Assessing the accuracy of the “two-point” dilution technique

The dilution technique is the most commonly used method to measure microzooplankton grazing rates in the ocean. A simplified version of the dilution technique termed the “two-point” dilution technique, has been increasingly used to reduce workload and to increase spatiotemporal coverage. Several variants of the “two-point” dilution technique have been used in the literature. The objective of this study is to assess which one is the best. Three versions of “two-point” dilution experiments with different number of replicates were simulated from a dataset of full dilution series experiments, with the rate estimates from the full dilution series experiments being treated as “true” values. We find that the approach of setting up a highly diluted bottle (the fraction of undiluted seawater ≤ 20%) and treating the net growth rate of phytoplankton of this bottle as the instantaneous growth rate yields the best result, for both experiments with linear and nonlinear behavior. With this approach, significant overestimates or underestimates occurred in only one-fourth of the experiments even if no replicates were set up. Setting up duplicates or more for each dilution level can give more reliable results, very close to those from full dilution series experiments.