Doing More with Less? Toward Increasing the Resolution of Protistan Grazing-rate Measurements.

The dilution method is the standard protocol to quantify phytoplankton grazing-mortality rates and has been key in developing an understanding of protistan grazing impact on ocean primary production. Although the method’s extensive use has facilitated the acquisition of a global dataset, its laborious application hinders the sampling resolution needed to fill knowledge gaps remaining at the geographical, seasonal, and vertical scales, and of the effects of climate-related factors influencing grazing magnitude. Here we present a rigorous assessment of an abbreviated method known as the 2-point. We analyzed unpublished results from 77 dilution experiments performed using a series of up to 5 dilutions under a wide range of chlorophyll concentrations and temperatures. We quantified the difference between estimates of both phytoplankton growth and grazing-mortality obtained based on the full dilution series and those obtained when the number of dilutions was reduced to 2. We considered the effect of non-linearity and chlorophyll concentration, and generated quantified estimates of trade-offs when choosing the fraction of seawater in the diluted treatment. Ultimately, we provide an assessment of the reliability of the 2-point method and recommendations on how to apply it.