Search

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.

This dissertation addresses the role of mesozooplankton feeding in influencing the trophic interactions in marine planktonic food web. Several related hypotheses were tested in the framework of five individual chapters. Research started with two field investigations studying the response of mesozooplankton biomass and composition to estuarine nutrient loading in Hong Kong coastal waters and the spatial and temporal grazing importance in northern South China Sea (SCS). Trophic cascades induced by mesozooplankton resulted from the release of microzooplankton grazing was demonstrated in laboratory feeding experiments involving simplified triangle food chains and the strengths of trophic cascades were examined for a range of prey concentrations and compared for different predator species with different feeding characteristics. Finally, I went back to field works to test the hypothesis that mesozooplankton carnivory causes a decrease of clearance rate on phytoplankton community. To address this hypothesis, feeding experiments were conducted at two contrasting sites of Hong Kong coastal waters over a two year period to measure mesozooplankton clearance rate and the identity of mesozooplankton grazers in feeding experiments was determined to estimate a net carnivory degree of each assemblage. Field investigation in Hong Kong water indicated that mesozooplankton biomass in the estuarine eutrophic station was not elevated by estuarine nutrient loading compared to the coastal mesotrophic station. Instead, estuarine eutrophic conditions significantly modified the composition of mesozooplankton by increasing the proportion of omnivorous copepods in community and by decreasing some particular groups that are primarily herbivorous such as marine cladocerans, bivalve larvae and gastropod larvae. Field investigation carried out in northern SCS indicated that mesozooplankton in shelf water generally have higher biomass and grazing impacts on phytoplankton than in oceanic water becau

Although grazer inclusion incubation experiments are commonly used to evaluate mesozooplankton grazing mortality of phytoplankton, few studies have employed molecular methods to examine the changes in the composition of the microbial community across different grazing treatments. We used denaturing gradient gel electrophoresis (DGGE) to study the impact of mesozooplankton-mediated trophic cascades on microbial food web components at two subtropical coastal sites with contrasting trophic conditions. Reciprocal changes in microbial food web components resulting from trophic cascades were observed, with decreases in >3 mu m eukaryotes, increases in <3 mu m eukaryotes and decreases in bacterio plankton. At the estuarine station, diatoms and dinoflagellates accounted for the majority of the micro and nanoplankton, and they, particularly dinoflagellates, were selectively grazed by mesozooplankton. Although the total band numbers of picoeukaryotes did not vary significantly, the relative abundance of different phylogenetic groups changed with the appearance of mesozooplankton grazers. Both total band brightness and species richness and diversity of bacterioplankton declined sharply under mesozooplankton grazing at the coastal site, but not at the estuarine site, suggesting that mesozooplankton-mediated trophic cascade effect was less pronounced in eutrophic estuarine waters. We concluded that DGGE could be a useful tool to track the community shifts of microbial food web in experimental settings and we have demonstrated in this study such shifts in both eukaryotic and prokaryotic microbial communities induced by mesozooplankton grazing. The magnitude of mesozooplankton-mediated trophic cascade effect transferred to picoplankton appears to be stronger at the mesotrophic coastal water than at the eutrophic estuarine water. (C) 2009 Elsevier B.V. All rights reserved.