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Microzooplankton 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. © 2020 The Authors. Limnology and Oceanography published by Wiley Periodicals. LLC on behalf of Association for the Sciences of Limnology and Oceanography.

In order to understand how mesozooplankton assemblages influenced phytoplankton in coastal and estuarine waters, we carried out a monthly investigation on mesozooplankton composition at two contrasting stations of Hong Kong coastal and estuarine waters and simultaneously conducted bottle incubation feeding experiments. The assemblage of mesozooplankton was omnivorous at both stations with varying carnivory degree (the degree of feeding preference of protozoa and animal food to phytoplankton) and the variations of carnivory degree were significantly associated with microzooplankton biomass (ciliates for the coastal station, both ciliates and dinoflagellates for the estuarine stations) and physical environmental parameters (primarily salinity). High carnivory was primarily due to high composition of noctilucales, Corycaeus spp., Oithona spp. and Acartia spp. Results of feeding experiments showed that grazing impacts on phytoplankton ranged from -5.9 to 17.7%, while the mean impacts were just < 4% at both stations. The impacts were size-dependent, by which mesozooplankton consumed around 9% of large-sized phytoplankton while indirectly caused an increase of 4% of small-sized phytoplankton. Mesozooplankton clearance rate on phytoplankton, calculated from the log response of chlorophyll a concentrations by the introduction of bulk grazers after 1-day incubation, was significantly reduced by increasing carnivory degree of the mesozooplankton assemblage. The mechanism for the reduction of mesozooplankton clearance rate with increasing carnivory degree was primarily due to less efficient of filtering feeding and stronger trophic cascades due to suppression of microzooplankton. The feeding rates of mesozooplankton on microzooplankton were not obtained in this study, but the trophic cascades indirectly induced by mesozooplankton carnivorous feeding can be observed by the negative clearance rate on small-sized phytoplankton. Overall, the main significance of this study is the

Diatoms are often a major food source for zooplankton and contribute significantly to vertical POC flux through sinking of dead cells, aggregates and zooplankton fecal pellets. The silica content of diatoms varies among different species and within a species growing under different environmental conditions and physiological status. However, to-date there has been no investigation of the effect of diatom silica content on zooplankton grazing, growth and reproduction. We conducted a series of experiments using the diatom Thalassiosira weissflogii with different silica content achieved by growth under high and low light and these cells were fed to a copepod, Parvocalanus crassirostris. Our results show that this copepod strongly preferred cells with low silica content over high silica-containing cells, with the ingestion rate on low silica diatoms being 2-3 times higher than that on high silica diatoms. Fecal pellet production rate was significantly higher for copepods feeding on highly silicified cells. Furthermore, copepod growth rate (measured as an increase in wet weight), egg production rate and hatching success were all severely compromised under a high silica diatom diet. Females of P. crassirostris feeding on a low silica diatom diet produced an average 90 eggs during a 1 day incubation, while those fed with high silica diatoms produced only 11 eggs per day. Similarly, the hatching success during a 3-day period was 82 ± 17 and 23 ± 36% for the low and high silica diatom treatments, respectively, with zero success observed in ~65% of the females feeding on high Si diatoms. Our findings have important ecological implications for the biological pump and may alter our previous view of the role of diatoms in planktonic food webs and the role of the degree of silicification in controlling amount of POC flux to deeper waters. © 2016 Liu, Chen, Zhu and Harrison.

Diatoms are often a major food source for zooplankton and contribute significantly to vertical POC flux through sinking of dead cells, aggregates and zooplankton fecal pellets. The silica content of diatoms varies among different species and within a species growing under different environmental conditions and physiological status. However, to-date there has been no investigation of the effect of diatom silica content on zooplankton grazing, growth and reproduction. We conducted a series of experiments using the diatom Thalassiosira weissflogii with different silica content achieved by growth under high and low light and these cells were fed to a copepod, Parvocalanus crassirostris. Our results show that this copepod strongly preferred cells with low silica content over high silica-containing cells, with the ingestion rate on low silica diatoms being 2-3 times higher than that on high silica diatoms. Fecal pellet production rate was significantly higher for copepods feeding on highly silicified cells. Furthermore, copepod growth rate (measured as an increase in wet weight), egg production rate and hatching success were all severely compromised under a high silica diatom diet. Females of P. crassirostris feeding on a low silica diatom diet produced an average 90 eggs during a 1 day incubation, while those fed with high silica diatoms produced only 11 eggs per day. Similarly, the hatching success during a 3-day period was 82 ± 17 and 23 ± 36% for the low and high silica diatom treatments, respectively, with zero success observed in ~65% of the females feeding on high Si diatoms. Our findings have important ecological implications for the biological pump and may alter our previous view of the role of diatoms in planktonic food webs and the role of the degree of silicification in controlling amount of POC flux to deeper waters. © 2016 Liu, Chen, Zhu and Harrison.