The contemporary distribution of benthic foraminifera in the Pearl River estuary, southeast China, and their use in reconstructing mid‐ to late‐Holocene fluvial flux

This thesis investigates the contemporary distribution of benthic foraminifera in the Pearl River estuary, southeast China, and establishes a new proxy record of estuarine salinity and fluvial flux over the mid‐ to late‐Holocene. Seventy‐six samples from the contemporary estuary yielded 118 benthic taxa. Ammonia beccarii dominates a large proportion of the estuary, with contributions from a number of other calcareous taxa, including Haynesina sp., Elphidium spp., Quinqueloculina spp. and Rotalinoides annectens, and agglutinated taxa, for example Haplophragmoides sp. and Ammobaculites formosensis. Species distribution maps, cluster analysis and unconstrained ordination suggest a link between foraminiferal assemblages and the magnitude of fluvial influence. Constrained ordination links the faunal data to recognised environmental gradients and highlights salinity as the most important factor in controlling species distributions. Transfer function development enables quantitative estimates of palaeosalinity to be obtained from fossil faunal data. A radiocarbon dated mid‐ to late‐Holocene sediment core from the same estuary yielded 102 benthic taxa, 83 % of which were also found in contemporary samples. Ammonia beccarii and Quinqueloculina akneriana dominate fossil assemblages. In light of relative stability in mid‐ to late‐Holocene sea level along the South China coast (Zong, 2004), the long‐term increase in taxa indicative of high salinity environments is primarily interpreted in terms of declining fluvial flux. The trends in quantitative palaeosalinity reconstructions are in agreement with qualitative interpretations, however absolute salinity values are subject to large error terms. The inferred decline in fluvial flux is in accord with bulk organic carbon isotope and diatom data from the same core (Zong et al., submitted a; Yu, 2009) and with records of weakening precipitation intensity derived from terrestrial proxies (e.g. An et al., 2000; Y. Wang et al., 2005; Hu et al., 2008). Millennial‐scale variability in the East Asian monsoon, linked to precession and obliquity cycles of solar insolation, is the hypothesised cause of these trends. A number of abrupt low discharge events, postulated to reflect northern hemisphere teleconnections, punctuate the Pearl River estuary record

Nitrate sources and dynamics in a salinized river and estuary : a δ15N-NO₃⁻ and δ18O-NO₃⁻ isotope approach

To trace NO3- sources and assess NO3- dynamics in salinized rivers and estuaries, three rivers (Haihe River: HH River, Chaobaixin River: CB River and Jiyun River: JY River) and two estuaries (HH Estuary and CJ Estuary) along the Bohai Bay (China) have been selected to determine dissolved inorganic nitrogen (DIN: NH4+, NO2- and NO3-. Upstream of the HH River, NO3- was removed 30.9 +/- 22.1% by denitrification, resulting from effects of the floodgate: limiting water exchange with downstream and prolonging water residence time to remove NO3-. Downstream of the HH River NO3- was removed 2.5 +/- 13.3% by NO3- turnover processes. Conversely, NO3- was increased 36.6 +/- 25.2% by external N source addition in the CB River and 34.6 +/- 35.1% by instream nitrification in the JY River. The HH and CY Estuaries behaved mostly conservatively excluding the sewage input in the CJ Estuary. Hydrodynamics in estuaries has been changed by the ongoing reclamation projects, aggravating the loss of the attenuation function of NO3- in the estuary

Validation and Application of SMAP SSS Observation in Chinese Coastal Seas

Using sea surface salinity (SSS) from the Soil Moisture Active Passive (SMAP) mission from September 2015 to August 2016, the spatial distribution and seasonal variation in SSS in the Chinese coastal seas were investigated. First, in situ salinity observation over Chinese East Sea was used to validate SMAP observation. Then, the SSS signature of the Yangtze River fresh water was analyzed using SMAP data and the river discharge data. The SSS around the Yangtze River estuary in the Chinese East Sea, the Bohai Sea and the Yellow Sea is significantly lower than that of the open ocean. The SSS of Chinese coastal seas shows significant seasonal variation, and the seasonal variation in the adjacent waters of the Yangtze River estuary is the most obvious, followed by that of the Pearl River estuary. The minimum value of SSS appears in summer while maximum in winter. The root-mean-squared difference of daily SSS between SMAP observation and in situ observation is around 3 psu in both summer and winter, which is much lower than the annual range of SSS variation. The path of fresh water from SMAP and in situ observation is consistent during summer time

Nitrification and inorganic nitrogen distribution in a large perturbed river/estuarine system: the Pearl River Estuary, China

We investigated the spatial distribution and seasonal variation of dissolved inorganic nitrogen in a large perturbed estuary, the Pearl River Estuary, based on three cruises conducted in winter (January 2005), summer (August 2005) and spring (March 2006). On-site incubation was also carried out for determining ammonium and nitrite oxidation rates (nitrification rates). We observed a year-round pattern of dramatic decrease in NH4+, increase in NO3-, but insignificant change in NO2- in the upper estuary at salinity similar to 0-5. However, species and concentrations of inorganic nitrogen at upper estuary significantly changed with season. In winter, with low runoff, the most upper reach of the Pearl River Estuary showed relatively low rates of ammonia oxidation (0-5.4 mu mol N L-1 d(-1)) and nitrite oxidation (0-5.2 mu mol N L-1 d(-1)), accompanied by extremely high concentrations of ammonia (up to >800 mu mol L-1) and nitrate (up to >300 mu mol L-1). In summer, the upper estuary showed higher nitrification rates (ammonia oxidation rate similar to 1.5-33.1 mu mol N L-1 d(-1), nitrite oxidation rate similar to 0.6-32.0 mu mol N L-1 d(-1)) with lower concentrations of ammonia ( <350 mu mol L-1) and nitrate ( <120 mu mol L-1). The Most Probable Number test showed relatively lower nitrifier abundance in summer at most sampling stations, indicating a greater specific nitrification rate per cell in the warm season. Temperature appeared to control nitrification rates to a large degree in different seasons. Spatial variability of nitrification rates appeared to be controlled by a combination of many other factors such as nutrient concentrations, nitrifier abundance and dissolved oxygen (DO) concentrations. In addition to aerobic respiration, nitrification contributed significantly to the consumption of DO and production of free CO2 at upper estuary. Nitrification-induced consumption accounted for up to approximately one third of the total water column community DO consumption in the upper estuary during the surveyed periods, boosting environmental stress on this large estuarine ecosystem.Natural Science Foundation of China [40521003, 40576036, 90711005, 90211020

Temporal variation in riverine organic carbon concentrations and fluxes in two contrasting estuary systems: Geum and Seomjin, South Korea

In this study, surface water samples were collected at sites located in the lowest reaches of closed (Geum) (i.e. with an estuary dam at the river mouth) and open (Seomjin) estuary systems between May 2016 and May 2018. We analyzed concentrations and stable isotopes of particulate organic carbon (POC) and dissolved organic carbon (DOC) to assess OC sources, to estimate fluxes of riverine OC, and to assess some of the factors driving OC exports in these two contrasting Korean estuary systems. Our geochemical results suggest that the contribution of the phytoplankton-derived POC to the total POC pool was larger in the Geum River than in the Seomjin River. Notably, a heavy riverine algae bloom occurred in the Geum River in August 2016, resulting in a high carbon isotopic composition (-19.4%) together with low POC/PN ratio (˂ 10) and POC/Chl-a ratio (˂ 100). In contrast, potential DOC sources in both the Geum River and the Seomjin River were a mixture of C3-derived forest soils and cropland organic matter. During the study period, the catchment area-normalized fluxes of POC and DOC were 0.40x10(-3) tC/km(2)/yr and 6.5x10(-2) tC/km(2)/yr in the Geum River and 5.2x10(-4) tC/km(2)/yr and 8.6x10(-4) tC/km(2)/yr in the Seomjin River, respectively. It appears that the POC flux was more weakly associated with the water discharge in the Geum River than in the Seomjin River, but the DOC fluxes were in general controlled by the water discharges in both rivers. Accordingly, the estuary dam of the Geum River might be one of the most strongly influencing factors on seasonal patterns in POC fluxes into the adjacent coastal seas, strongly modifying water residence times and thus biogeochemical processes.We would like to thank Dokyun Kim, Ji Hwan Hwang, Jong-Ku Gal, Dong-Hun Lee, Dahae Kim, and Solbin Kim for their assistance during fieldwork. This work was supported by the National Research Foundation of Korea (NRF) grants funded by the Ministry of Science and ICT (MSIT) -South Korea [NRF-2016R1A2B3015388, KOPRI-PN19100]

Temporal and spatial variations in nutrient stoichiometry and regulation of phytoplankton biomass in Hong Kong waters : influence of the Pearl River outflow and sewage inputs

Author Posting. © Elsevier B.V., 2008. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Marine Pollution Bulletin 57 (2008): 335-348, doi:10.1016/j.marpolbul.2008.01.020.In 2001, the Hong Kong government implemented the Harbor Area Treatment Scheme (HATS) under which 70% of the sewage that had been formerly discharged into Victoria Harbor is now collected and sent to Stonecutters Island Sewage Works where it receives chemically enhanced primary treatment (CEPT), and is then discharged into waters west of the Harbor. The relocation of the sewage discharge will possibly change the nutrient dynamics and phytoplankton biomass in this area. Therefore, there is a need to examine the factors that regulate phytoplankton growth in Hong Kong waters in order to understand future impacts. Based on a historic nutrient data set (1986-2001), a comparison of ambient nutrient ratios with the Redfield ratio (N:P:Si=16:1:16) showed clear spatial variations in the factors that regulate phytoplankton biomass along a west (estuary) to east (coastal/oceanic) transect through Hong Kong waters. Algal biomass was constrained by a combination of low light conditions, a rapid change in salinity, and strong turbulent mixing in western waters throughout the year. Potential stoichiometric Si limitation (up to 94% of the cases in winter) occurred in Victoria Harbor due to the contribution of sewage effluent with high N and P enrichment all year, except for summer when the frequency of stoichiometric Si limitation (48%) was the same as P, owing to the influence of the high Si in the Pearl River discharge. In the eastern waters, potential N limitation and N and P co-limitation occurred in autumn and winter respectively, because of the dominance of coastal/oceanic water with low nutrients and low N:P ratios. In contrast, potential Si limitation occurred in spring and a switch to potential N, P and Si limitation occurred in eastern waters in summer. In southern waters, there was a shift from P limitation (80%) in summer due to the influence of the N-rich Pearl River discharge, to N limitation (68%) in autumn, and to N and P co-limitation in winter due to the dominance of N-poor oceanic water from the oligotrophic South China Sea. Our results show clear temporal and spatial variations in the nutrient stoichiometry which indicates potential regulation of phytoplankton biomass in HK waters due to the combination of the seasonal exchange of the Pearl River discharge and oceanic water, sewage effluent inputs, and strong hydrodynamic mixing from SW monsoon winds in summer and the NE monsoon winds in winter.Financial support for this research was provided by the University Grants Council of Hong Kong AoE project (AoE/P-04/0401), and RGC project HKUST6478/05M. Support was also provided (to DMA) by U.S. National Science Foundation grants OCE-0402707 and OCE-0430724 and by NIEHS grant 1 P50-ES01274201

Downwelling wind, tides, and estuarine plume dynamics

Author Posting. © American Geophysical Union, 2016. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 121 (2016): 4245–4263, doi:10.1002/2015JC011475.The estuarine plume dynamics under a downwelling-favorable wind condition were examined in the windy dry season of the Pearl River Estuary (PRE) using the PRE primitive-equation Finite-Volume Community Ocean Model (FVCOM). The wind and tide-driven estuarine circulation had a significant influence on the plume dynamics on both local and remote scales. Specifically, the local effect of downwelling-favorable winds on the plume was similar to the theoretical descriptions of coastal plumes, narrowing the plume width, and setting up a vertically uniform downstream current at the plume edge. Tides tended to reduce these plume responses through local turbulent mixing and advection from upstream regions, resulting in an adjustment of the isohalines in the plume and a weakening of the vertically uniform downstream current. The remote effect of downwelling-favorable winds on the plume was due to the wind-induced estuarine sea surface height (SSH), which strengthened the estuarine circulation and enhanced the plume transport accordingly. Associated with these processes, tide-induced mixing tended to weaken the SSH gradient and thus the estuarine circulation over a remote influence scale. Overall, the typical features of downwelling-favorable wind-driven estuarine plumes revealed in this study enhanced our understanding of the estuarine plume dynamics under downwelling-favorable wind conditions.National Natural Science Foundation of China Grant Number: (41206005); Ocean Public Welfare Scientific Research Project, State Oceanic Administration of the People's Republic of China Grant Number: (201305019-3)2016-12-2

Phytoplankton abundance and size-fractionated structure in three contrasting periods in the Pear River Estuary

Phytoplankton abundance, composition and size-fractionated chlorophyll a (micro-, nano- and pico-chl. a) in the Pearl River Estuary (PRE), South China were assessed in three surveys (Aug. 2010, Jan. 2011 and Aug. 2011) to identify key environmental indicators that shape their distribution patterns. Non-metric multidimensional scaling (MDS) showed that the PRE formed three regional clusters that were characterized by a transition from dominant freshwater species to estuarine diatoms in normal summer flow conditions (Aug. 2010). With a reduced river flow in winter (Jan. 2011), the inner estuary was categorized as one group marked by the predominance of the nanoflagellate, Plagioselmis prolonga. This small-sized cryptophyte was first recorded in the PRE and was likely to outcompete other phytoplankton species in turbid or well-mixed waters. During the extreme drought of summer of 2011 (Aug. 2011), the estuarine plume was limited and regional division by MDS was similar to winter patterns, as some marine species were observed in the upper estuary. We considered that the higher phytoplankton density in the summer of 2011 was presumably a combined effect of longer residence time and higher phosphate concentration. With respect to the size-fractionated phytoplankton composition, our results showed that almost half of the chl. a in the estuary was contained in microplankton particles, while the proportion of pico-chl. a increased from the upstream of the PRE towards the estuary boundary (Wanshan Islands). Furthermore, phytoplankton abundance, three size classes of chl. a and environmental factors were explored by principal component regression (PCR) analysis. In three surveys, pico-chl. a was negatively correlated with the first principal component (PCL, positively loaded with salinity and inversely with inorganic nutrients), which indicated a negative influence of the riverine and coastal waters on picophytoplankton and the specific oligotrophic niche of picophytoplankton. Similarly, PC1 was also considered as the key environmental variable basis controlling micro-chl. a in summer of 2010, while in the summer of 2011, zooplankton and copepods were positively associated with phytoplankton abundance, suggesting a resource effect of phytoplankton on zooplankton development

Comunidad de cianobacterias en los sedimentos del estuario del río Perla en China

Cyanobacterial community diversity in the sediment of the Pearl River Estuary in China was evaluated in this study by denaturing gradient gel electrophoresis (DGGE) during the wet and dry seasons. Nucleotide sequences obtained from DGGE bands were classified into five cyanobacterial clusters, including Synechococcus, Cyanobium, Chroococcus, Prochlorales and Tolypothrix. Synechococcus was identified as the dominant cyanobacterial group in the sediment samples; its distribution varied from the inner estuary to the outer estuary, with a wide range of salinity adaptation. Observed patterns of cyanobacterial communities changed markedly between sampling sites and seasons, suggesting that most cyanobacteria were not delivered via fresh water. Canonical correspondence analysis was conducted to determine the relationship between environmental variables and bacterial community structures during the dry season. The results suggested that the cyanobacterial community was significantly influenced by pH, salinity, PO4-P and NO3-N in sediments.La diversidad de la comunidad de cianobacterias en el sedimento del estuario del río Perla en China fue evaluada en este estudio por electroforesis en gel de gradiente desnaturalizante (DGGE) durante las estaciones húmeda y seca. Las secuencias de nucleótidos obtenidas de bandas DGGE se clasificaron en cinco grupos de cianobacterias, incluyendo Synechococcus, Cyanobium, Chroococcus, Prochlorales y Tolypothrix. Synechococcus fue identificado como el grupo dominante de cianobacterias en las muestras de sedimento, su distribución varió desde la parte interna del estuario hasta la externa, con un amplio rango de adaptación a la salinidad. Los patrones observados de las comunidades de cianobacterias cambiaron marcadamente entre diferentes sitios de muestreo en diferentes estaciones del año y sugirió que la mayoría de las cianobacterias no provenían a través del agua dulce. Se realizó un análisis de correspondencia canónica (CCA) para determinar la relación entre variables ambientales y estructuras de las comunidades bacterianas durante la estación seca. Los resultados sugirieron que las distintas comunidades de cianobacterias estaban significativamente influenciadas por el pH, salinidad, PO4-P y NO3-N en los sedimentos