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Mesozooplankton are key components in the marine environment, linking the microbial food web and the classic food chain. Yet uncertainties remain on how mesozooplankton mediate the dynamics of prey communities through their complex feeding patterns. To examine mesozooplankton-mediated trophic interactions, we performed shipboard incubations using food removal and dilution methods during 4 cruises in the Pearl River estuary (PRE), Southern China. Our results revealed that mesozooplankton had diverse effects on different size fractions and taxonomic groups of phytoplankton via a combination of strong feeding selectivity and trophic cascades. High ingestion rates by mesozooplankton suppressed the accumulation of microphytoplankton (>20 µm), whereas low ingestion rates by mesozooplankton and resultant trophic cascades promoted the biomass of nano-sized (2-20 µm) and pico-sized (0.7-2 µm) phytoplankton. Among phytoplankton groups, diatoms were passively selected by mesozooplankton despite their high concentrations in natural seawater, whereas dinoflagellates and cryptophytes were actively preferred by mesozooplankton in spring and autumn. Similarly, ciliates were also preferred by mesozooplankton despite their lower biomass compared to phytoplankton, which induced a trophic cascade that indirectly increased the biomass of smaller phytoplankton. The overall feeding effect of mesozooplankton on phytoplankton was determined by the balance between direct grazing rates and indirect compensation with trophic cascades. The degrees of carnivory of the mesozooplankton, which determined the strength of trophic cascades, varied among seasons, resulting in weak control of algal blooms by mesozooplankton. Our findings provide insights into the complex trophic interactions between mesozooplankton and other plankton groups in dynamic natural ecosystems.

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.

Small marine copepods are key components of the pelagic food webs in Chinese coastal waters, but very few studies have addressed their trophodynamics, with even fewer studies addressing their diel feeding rhythms. In this study, the diel feeding rhythm and grazing selectivity of the copepod assemblage in Daya Bay during September 30 to October 2, 2014, were studied based on gut pigment analysis. Small copepods (body length < 1.5 mm) including Paracalanus parvus, Temora turbinata, Acrocalanus gibber, Temora stylifera, Euterpe acutifrons, and Acrocalanus gracilis, accounted for 73.9–100% of the total copepod abundance. The copepod assemblage generally exhibited a diurnal feeding pattern, characterized by a higher gut pigment content and ingestion rate during the daytime, consistent with variation in the ambient Chl α concentration. Fifty-five percent of the phytoplankton standing stock per day was consumed by the copepod assemblage, wherein diatoms, prymnesiophytes, and cyanobacteria were the main prey items with average contributions of 19.4–32.9% to the gut pigment contents. The copepod assemblage showed a strong feeding preference for prymnesiophytes, a weak feeding preference for diatoms, and avoidance of cyanobacteria. These results suggest a strong top-down control on phytoplankton community, especially on small groups from small copepods in the Daya Bay ecosystem.

Spatial distribution of phaeopigment and size-fractionated chlorophyll a (Chl a) concentrations were examined in relation to hydrographic conditions in the northern South China Sea (NSCS) during a survey from 20 August to 12 September, 2014. The total Chl a concentration varied from 0.006 to 1.488 µg/L with a mean value of 0.259±0.247 (mean±standard deviation) µg/L. Chl a concentration was generally higher in shallow water ( 200 m), with mean values of 0.364±0.311 µg/L and 0.206±0.192 µg/L respectively. Vertically, the maximum total Chl a concentration appeared at depths of 30–50 m and gradually decreased below 100 m. The size-fractionated Chl a concentrations of grid stations and time-series stations (SEATS and J4) were determined, with values of pico- (0.7–2 µm), nano- (2–20 µm) and micro-plankton (20–200 µm) ranging from 0.001–0.287 (0.093±0.071 µg/L), 0.004–1.149 (0.148±0.192 µg/L) and 0.001–0.208 (0.023±0.036 µg/L), respectively. Phaeopigment concentrations were determined at specific depths at ten stations, except for at station A9, and varied from 0.007 to 0.572 (0.127±0.164) µg/L. Nano-and pico-plankton were the major contributors to total phytoplankton biomass, accounting for 50.99%±15.01% and 39.30%±15.41%, respectively, whereas microplankton only accounted for 9.39%±8.66%. The results indicate that the contributions of microplankton to total Chl a biomass were less important than picoplankton or nanoplankton in the surveyed NSCS. Different sized-Chl a had similar spatial patterns, with peak values all observed in subsurface waters (30–50 m). The summer monsoon, Kuroshio waters, Zhujiang (Pearl) River plume, and hydrological conditions are speculated to be the factors controlling the abundance and spatial heterogeneity of Chl a biomass in the NSCS.

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 less than 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 one-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 empirical relationship between carnivory degree and clearance rate, which allow researchers to potentially predict the herbivory of mesozooplankton in the nature without conducting feeding experiments.

Noctiluca scintillans, a heterotrophic dinoflagellate responsible for most of the red tides in Hong Kong waters was investigated to determine the influence of biotic and abiotic factors in determining its population dynamics. N. scintillans first occurred in January when temperature was ~17°C, and reached a maximum of 1980cellsL-1 one month later. N. scintillans interacted with trophic compartments from picoplankton to mesozooplankton, but its population development was largely related to the availability of phytoplankton, especially diatoms. Growth rates in microcosms (with mainly biological factors) ranged from -0.38 to 1.11d-1 and were similar to the field in situ growth rates (physical and biological factors). Thus, optimum hydrographical conditions (temperature and water stability), and a rich food supply were necessary for N. scintillans to bloom. Its spatial distribution pattern was a result of biological and physical coupling, but mainly controlled by physical accumulation processes such as winds, tides and currents.

Jia, H.; Chen, M.; Su, W.; Zhang, S., and Zhao, K., 2019. Structural characteristics and associated factors influencing phytoplankton abundance and species composition in Huangmaohai Bay, Pearl River Estuary. Journal of Coastal Research, 35(1), 72–81. Coconut Creek (Florida), ISSN 0749-0208.Temporal–spatial variations of phytoplankton abundance and species composition, as well as associated environmental factors, were investigated to understand phytoplankton succession at Huangmaohai Bay in December 2010 and August 2011. Concentrations of both chlorophyll-a and nutrient showed apparent spatial variability with significantly higher levels in the upstream area of the estuary than in the offshore area. Temporal variation of the chlorophyll-a concentration showed that the concentration in the summer was 1.25 times higher than that in the winter. Significantly higher abundances of cyanobacteria and chlorophytes were found at the upstream sites where the system was influenced by freshwater input. In contrast, diatoms and dinoflagellates primarily dominated at the downstream sites. Phytoplankton abundance was higher in the summer (629.66 × 104 cells/m3) than in the winter (466.75 × 104 cells/m3). A canonical correspondence analysis of phytoplankton community and environmental factors indicated that dissolved inorganic phosphorus, nitrogen, and salinity were the key environmental factors influencing the spatial variation of phytoplankton. Dinoflagellates, cyanobacteria, and chlorophytes were generally sensitive to salinity, whereas diatoms were generally sensitive to nutrients. Because Huangmaohai Bay is part of the Pearl River Estuary, this study not only fills a gap toward understanding this region but also aids marine protection department personnel in making effective management plans that can be put into action.

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 one 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.

The ciliate community together with physical-chemical variables was investigated in a subtropical, eutrophic bay of the South China Sea from January 7–10 (dry season) and April 27–30 (wet season), 2012. Both temperature and salinity exhibited typical seasonality. A total of 27 species of ciliates belonging to 19 genera were identified, and the dominant species were Strombidium globosaneum in January and Codonella rapa in April. The composition and abundance exhibited spatial and temporal fluctuations, with total mean abundance of ciliates in April (1219 ± 919 ind l-1 ), was much higher than that in January (283 ± 137 ind l-1 ). Spatially, high species numbers were found mainly near the mouth of the estuary, and the abundance decreased from inside to the mouth of the bay. Statistical analysis showed that ciliate assemblages were closely related to the environmental variables. The distribution patterns of salinity and chlorophyll a concentration were similar, suggesting that, prey abundance may correlate significantly with ciliate abundance, and salinity may not be the main factor explaining the spatial variation in the ciliate community in the study area.