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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.

Biomass and grazing impact of three size fractions of mesozooplankton were investigated across a wide range of trophic conditions in northern South China Sea (SCS) during four seasonal cruises from July 2009 to May 2011. The grazing impact of mesozooplankton was determined by measurements of gut fluorescence and experimentally-derived gut clearance coefficients. In the northern SCS, the variation in mesozooplankton biomass was influenced by bottom-up effects. Due to riverine runoff to the shelf and winter mixing in oceanic areas, an increase in mesozooplankton biomass was observed in late spring corresponding to an increase in Chl a concentrations. However, because a significant portion of the Chl a came from pica-sized phytoplankton cannot be directly consumed by most mesozooplankton, especially in the oceanic waters during autumn and spring, the bottom-up effect was rather weak. Mesozooplankton consumed 0.7-215% of phytoplankton Chl a standing stocks daily, and it was higher in shallow shelf waters than in oceanic waters during spring and summer due to combined effects of mesozooplanIcton feeding behavior (higher herbivory in shelf waters) and phytoplanlcton composition (higher contribution of pico-sized phytoplankton in oceanic waters). If only the >2 mu m phytoplankton were considered, the grazing impact by mesozooplanlcton became substantial, reaching similar to 31% d(-1). Among the three size fractions of mesozooplanIcton, large-sized mesozooplankton (> 1000 mu m) dominated the total biomass and contributed more than 50% of the herbivory. (C) 2015 Elsevier Ltd. All rights reserved.

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

To study the effect of hydrographic factors on the spatial distributions of chlorophyll a (Chl a), an investigation was carried out in the tropical eastern Indian Ocean (80A degrees-100A degrees E along 7A degrees S, and 7A degrees-18A degrees S along 80A degrees E) in December 2010. The fluorescent method was used to obtain total Chl a and size-fractioned Chl a at the 26 stations. The results show that surface Chl a concentration averaged at (0.168 +/- 0.095) mg/m(3) s.d. (range: 0.034-0.475 mg/m(3)), concentrations appeared to be higher in the west for longitudinal variations, and higher in the north for latitudinal variations. Furthermore, the surface Chl a concentration was lower (0.034-0.066 mg/m(3)) in the region to the south of 16A degrees S. There was a strong subsurface Chl a maximum layer at all stations and the depth of the Chl a maximum increased towards to the east and south along with the respective nitracline. The spatial variation of Chl a was significant: correlation and regression analysis suggests that it was primarily affected by PO (4) (3-) , N(NO3-N+NO2-N) and temperature. Size-fractionated Chl a concentration clearly showed that the study area was a typical oligotrophic open ocean, in which picophytoplankton dominated, accounting for approximately 67.8% of total Chl a, followed by nanophytoplankton (24.5%) and microphytoplankton (7.6%). The two larger fractions were sensitive to the limitation of P, while picophytoplankton was primarily affected by temperature.

Phytoplankton uptake rates of ammonium (NH4 (+)), nitrate (NO3 (-)), and urea were measured at various depths (light levels) in Hong Kong waters during the summer of 2008 using N-15 tracer techniques in order to determine which form of nitrogen (N) supported algal growth. Four regions were sampled, two differentially impacted by Pearl River discharge, one impacted by Hong Kong sewage discharge, and a site beyond these influences. Spatial differences in nutrient concentrations, ratios, and phytoplankton biomass were large. Dissolved nutrient ratios suggested phosphorus (P) limitation throughout the region, largely driven by high N loading from the Pearl River in summer. NH4 (+) and urea made up generally a parts per thousand yen50% of the total N taken up and the f ratio averaged 0.26. Even at the river-impacted site where concentrations of NO3 (-) were > 20 mu M N, NH4 (+) comprised > 60% of the total N uptake. Inhibition experiments demonstrated that NO3 (-) uptake rates were reduced by 40% when NH4 (+) was > 5 mu M N. The relationship between the total specific uptake rates of N (sum of all measured substrates, V, per hour) and the chlorophyll a-specific rates (micromolars of N per microgram of Chl a per hour) varied spatially with phytoplankton biomass. Highest uptake rates and biomass were observed in southern waters, suggesting that P limitation and other factors (i.e., flushing rate) controlled production inshore and that the unincorporated N (mainly NO3 (-)) was transported offshore. These results suggest that, at the beginning of summer, inshore algal blooms are fueled primarily by NH4 (+) and urea, rather than NO3 (-), from the Pearl River discharge. When NH4 (+) and urea are depleted, then NO3 (-) is taken up and can increase the magnitude of the bloom.

Trophic cascades in the marine pelagic food web, especially in the microbial food web in which copepods are the top controllers, are difficult to quantify and are generally overlooked. In this study, we simulated a simple pelagic food web in laboratory feeding experiments to demonstrate the trophic interactions among copepods, heterotrophic dinoflagellates and chain-forming diatoms and to quantify the trophic cascade effect and the true feeding rate of copepods. Results from the study showed that heterotrophic dinoflagellates play a central role in the lower trophic level of the marine food web by consuming diatoms and by serving as a quality food source for copepods, especially during the period of diatom blooms. Copepod omnivory was mediated by the concentration of diatoms, with the highest ingestion of diatoms occurring at intermediate diatom abundances and the selectivity toward dinoflagellates increasing as the diatom concentration increased. The trophic cascade was surprisingly low and even negative in treatments with low diatom concentrations, suggesting a competition effect when there was not enough diatom food. In this situation, the omnivorous copepods together with heterotrophic dinoflagellates exhibited strong herbivory and were able to control diatoms. In contrast, a high trophic cascade effect occurred at high diatom concentrations in which copepods had a high ingestion of heterotrophic dinoflagellates. We conclude that the role of copepods in the food web structure is mediated by the concentrations of diatoms, but one of the most important biological factors that determine the strength of trophic cascades is the ingestion rates of both copepods and heterotrophic dinoflagellates. (C) 2011 Published by Elsevier B.V.

A monthly comparative study of mesozooplankton biomass and composition between a eutrophic Pearl River estuarine site (WE) and a mesotrophic coastal-oceanic site (EO) in Hong Kong waters was conducted to examine the response of mesozooplankton to nutrient-rich riverine discharge. Although the annual average mesozooplankton biomass was higher at WE than at EO, they were not statistically significant. Variations of mesozooplankton biomass at both stations followed similar trends of Chl a concentrations, with the exception of July at WE where mesozooplankton biomass was low but total Chl a was high. This mismatch may be due to the high flushing effect of the Pearl River discharge in summer and a time lag in mesozooplankton population growth. On the other hand, the composition of mesozooplankton was significantly modified by riverine discharge and eutrophication conditions. While small copepods dominated at both sites, the eutrophic estuarine water had a high abundance of barnacle and polychaete larvae, while cladocerans, bivalve larvae, gastropod larvae and chaetognaths mainly occurred at EO. Eutrophication increased the top-down role of copepods in the grazing community, revealed by an increase in the percentage of copepods in the total metazoan mesozooplankton, especially during the period of high river discharge. Moreover, mesozooplankton diversity at the two stations was similar, and they both showed relatively higher diversity during autumn and winter and lower diversity during summer, especially at WE. These results suggest that, despite high nutrient and Chl a concentrations in estuarine waters, mesozooplankton biomass were not enhanced compared to coastal waters with no river impact, possibly due to poor food quality and increased predation in the eutrophic estuarine waters. (C) 2011 Elsevier Ltd. All rights reserved.

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.