limiting environment.
► Ammonia and phosphate–P could stimulate the growth of Anabaena sp. more than Chaetoceros simplex.
► Lower chlorophyll-b/a ratios in phytoplankton indicated nitrogen deficiency.
► Chlorophyll specific carbon assimilation was higher for Anabaena sp. relative to Chaetoceros simplex.