Possible limitation of the dilution technique for estimating growth and grazing mortality rates of picoplanktonic cyanobacteria in oligotrophic tropical waters

Abstract

In total, ten dilution experiments were conducted to determine growth and grazing mortality rates of picoplanktonic cyanobacteria. Concentrations of nitrogenous nutrients (NO 3 + NO 2 + NH 4) and phosphate (PO 4) were relatively low, being in the range between 0.09–0.82 μM and between 0.05–0.11 μM, respectively. Specific growth and grazing mortality rates of cyanobacteria were estimated as 0.0101–0.0424 h −1 and < 0–0.0069 h −1, respectively. The reliability of these estimates were, however, rather questionable. One of the critical assumptions in the dilution technique is that phytoplankton growth rates are identical in diluted and undiluted water. This assumption was not always valid for cyanobacterial populations sampled. In seven of ten experiments, the day-time peak of the frequency of dividing cells (FDC) of cyanobacteria was lower in diluted water than in undiluted water, with the difference in mean value being in the range between 1.3–3.9%. No such difference was, however, observed in nutrient enriched treatments. This suggests the likelihood of nutrient limitation and the need for including nutrient enriched treatments in the experimental design. Incorporating FDC measurements into the experimental design is also a worthwhile practice as an independent check of the reliability of growth and grazing mortality rate estimates, particularly those for cyanobacterial populations in oligotrophic tropical waters.