Scaling of growth rate and mortality with size and its consequence on size spectra of natural microphytoplankton assemblages in the East China Sea

摘要

Allometric scaling of body size versus growth rate and mortality has beensuggested to be a universal macroecological pattern, as described by themetabolic theory of ecology (MTE). However, whether such scaling generallyholds in natural assemblages remains debated. Here, we test the hypothesisthat the size-specific growth rate and grazing mortality scale with the bodysize with an exponent of −1/4 after temperature correction, as MTEpredicts. To do so, we couple a dilution experiment with the FlowCAM imagingsystem to obtain size-specific growth rates and grazing mortality of naturalmicrophytoplankton assemblages in the East China Sea. This novel approachallows us to achieve highly resolved size-specific measurements that would bevery difficult to obtain in traditional size-fractionated measurements usingfilters. Our results do not support the MTE prediction. On average, thesize-specific growth rates and grazing mortality scale almost isometricallywith body size (with scaling exponent ∼0.1). However, this findingcontains high uncertainty, as the size-scaling exponent varies substantiallyamong assemblages. The fact that size-scaling exponent varies amongassemblages prompts us to further investigate how the variation ofsize-specific growth rate and grazing mortality can interact to determine themicrophytoplankton size structure, described by normalized biomass sizespectrum (NBSS), among assemblages. We test whether the variation ofmicrophytoplankton NBSS slopes is determined by (1) differential grazingmortality of small versus large individuals, (2) differential growth rate ofsmall versus large individuals, or (3) combinations of these scenarios. Ourresults indicate that the ratio of the grazing mortality of the large sizecategory to that of the small size category best explains the variation ofNBSS slopes across environments, suggesting that higher grazing mortality oflarge microphytoplankton may release the small phytoplankton from grazing,which in turn leads to a steeper NBSS slope. This study contributes tounderstanding the relative importance of bottom-up versus top-down control inshaping microphytoplankton size structure.