Microzooplankton grazing and its key group composition in subtropical eutrophic coast of Southern China: in relation to environmental changes.

Microzooplanton play a crucial role in marine ecosystems, as they transfer matter and energy from pico- and nano-phytoplankton to mesozooplankton. In this study, we explored the seasonal variations of microzooplankton grazing derived from dilution experiments in a typical eutrophic coast of Southern China, as well as the abundance and biodiversity of its key group (ciliate), to further understand its function in the subtropical coastal food web associated with potential regulation factors. A total of 29 ciliate species belonging to 18 genera were identified, with the dominating species of Mesodinium rubrum, Strombidium globosaneum and Strombidium conicum. The spatial difference of ciliates abundance was attributed by the changes of temperature and salinity. Phytoplankton growth rate (µ) and microzooplankton grazing rate (m) ranged from 0.03 to 1.36 d-1 and 0.10 to 1.57 d-1, respectively, and both µ and m showed the highest values in summer and the lowest in winter. Moreover, microzooplankton grazing pressure on the phytoplankton standing stocks and potential primary production ranged from 10% to 79% and 58% to 471%, respectively. Our results indicated that temperature is the main environmental driving force for the seasonal changes of µ and m, and that the impacts of run-offs from the Pearl River and offshore seawater intrusion from the South China Sea are responsible for the spatial-temporal variations of phytoplankton growth and microzooplankton grazing.

Synechococcus bloom in the Pearl River Estuary and adjacent coastal area-With special focus on flooding during wet seasons.

Based on the field surveys aimed at understanding the variations of Synechococcus (Syn) abundance in the Pearl River Estuary during different seasons. We found that heavy terrestrial precipitation result in significant riverine runoffs and promote Syn growth, extension and blooms during warm and wet seasons. To understand the ecological role of Syn play in this estuary during wet seasons, we combined flow cytometry and high throughput sequencing (HTS) of 16S rDNA to investigate the phytoplankton distribution patterns and the potential shaping mechanisms during a typical wet season. During the cruise, picophytoplankton, especially Syn, and Nano-eukaryotes contributed importantly to the total phytoplankton biomass of the estuary. Syn can be further divided into phycoerythrin (PE)-rich Syn and phycocyanin (PC)-rich Syn, with PC-rich Syn about 1.5 times higher than PE-rich Syn in abundance. Both PE-rich Syn (60.75 × 103 cells ml-1) and PC-rich Syn (604.05 × 103 cells ml-1) reach the highest abundance at the lower part of the estuary. Moreover, PE-rich Syn can be divided into two subgroups which showed different salinity preference, with PE1 distributed in the high salinity area (with salinity >25) while PE2 in the middle salinity area (with salinity 7-20). Our results from the 16S rDNA sequencing also indicated abundant diversity and different niche adaptation of Syn with the operational taxonomic units (OTUs) along the estuary. Besides, analysis also indicated a tight correlation between estuarine Syn and active heterotrophic bacteria, especially groups of Rhodobacteria and Actionobacteria.

Bacterial growth efficiency in a partly eutrophicated bay of South China Sea: Implication for anthropogenic impacts and potential hypoxia events.

Bacterial metabolism plays a dual role [bacterial production (BP) and bacterial respiration (BR)] in the aquatic ecosystem and potentially leads to hypoxia in the coastal eutrophic area. Bacterial growth efficiency (BGE) is an important index showing the contribution of bacterial metabolism to marine biological production and carbon budget in the pelagic ecosystem. In this study, the spatial and seasonal variety as well as diurnal variation dynamics of BGE and associated ecological characteristics were investigated in a partly eutrophicated subtropical bay (the Daya Bay) located in the northern South China Sea. Furthermore, the relationship between bacterial metabolism and potential hypoxia event was analyzed. The average BGE was 0.14 and 0.22 in summer and winter, respectively, which was lower than the mean value ever reported in other coastal and estuarine waters. The diurnal variations of BGE and BP were widely fluctuated in the Daya Bay, with approximately 3-8 fold variation of BP and 2-3 fold variation of BR in different seasons, suggesting the importance of short-term ecological dynamics on evaluating the long-term ecological processes in the coastal waters. BR was the predominant contributor to the bacterial carbon demand; however, the variation of BGE was controlled by BP in both seasons. BGE was always high in the near-shore waters with higher eutrophic level and more active BP and BR. The bacterial metabolism could deplete dissolved oxygen (DO) in the Daya bay within about 9 days when the water body was enclosed and photosynthesis was prohibited. Therefore, low DO concentration and potential hypoxia was more likely to be found in the near-shore waters of the Daya Bay in summer, since the water was stratified and enclosed with poor water exchange capacity in this area. While in winter, hypoxia seldom occurred due to vertical mixing throughout the water column. Further biological-physical coupling research is recommended to find out the detailed formation mechanism of hypoxia in the bay, and to predict the potential hypoxia events and their environmental impacts in the future.

Planktonic community structure during a harmful bloom of Phaeocystis globosa in a subtropical bay, with special reference to the ciliate assemblages.

Planktonic community structure was investigated during outbreak of harmful Phaeocystis globosa bloom in a subtropical bay, the Maowei Sea, South China Sea. The phytoplankton assemblage was numerically dominated by colonial P. globosa, with its abundance ranging from 1.23 × 10(8) to 11.12 × 10(8) cells m(-3) and contributing nearly 90 % to the total abundance. Totally 66 mesozooplankton (>169 µm) and 19 ciliates species were recorded, with the densities ranged from 169 to 1633 ind m(-3) and 74 to 1118 cells L(-1), respectively. The dominant species for mesozooplankton were Copepoda (larvae), Bestiola sinicus, B. amoyensis, Macrura (larvae) and Acartia spinicauda, respectively. The ciliate assemblage was numerically dominated by Codonella rapa, Strombidium globosaneum and Mesodinium rubrum. During the bloom, P. globosa seemed to be negatively affected by the nutrient phosphate significantly (p < 0.05). However, no correlation between P. globosa and ciliate assemblage was detected, but P. globosa was negatively correlated with total biomass of mesozooplankton and abundance of B. sinicus (p < 0.05), suggesting that P. globosa was uncoupled from the grazing by both ciliates and mesozooplankton when appearing as colonies form. On the other hand, both positive and negative correlations among the dominant groups of mesozooplankton and ciliates were observed (p < 0.05) which possibly indicated that the predation of mesozooplankton upon ciliates might be strengthened during the Phaeocystis bloom and the complex effect also varied from species to species.

Biotransformation and detoxification of inorganic arsenic in Bombay oyster Saccostrea cucullata.

Arsenic (As) exists as the toxic inorganic forms in marine water and sediment, while marine oysters usually accumulate high As contents mostly as the less toxic organic forms. It has not yet been clear that how As is biotransformed in marine oysters. This study therefore investigated the biotransformation and detoxification of two inorganic As forms (As(III) and As(V)) in Bombay oyster Saccostrea cucullata after waterborne exposures for 30 days. Seven treatments of dissolved As exposure (clean seawater, 1, 5, 20 mg/L As(III), and 1, 5, 20 mg/L As(V)) were performed. Body As concentration increased significantly after all As exposure treatments except 1mg/L As(V). Total As, As(III), and As(V) concentration were positive correlated with glutathione-S-transferases (GST) activities, suggesting GST might play an important role in the As biotransformation and detoxification process. Organic As species were predominant in control and the low As exposed oysters, whereas a large fraction of As was remained as the inorganic forms in the high As exposed oysters, suggesting As could be biotransformed efficiently in the oysters in clean or light contaminated environment. The results of As speciation demonstrated the As biotransformation in the oysters included As(V) reduction, methylation to monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA), and subsequent conversion to arsenobetaine (AsB). More As was distributed in the subcellular metallothionein-like proteins fraction (MTLP) functioning sequestration and detoxification in the inorganic As exposed oysters, suggesting it was also a strategy for oysters against As stress. In summary, this study elucidated that marine oysters had high ability to accumulate, biotransform, and detoxify inorganic As.

[Characteristics of net phytoplankton community and their relationships to environmental factors in the waters around Nansha Islands].

Based on samples collected in the waters around Nansha Islands from August 25 to September 28, 2011, the characteristics of net phytoplankton community and their relationships to environmental factors were investigated. A total of 113 species, belonging to 34 genera of 3 phyla were identified, among which 57.5% belonged to Pyrrophyta and 40.7% belonged to Bacillariophyta. Ceratium in Pyrrophyta had the most species accounting for 30.1% of the 113 species. The average abundance of net phytoplankton was 2.12 x 10(4) cell x m(-3) and high abundances were encountered in the complex gyre adjacent to Reed Tablemount and in the Cyclonic Gyre adjacent to Wan'an Tan. Trichodesmium in Cyanophyta was the dominant functional group, taking up 77.0% of the total net phytoplankton abundance. Trichodesmium thiebautii, T. erythraeum and Pyrocystis noctiluca were the major dominant species. The dominant species varied with locations. Cyanophyta widely dominated at stations 3, 5, 6 and 10-14, Pyrrophyta were the dominant phytoplankton in the central locations at stations 4 and 7-9, while Bacillariophyta dominated only at the southernmost stations 1 and 2. The values of Shannon index and Pielou evenness index of net phytoplankton community were 3.10 and 0.62, respectively. The salinity, water temperature, contents of ammonium, nitrite, phosphate and silicate, as well as mesoscale gyres and the west Nansha coastal current were the important environmental factors affecting the characteristics of net phytoplankton community. The ordination plots by canonical correspondence analysis could well display the characteristics of net phytoplankton community and their relationships to environmental factors.