Estimating size-dependent growth and grazing rates and their

Darcy A.A. Taniguchi, Peter J.S. Franks, Michael R. Landry

Limnol. Oceanogr. Methods 10:868-881 (2012) | DOI: 10.4319/lom.2012.10.868

ABSTRACT: Size-dependent properties are pervasive in nature but difficult to measure for natural communities. Here, we develop a technique to estimate size-specific phytoplankton growth and grazing rates based on the two point dilution method, enhanced by the acquisition of the size spectra of the phytoplankton in the samples. We describe a way to estimate standard deviations associated with the rate estimates, which can be applied either to the size-dependent or total community rates. We tested the accuracy of rates estimated using the size-dependent dilution method by applying it to dilution experiments simulated using a complex size-structured ecosystem model. The strong agreement between model and size dependent dilution method rates (two-sample Kolmogorov-Smirnov test, P = 1) supports the accuracy of this new technique. Because size-dependent rates vary with the size interval over which they are calculated, we display the size-dependent growth and grazing rates and their standard deviations as a function of the size interval. This technique easily allows the assessment of rates for any size class of interest. Finally, we apply the size-dependent dilution method to data collected in the equatorial Pacific. There is a general agreement between size-based and previously published taxonomic-based rates, with differences reflecting the extent to which size classes are mixtures of taxa. The use of the size-dependent dilution method will provide new insights into the structure and dynamics of planktonic communities. Future applications of this method to other natural communities will help in assessing the size-dependencies of phytoplankton growth and grazing rates in their environments.