Recovering growth and grazing rates from nonlinear dilution experiments

Abstract

AbstractBiological rate measurements provide critical information for understanding key processes and modeling future states of marine ecosystems. Experimentally derived rates can be challenging to interpret when methodological assumptions are untested or potentially violated under variable natural conditions, such as the assumed linear grazing response of the dilution technique for estimating rates of phytoplankton growth and microzooplankton grazing impact. Here, we show that grazing nonlinearity can be related to the ratio of initial phytoplankton biomass to the half‐saturation parameter in the Holling II model, while not being affected by varying grazer biomass during dilution experiments. From this, we present theory to recover growth and grazing rates from multi‐treatment dilution experiments with nonlinear grazing results. We test our analyses with data collected during the California Current Ecosystem‐Long‐Term Ecological Research (CCE‐LTER) program. We show that using a linear regression in 2‐treatment dilution experiments may lead to underestimates of microzooplankton grazing rates, particularly in high‐phytoplankton‐biomass coastal regions where grazing can be saturated. Using the Holling II grazing model and a correction factor, growth and grazing rates from 2‐treatment experiments can also be estimated, as illustrated by application to Lagrangian water‐tracking studies of growth and grazing dynamics in the CCE.