Phytoplankton settling depends on cell morphological traits, but what is the best predictor?

Our objective was to determine how different morphological traits of microalgae influence their settling rates, since settling is important to population dynamics and the loss of individuals affects phytoplankton communities. We hypothesized that different morphological characteristics can affect phytoplankton settling rate, and that surface to volume ratio (S:V) would be especially important in determining the settling rhythm than any other morphological trait alone. Experiments were conducted in settling columns, which generate subsamples for quantification of phytoplankton and allow the calculation of settling rate. The dependence of settling rate on various cell characteristics was assessed by predictive models generated by non-linear regression analyses in which settling rate was the independent variable, while the dependent variable was biovolume, surface area or S:V ratio. The resulting statistical coefficients were used to compare models in terms of their capacity to describe and predict settling. The results show that phytoplankton did not follow Stokes Law of particle settling. As expected, the model based on S:V ratio was the best predictor of phytoplankton settling rate. The balance between volume and surface was more important than each characteristic alone because although increasing volume could positively affect settling rates, surface area is fundamental to determine resistance to settling.