Abstract
AbstractMicrozooplankton are the primary herbivores and nutrient regenerators in the marine food web, but their importance is often underestimated, and the quantitative relationships between environmental factors and the biomass and herbivory rate of microzooplankton remain obscure. To fill this gap, we conducted 224 dilution experiments to measure microzooplankton biomass and herbivory rate across a vast area of the marginal seas of China. To gain the potential mechanisms controlling microzooplankton herbivory, we also use a model that combines the Metabolic Theory of Ecology and the functional responses of grazing to quantify the effects of temperature, phytoplankton biomass, and microzooplankton biomass on microzooplankton grazing rate. We estimate an activation energy of 0.51 eV of microzooplankton and found that the Holling III function best described the functional response of microzooplankton grazing with a maximal ingestion rate of 4.76 d−1 at 15°C and a half‐saturation constant of 0.27 μM N. We also find that microzooplankton biomass scales with phytoplankton biomass with an exponent of 0.77, consistent with the general 3/4 scaling law found in other ecosystems. This scaling relationship is accompanied by a shift from ciliates to heterotrophic dinoflagellates with increasing phytoplankton biomass. Our results provide empirical patterns that will be vital to parameterize and validate marine ecosystem models, particularly in China seas.
Highlights
Microzooplankton, defined as heterotrophic and mixotrophic organisms with a size range of 20-200 μm, primarily consist of phagotrophic protists, including ciliates, dinoflagellates, and mesozooplankton nauplii (Sieburth et al 1978)
We explored the theoretical prediction of how temperature affects the ratio of microzooplankton to phytoplankton biomass
Environmental controls on microzooplankton biomass--We find that microzooplankton biomass is significantly correlated with phytoplankton biomass (Spearman r = 0.61, p < 0.001; Fig. 2; Detailed results for each experiment are summarized in the Appendix)
Summary
Microzooplankton, defined as heterotrophic and mixotrophic organisms with a size range of 20-200 μm, primarily consist of phagotrophic protists, including ciliates, dinoflagellates, and mesozooplankton nauplii (Sieburth et al 1978). The number of observations of microzooplankton herbivory rate is around 1600 globally (Chen et al 2012; Schmoker et al 2013), far less than that of primary productivity (>50000; Buitenhuis et al.2013) This makes validating and optimizing the grazing function of microzooplankton difficult in ocean ecosystem models. Microzooplankton have been rarely validated by observational data in mainstream ocean models (but see Buitenhuis et al 2010) Promoted by such gap, Buitenhuis et al (2010) suggested that “the most effective progress which can be made in defining the role of microzooplankton in global biogeochemical cycles is to make microzooplankton biomass a standard oceanographic measurement, in particular during dilution grazing experiments”
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