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Recent research has shown that jellyfish blooms occur more frequently near nuclear power plants (NPPs), and a relationship between thermal discharges from the NPPs and the blooms has been hypothesized. There is, however, no direct evidence that links the thermal discharges with jellyfish blooms. In this study, we conducted 3 cruises (pre-bloom, bloom, and post-bloom) during July to September 2017 in the area surrounding the largest NPP in northern China. The water temperature near the cooling water outfall rose by 2 °C. Although we found no significant impact on phytoplankton abundance or community composition near the outfall, a broader analysis of monitoring data revealed significant correlations between temperature and the abundance of Nemopilema nomurai , cyanobacteria, and haptophytes. Temperature increases may have a stimulatory effect on key stages in the life history of jellyfish, and the abundance of jellyfish prey may be the factor that triggers jellyfish blooms. Blooms of jellyfish were accompanied by a decline of diatom dominance and an increase of the concentrations of small phytoplankton such as cyanobacteria and haptophytes. This study revealed for the first time a significant positive correlation ( R 2 = 0.39, P < 0.001) between jellyfish abundance and concentrations of dissolved organic nutrients. This correlation suggested that the blooms of jellyfish resulted in the release of large amounts of organic nutrients. The results provided a comprehensive perspective and suggested a need for further study of the dynamics of planktonic ecosystems to link thermal discharges and jellyfish blooms.

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.

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.

Trophism (i.e., food resources and trophic levels) of different copepod groups was assessed along a salinity gradient in the temperate estuarine Gwangyang Bay of Korea, based on seasonal investigation of taxonomic results in 2015 and stable isotope analysis incorporating multiple linear regression models. The δ 13 C and δ 15 N values of copepods in the bay displayed salinity-associated spatial heterogeneity as well as temperature-related seasonal variations. Both spatial and temporal variations reflected those in isotopic values of food sources. Three major groups (marine calanoids, brackish water calanoids and cyclopoids) had a mean trophic level of 2.2 relative to nanoplankton as the basal food source, similar to the bulk copepod assemblage; however, they had dissimilar food sources based on the different δ 13 C values. Calanoid isotopic values indicated a mixture of different genera including species with high δ 15 N values (e.g., Sinocalanus and Labidocera ) and relatively low δ 15 N values ( Paracalanus and Acartia ). Feeding preferences of different copepods probably explain these seasonal and spatial patterns of the community trophic niche. Bayesian mixing model calculations based on source materials of two size fractions of particulate organic matter (nanoplankton at Acartia preferred large particles, Paracalanus and Pseudodiaptomus apparently preferred small particles, and Corycaeus was typically omnivorous with low selectivity on particle size. In addition, the carnivorous genus Tortanus predated on copepods without apparent selectivity, Labidocera preferred Acartia to Paracalanus , and Sinocalanus preferred Paracalanus to Acartia and cyclopoids. Overall, our results depict a simple energy flow of the planktonic food web of Gwangyang Bay: from primary producers (nanoplankton) and a mixture of primary producers and herbivores (microplankton), through omnivores ( Acartia , Paracalanus , and Corycaeus ) and detrivores ( Pseudodiaptomus and Euterpina ) to carnivores ( Tortanus , Labidocera , and Sinocalanus ).

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.

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 (80°-100°E along 7°S, and 7°-18°S along 80°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/m3 s.d. (range: 0.034–0.475 mg/m3), 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/m3) in the region to the south of 16°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.

University Grants Committee of Hong Kong's AoE [AoE/P-04/04]; Hong Kong Research Grants Council's (RGC) GRF [661809, 661610]; TUYF Charitable Trust [TUYF10SC08]; Xiamen University [2011121007]

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 15N 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 ≥50% 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 μM N, NH4+ comprised >60% of the total N uptake. Inhibition experiments demonstrated that NO3− uptake rates were reduced by 40% when NH4+ was >5 μM 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.

Measurements of uptake rates of inorganic (NO3- and NH4+) and organic (urea, glycine, and glutamic acid) N, and indirect estimates of total N uptake by bacteria, were made in four contrasting environments in sub-tropical Hong Kong waters in summer of 2008. In addition, the effects of several days of rain on N uptake rates were studied in eastern waters. Although ambient NO3- was the dominant form of N in Hong Kong waters, the dominant N form taken up by phytoplankton was usually NH4+ and organic N, including urea and amino acids, rather than NO3-. Hence, because of the low NO3- uptake, there was a long turnover time for NO3- (100 days), and most of the NO3- was apparently transported offshore into deeper shelf waters. In eastern waters where NH4+ was undetectable, NO3- uptake rates were positively correlated with phytoplankton cell size. In contrast, potential rates of glutamic acid uptake were negatively correlated with phytoplankton size. N uptake rates in the smaller size fraction (0.7-2.8 mu m) were less affected by the rain event, and smaller phytoplankton appeared to outcompete larger cells after several days of rain. The surface (PN)-specific N uptake rates in the >8-mu m fraction decreased from 0.02 to 0.0001 h(-1), while the smaller fraction only exhibited a one-to threefold decrease after the rainfall. In contrast, bacterial production and N uptake were not affected by the rain event, and bacteria N uptake accounted for 10-60% of the total N uptake by phytoplankton.

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.

A 10-week enclosure experiment was conducted in the dry season (October to December) to assess the impact of fish stocking on water quality and phytoplankton community at three stocking levels of silver carp (Hypophthalmichthys molitrix) (0, 16 and 50 g m–3) in a shallow, eutrophic tropical reservoir (Guangdong, South China). The nanophytoplankton (< 20 μm) accounted for 60 ± 5% of total biomass and Scenedesmus quadricauda was the predominant species (size: 16.5 ± 5 µm, relative biomass 27 ± 10%). Phytoplankton biomass was markedly suppressed in high fish stocking level and the grazing effect of silver carp on the nanoplankton was significant from the sixth week to the end of the experiment. The high fish density treatment significantly increased water transparency and somewhat reduced total phosphorus. Indirect effects of silver carp on the phytoplankton community involved excretion of nitrogen that favored small species of Chlorophyta. Colonial Cyanobacteria were reduced in the high fish treatment, but enhanced in the low fish treatment. Our results suggest that the widespread practice of harvesting fish from October to December in reservoirs of South China releases phytoplankton from consumption by filter-feeding fish, and consequently prompts algal blooms. Conversely, harvesting fish just before monsoon, around the time of restocking with small fish, would make more sense since no interruption in grazing pressure would occur. This would help to control excessive phytoplankton biomass just before monsoon. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Phytoplankton community composition and mesozooplankton selective feeding on dif- ferent phytoplankton taxa were studied by HPLC pigment analysis at 2 sites with contrasting hydro- graphy in the coastal areas of Hong Kong. The site (WE) on the western part of Hong Kong is located in the Pearl River estuary, whereas the site (EO) on the eastern part of Hong Kong is influenced by oceanic waters. Despite the strong difference in hydrographic conditions, diatoms were the dominant phytoplankton at both sites throughout most months of the year, contributing on average more than 40% of the total phytoplankton in terms of chlorophyll a (chl a). Cryptophytes were the second most abundant phytoplankton group at both sites, whereas contributions from haptophytes, green algae, cyanobacteria and dinoflagellates were small. Diatoms and cryptophytes, the 2 most dominant phyto- plankton groups, accounted for about 80 and 75% of the diets of mesozooplankton at EO and WE, respectively. One consistent pattern observed throughout the study period was that mesozooplank- ton assemblages strongly preferred dinoflagellates over other types of phytoplankton, especially in estuarine waters, even though dinoflagellates only contributed to a small fraction of total chl a. On the other hand, phytoplankton groups with relative small cell sizes, such as green algae and cyanobacte- ria, were not efficiently ingested. Overall, mesozooplankton feeding selectivity in subtropical eutrophic coastal waters, where planktonic food is sufficient, appears to be influenced by a combina- tion of different factors, including the compositions of predators and the size and quality of the prey.

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 µm eukaryotes, increases in

To investigate the effects of enhanced nutrient loading in estuarine waters on phytoplankton growth and microzooplankton grazing, we conducted monthly dilution experiments at 2 stations in Hong Kong coastal waters with contrasting trophic conditions. The western estuarine station (WE) near the Pearl River estuary is strongly influenced by freshwater discharge, while the eastern oceanic station (EO) is mostly affected by the South China Sea. Growth rates of phytoplankton were often limited by nutrients at EO, while nutrient limitation of phytoplankton growth seldom Occurred at WE due to the high level of nutrients delivered by the Pearl River, especially in the summer rainy season. Higher chlorophyll a, microzooplankton biomass, phytoplankton growth and microzooplankton grazing rates were found at WE than at EO. However, the increase in chlorophyll greatly exceeded the increase in phytoplankton growth rate, reflecting different response relationships to nutrient availability. Strong seasonality was observed at both stations, with temperature being an important factor affecting both phytoplankton growth and microzooplankton grazing rates. Picophytoplankton, especially Synechococcus, also exhibited great seasonality at EO, with summer abundances being 2 or 3 orders of magnitude higher than those during winter, Our results confirm that in eutrophic coastal environments, microzooplankton grazing is a dominant loss pathway for phytoplankton, accounting for the utilization of >50%, of primary production on average.

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.