Coastal East China Sea Research Articles

Aerosol sources and transport paths co-control the atmospheric bacterial diversity over the coastal East China Sea

Airborne bacteria along with chemical composition of aerosols were investigated during five sampling seasons at an offshore island of the East China Sea. Bacterial diversity was the lowest in spring, the highest in winter, and similar between the autumns of 2019 and 2020, suggesting remarkably seasonal variation but little interannual change. Geodermatophilus (Actinobacteria) was the indicator genus of mineral dust (MD) showed higher proportion in spring than in other seasons. Mastigocladopsis_PCC-10914 (Cyanobacteria) as the indicator of sea salt (SS) demonstrated the highest percentages in both autumns, when the air masses mainly passed over the ocean prior to the sampling site. The higher proportions of soil-derived genera Rubellimicrobium and Craurococcus (both Proteobacteria) and extremophile Chroococcidiopsis_SAG_2023 (Cyanobacteria) were found in summer and winter, respectively. Our study explores the linkage between aerosol source and transport path and bacterial composition, which has implication to understanding of land-sea transmission of bacterial taxa.

Changes in dinoflagellate and diatom blooms in the East China Sea over the last two decades, under different spatial and temporal scale scenarios

Frequent algal blooms in the nearshore area of the East China Sea (ECS) pose a serious threat to both the marine environment and human health. Climate and environmental changes play an important role in the occurrence of diatoms and dinoflagellates blooms. Using the MODIS-Aqua 1-km satellite observations, the outbreaks of dinoflagellate and diatom blooms in the ECS coast in summer during 2003–2022 were mapped. Our results found that although the bloom frequency of dinoflagellate was consistently higher than diatoms, its bloom intensity showed a slightly decline trend in recent decades. The driving factors analysis showed that river runoff and sediments discharge played different effect on the formation of diatom and dinoflagellate blooms. Besides, our results compared the effect of El Niño and La Niña on bloom occurrences. This study was supposed to provide detailed insights into algal blooms, with important implications for relevant meteorological and climate changes in coastal regions.

Distribution and sources of polycyclic aromatic hydrocarbons in surface sediments of the East China marginal seas: Significance of the terrestrial input and shelf mud deposition

To investigate the distribution, sources, influencing factors, and ecological risk of polycyclic aromatic hydrocarbons (PAHs) in East China Marginal Seas (ECMSs) sediments, we measured the concentrations of 16 PAHs in 104 surface sediment samples collected from the ECMSs in 2014 and 2016. Total PAH concentration (∑PAHs) ranged from 4.49 to 163.66 ng/g dry weight (dry w), with 65.98 ± 33.00 (mean ± SD) ng/g dry w. The highest PAH concentrations and total organic carbon were observed in areas with fine-grained sediments in the Bohai Sea (BS), Yellow Sea (YS), and coastal East China Sea (ECS), indicating the prominent influence of regional hydrodynamics and sediment properties. The dominant PAH congener in BS and YS was BbF, whereas coastal ECS was Phe. The heterogeneity of PAH sources implies that terrestrial PAH input and shelf mud deposition have crucial roles in the source–sink processes of PAHs in a strongly human-influenced marginal sea.

Considering species functional and phylogenetic rarity in the conservation of fish biodiversity

AbstractAimRare species make substantial contributions to coastal ecosystem functions. Functional rarity (FR) and phylogenetic rarity (PR) are important features for biodiversity conservation. This work aimed to discuss the necessity and reasonableness of conserving fish FR and PR in coastal seas.LocationChina.MethodsBy compiling historical fish investigation data, joint species distribution modelling (JSDM) was applied to model fish communities in coastal China Seas. Biogeographic patterns of FR and PR were explored, and the effectiveness of current MPA networks in terms of match/mismatch with the hotspots of rarity was assessed.ResultsA total of 44 functionally rare species and 22 phylogenetically rare species were identified. Six of these species were both functionally and phylogenetically rare, and only one was listed as endangered on the Red List of the International Union for Conservation of Nature (IUCN). Functional rarity hotspots covered 10.27% of the coastal areas, which geographically converged in the southern and eastern coast of Taiwan, the Yangtze River Estuary and the Yellow River Estuary. Phylogenetic rarity hotspots only covered 3.06% of the coastal areas, which were sporadically distributed in the coastal East China Sea, the Bohai Sea and the northern Yellow Sea. Current marine‐protected areas (MPAs) only represented 16.16% of the FR hotspots and 20.48% of the PR hotspots, indicating substantial mismatched areas between the MPAs and the hotspots of FR and PR.Main ConclusionsOnly considering threatened species in conservation practices will omit functionally and phylogenetically rare species because FR and PR are not necessarily correlated with species threat status on the IUCN Red List. Functional rarity hotspots do not necessarily overlap with PR hotspots, and current MPAs mismatch the majority of these areas. We therefore advocate that conservation prioritization and expansion of MPA networks should account for FR and PR both at the species and site levels.

East Asian monsoon manipulates the richness and taxonomic composition of airborne bacteria over China coastal area

Airborne bacteria may have significant impacts on aerosol properties, public health and ecosystem depending on their taxonomic composition and transport. This study investigated the seasonal and spatial variations of bacterial composition and richness over the east coast of China and the roles of East Asian monsoon played through synchronous sampling and 16S rRNA sequencing analysis of airborne bacteria at Huaniao island of the East China Sea (ECS) and the urban and rural sites of Shanghai. Airborne bacteria showed higher richness over the land sites than Huaniao island with the highest values found in the urban and rural springs associated with the growing plants. For the island, the maximal richness occurred in winter as the result of prevailing terrestrial winds controlled by East Asian winter monsoon. Proteobacteria, Actinobacteria and Cyanobacteria were found to be top three phyla, together accounting for 75 % of total airborne bacteria. Radiation-resistant Deinococcus, Methylobacterium belonging to Rhizobiales (related to vegetation) and Mastigocladopsis_PCC_10914 originating from marine ecosystem were indicator genera for urban, rural and island sites, respectively. The Bray-Curits dissimilarity of taxonomic composition between the island and two land sites was the lowest in winter with the representative genera over island also typically from the soil. Our results reveal that seasonal change of monsoon wind directions evidently affects the richness and taxonomic composition of airborne bacteria in China coastal area. Particularly, prevailing terrestrial winds lead to the dominance of land-derived bacteria over the coastal ECS which may have a potential impact on marine ecosystem.

Investigation of physiological energetic response of the thick shell mussel, Mytilus coruscus, to microplastics and low salinity: Potential countermeasures to multi-environmental changes

Microplastics (MPs) contamination and salinity fluctuation are two common phenomena on the East China Sea coast. The purpose of this experiment was to evaluate the combined effects of MPs and salinity on the energy budget of the thick shell mussel Mytilus coruscus. Juvenile mussels were exposed to six combined treatments with three MPs levels (0, 10 and 1000 items L−1) and two salinity levels (15, 25) for 28 d. The clearance rate, food absorption efficiency, respiration rate, ammonium excretion rate, and O:N ratio were significantly reduced, and the fecal organic dry weight ratio was significantly increased at salinity 15, inducing a reduction in the scope for growth. Under salinity 15, 1000 items L−1 MPs had significant negative effects on growth at Day 28. However, under salinity 25, the results were opposite in that the level of 1000 items L−1 MPs brought an obvious increment in scope for growth at Day 28. This meant that there were interactions between salinity and MPs in scope for growth at Day 28. These results suggest that the physiological energetics of juvenile M. coruscus can acclimate to MPs contamination with little physiological effect under normal salinity. This experiment has revealed that although marine MPs contamination is of concern, marine organisms likely have a significant capacity to adapt to it.

Japan, Okinawa, East China Sea Coast

Japan, Okinawa, East China Sea Coast

Gonyaulax geomunensis sp. nov. and two allied species (Gonyaulacales, Dinophyceae) from Korean coastal waters and East China Sea: morphology, phylogeny and growth response to changes in temperature and salinity

ABSTRACT Six strains of three different Gonyaulax species were established by isolating cells from the Korean coastal area and the East China Sea, and their morphologies and molecular phylogenies based on SSU and LSU rRNA gene sequences were examined. In addition, the growth responses of the Gonyaulax species to changes in temperature and salinity were investigated. Based on morphological features and phylogenetic positions, Gonyaulax whaseongensis and G. polygramma were identified, and G. geomunensis sp. nov. is proposed in this study. These species displayed the plate formula typical for Gonyaulax, but G. polygramma and G. geomunensis sometimes showed a small intercalary plate (1a) surrounded by plates 2’, *3’ and 3’’. G. geomunensis was morphologically characterized by an S-type ventral organization, descending with a displacement of one cingulum width and bearing one, two or three prominent antapical spines of similar size. The cell surface, which was distinct from other Gonyaulax species, was thick and heavily reticulated into numerous polygonal areas. The reticulation was deeply excavated. The phylogenetics revealed that G. geomunensis and G. whaseongensis belong to different clades, and that there are two ribotypes of G. polygramma, which were morphologically indistinguishable. These species had a close phylogenetic relationship to G. hyalina, and all of them were characterized by dextral torsion. G. whaseongensis, G. polygramma and G. geomunensis had different growth responses to changes of temperature and salinity, respectively, indicating that morphological and phylogenetic identification of Gonyaulax species can be supported by ecological niches.

Effect of dissolved organic nutrients on the bloom of Prorocentrum donghaiense in the East China Sea coastal waters

Prorocentrum donghaiense blooms occur annually in the East China Sea coastal waters, degrading ecosystem functions and impeding economic development. Dissolved organic nitrogen and phosphorus (DON and DOP) are the main components in the marine nutrient pools and are closely related to harmful algal blooms. From April to June 2019, a survey was conducted along the East China Sea coast (Sansha and Lianjiang counties) to investigate the relationship between dissolved organic nutrients and P. donghaiense bloom. Our findings showed that dinoflagellates dominated the phytoplankton community, and dissolved organic nutrients were the major factors influencing community structure during the P. donghaiense bloom. Redundancy analysis indicated that P. donghaiense abundance was primarily affected by DON in the Sansha area while it was primarily affected by DON and DOP in the Lianjiang area. Correlation analysis also confirmed a strong positive correlation between dissolved organic nutrients and P. donghaiense abundance both in the Sansha and Lianjiang coastal areas (p < 0.001). Furthermore, a culture experiment was carried out during the bloom to further investigate the effect of dissolved organic nutrients on the phytoplankton community structure. After 10 days of culture, dinoflagellates' relative abundance decreased from 97.1% to 28.2% in the inorganic treatment, whereas dinoflagellates continued to dominate the phytoplankton community in the organic treatment (76.9%). As a result, we propose that dissolved organic nutrients are responsible for the P. donghaiense bloom outbreak and promote the phytoplankton community shift from diatoms to dinoflagellates.

Archaeobotanical evidence of plant cultivation from the Sanbaopi site in south-western Taiwan during the Late Neolithic and Metal Age

Despite decades of lively debate about Taiwan’s role in the spread of early agriculture, crops and cultivation practices to the Indo-Pacific region, there is little archaeobotanical data from the island. Here we present the first directly dated and systematically analysed macrobotanical records from Taiwan obtained by flotation at the archaeological site Sanbaopi 5 (23°07′03′′N, 120°15′32′′E), representing the Dahu (1400 BCE–100 CE) and Niaosong (100–1400 CE) culture periods. The results suggest that Middle Dahu (900–100 BCE) communities in the study area practiced rainfed crop cultivation with mainly foxtail ( Setaria italica) and broomcorn ( Panicum miliaceum) millet and rice ( Oryza sativa). Pulses ( Vigna angularis, Glycine soja/ max) were also part of the subsistence of local farmers and used as supplementary food and/or green manure. The archaeobotanical record together with archaeological site data for prehistoric China substantiates evidence that the Dahu culture originates in the Lower Yellow River region and migrated to Taiwan along the East China Sea coast. The emergence of the Dahu culture coincided with the spread of mixed millet-rice farming to the Korean Peninsula and Japan and was possibly related to enhanced economic and political expansion of the Shang and Western Zhou dynasties and the long-term weakening of summer monsoon precipitation. Pigeon pea ( Cajanus cajan) and mung bean ( V. radiata var. radiata) assemblages from the sixth century CE Niaosong period highlight the influx of goods, crops, knowledge and people from South and Southeast Asia via southern routes in the context of enhanced exchange across the South China Sea region.

The influence of anthropogenic activities on heavy metal pollution of estuary sediment from the coastal East China Sea in the past nearly 50 years

Historical records of eight heavy metals (HMs: Cu, Zn, Cr, Ni, Pb, As, Hg and Cd) were analyzed in two dated sediment cores collected from the coastal East China Sea (Meishan Bay, MSB) to investigate the influence of anthropogenic activities on estuary sediment from 1972 to 2019. The sedimentary records of HMs in the two cores reflected the development of economy and change of energy consumption in China's east coastal areas. The contents of most HMs in sediments had no change or showed a downward trend before the 1980s. After the 1980s, the contents of HMs showed an increasing trend, mainly because of the rapid development of aquaculture, industry, and agriculture. The distribution of HMs outside the bay (OB) was mainly affected by industry, shipping, and agriculture (vegetable planting), while HMs in sediments inside the bay (IB) was affected by shipping, agriculture, and aquaculture. Principle component analysis (PCA) showed that Cd, Cr, and Ni could be attributed to industrial discharge, while As, Cu, Pb, and Zn were associated with shipping, agriculture, and aquaculture. Sediments were slightly polluted with Cd (Igeo:0.10–0.71, CF:1.90–7.74) and Ni (Igeo:0.08–0.92, CF:1.25–12.55), and seriously polluted with Hg (Igeo:0.95–1.76, CF:13.43–85.65). This study provides insights for the local governments to raise awareness of HM pollution in the coastal East China Sea and formulate corresponding pollution control measures.

Linking Bacterial Communities to Optical-Derived Properties of Porewater DOM in Sediments in the Coastal East China Sea

Bacterial communities and porewater dissolved organic matter (DOM) pool are intimately interactive in sedimentary environments. Estuarine coastal regions are an interactive area between terrestrial and marine influences in terms of DOM origins and freshness. Yet, we know little about the relationships between the bacterial communities and DOM in those regions. In this study, porewater DOM samples were collected from 42 sites in the coastal East China Sea. The porewater DOM optical properties were determined by fluorescence and absorption spectra, while the corresponding bacterial community compositions of those sediments were examined by 16S rRNA gene high-throughput sequencing. The results showed that bacterial species richness was positively correlated with multiple terrestrial indicators based on the optical properties of DOM, which implied that heterogeneous DOM from terrestrial origins might harbor a wider spectrum of bacterial taxa in marine sediments. The analysis of the co-occurrence network of the bacterial communities showed that the edges and density for samples with low DOM freshness were 3.4 times and 3 times those for the samples with high DOM freshness, respectively. This suggested that the connection among the bacterial taxa under the lower DOM freshness condition were enhanced and that reduced freshness of DOM may encourage more complimentary utilization of resources. The findings provide a new insight into such interactive processes of heterogeneous organic matter utilization meditated by microorganisms in coastal sediments.

Mercury and selenium bioaccumulation in wild commercial fish in the coastal East China Sea: Selenium benefits versus mercury risks

This study investigated the contents of total mercury (THg), methylmercury (MeHg) and selenium (Se) in 22 fish species and 10 invertebrate species from the coastal East China Sea. The THg and MeHg contents were significantly higher in benthic fishes. Both Hg and Se biomagnified in the food webs, with evidences of associations during trophic transfer. In addition, Se:Hg molar ratio and Se health benefit value (HBVSe) were used as novel criteria for Hg exposure risk assessments, showing that Se presented in molar excess of Hg in all samples, which would negate the risks of Hg toxicity. HBVSe provided more informative results than Se:Hg molar ratio, pointing to possibly lower health risks for some fishes containing high levels of Hg and Se. Although the HBVSe results challenge the traditional Hg health risk assessment, its future application still requires worldwide comprehensive investigations.

Transparent exopolymer particle (TEP) and its impact on marine carbon transport along the East China Sea coast

Transparent exopolymer particle (TEP) is a transparent jellylike substance characterized with high carbon concentration, playing an important role in the marine carbon cycle. Based on a survey of the East China Sea coast in summer of 2020, the distribution, controlling factors of TEP and its impact on marine carbon transportation were discussed. The results showed that the TEP in the surface decreases from northwest to southeast gradually with an average concentration of 214.90 µg Xeq./L, which is basically consistent with the distribution of POC, Chl a and NO3-N. The TEP in the water of 10 m, 20 m, and 30 m also gradually decreases from northwest to southeast horizontally, while it is high in the surface and bottom layer, and low in the middle water layer vertically. The hydrological situation of the study area is complex and is the main factor affecting the distribution of various parameters including TEP. The TEP in the seawater has the characteristics of high TEP-C/POC, low settling ratio and high degradation ratio. The TEP-C/POC in the seawater is 32.03%, which is extremely higher than that of 14.46% in the surface sediments; the settling ratio of TEP is 17.67%, which is extremely lower than that of 50.31% for POC; the degradation ratio of TEP is 82.79%, which is extremely higher than that of 49.69% for POC. These results together indicated that TEP in the seawater has a longer residence time and is more easily degraded than other forms of POC.

The distribution of intact polar lipid-derived branched tetraethers along a freshwater-seawater pH gradient in coastal East China Sea

In view of the application of microbial branched tetraether lipids (i.e., brGDGTs) in terrestrial pH reconstructions, their potential as seawater pH proxy is investigated from the Yangtze River Estuary (YRE) to the shelf region in the East China Sea (ECS). BrGDGTs occurring as ‘fossil’ core and in ‘living’ intact polar lipids (CL-brGDGTs and IPL-brGDGTs, respectively) in surface sediments are separately analyzed. The results show that riverine IPL-brGDGTs are rapidly lost at the transition from the YRE to the marine settings and sedimentary IPL-brGDGTs are predominantly produced in situ in brackish water environments. Among environmental parameters, surface water pH is a determinant factor controlling the cyclization of IPL-brGDGTs, with higher cyclization at higher pH. Hereby, two IPL-brGDGTs-based seawater pH calibrations are established in the brackish seawater environment in this study: surface pH = 7.28 + 1.08 × #Ringstetra (n = 18, r2 = 0.87, RMSE = 0.05, p < 0.01) and surface pH = 7.13 + 3.82 × f(Ic) + 4.29 × f(IIb') + 4.21 × f(IIIa) (n = 18, r2 = 0.90, RMSE = 0.04, p < 0.01), which are different from those established in terrestrial environments. This is the first work demonstrating that in situ produced microbial brGDGTs can trace seawater pH change in marine environment, which makes it promising to apply brGDGTs for seawater pH reconstructions in marine settings where the terrestrial inputs are negligible.

Non‐Marine Sources Contribute to Aerosol Methanesulfonate Over Coastal Seas

AbstractMethanesulfonate (MSA) in the marine boundary layer is commonly considered to be solely contributed by the oxidation of ocean‐derived dimethyl sulfide (DMS) and often used as an indicator of marine biogenic sources. But whether this judgment is valid in coastal seas and how the validity is affected by air mass transport history have been less discussed. Based on multi‐year observations of aerosol MSA in the coastal East China Sea (ECS) and the Gulf of Aqaba (GA), as well as the analysis of air mass transport pattern and exposure to ocean surface phytoplankton biomass, we found that terrestrial sources made a non‐negligible contribution to MSA over the ECS but not over the GA. The abundant MSA in winter over the coastal ECS was likely associated with substantial emissions of volatile organic sulfur compounds from both anthropogenic and natural sources in eastern China and significant terrestrial transport influenced by the East Asian Monsoon. Good correlations between aerosol MSA and air mass exposure to surface phytoplankton biomass were established by removing the influence of terrestrial transport and confining the air transport height within boundary layer, which makes it possible to construct parameterizations for obtaining the spatiotemporal distributions of marine biogenic aerosol components using satellite ocean color datasets.

Metagenomic insights into co-proliferation of Vibrio spp. and dinoflagellates Prorocentrum during a spring algal bloom in the coastal East China Sea

Coastal harmful algal blooms (HABs), commonly termed ‘red tides’, have severe undesirable consequences to the marine ecosystems and local fishery and tourism industries. Increase in nitrogen and/or phosphorus loading is often regarded as the major culprits of increasing frequency and intensity of the coastal HAB; however, fundamental understanding is lacking as to the causes and mechanism of bloom formation despite decades of intensive investigation. In this study, we interrogated the prokaryotic microbiomes of surface water samples collected at two neighboring segments of East China Sea that contrast greatly in terms of the intensity and frequency of Prorocentrum-dominated HAB. Mantel tests identified significant correlations between the structural and functional composition of the microbiomes and the physicochemical state and the algal biomass density of the surface seawater, implying the possibility that prokaryotic microbiota may play key roles in the coastal HAB. A conspicuous feature of the microbiomes at the sites characterized with high trophic state index and eukaryotic algal cell counts was disproportionate proliferation of Vibrio spp., and their complete domination of the functional genes attributable to the dissimilatory nitrate reduction to ammonia (DNRA) pathway substantially enriched at these sites. The genes attributed to phosphorus uptake function were significantly enriched at these sites, presumably due to the Pi-deficiency induced by algal growth; however, the profiles of the phosphorus mineralization genes lacked consistency, barring any conclusive evidence with regard to contribution of prokaryotic microbiota to phosphorus bioavailability. The results of the co-occurrence network analysis performed with the core prokaryotic microbiome supported that the observed proliferation of Vibrio and HAB may be causally associated. The findings of this study suggest a previously unidentified association between Vibrio proliferation and the Prorocentrum-dominated HAB in the subtropical East China Sea, and opens a discussion regarding a theoretically unlikely, but still possible, involvement of Vibrio-mediated DNRA in Vibrio-Prorocentrum symbiosis. Further experimental substantiation of this supposed symbiotic mechanism may prove crucial in understanding the dynamics of explosive local algal growth in the region during spring algal blooms.

Diurnal Variation of the Diffuse Attenuation Coefficient for Downwelling Irradiance at 490 nm in Coastal East China Sea

The diurnal variation of the diffuse attenuation coefficient for downwelling irradiance at 490 nm (Kd(490)) has complex characteristics in the coastal regions. However, owing to the scarcity of in situ data, our knowledge on the diurnal variation is inadequate. In this study, an optical-buoy dataset was used to investigate the diurnal variation of Kd(490) in the coastal East China Sea, and to evaluate the Kd(490) L2 products of geostationary ocean color imager (GOCI), as well as the performance of six empirical algorithms for Kd(490) estimation in the Case-2 water. The results of validation show that there was high uncertainty in GOCI L2 Kd(490), with mean absolute percentage errors (MAPEs) of 69.57% and 68.86% and root mean square errors (RMSEs) of 0.70 and 0.71 m−1 compared to buoy-measured Kd12(490) and Kd13(490), respectively. Meanwhile, with the coefficient of determination (R2) of 0.71, as well as the lowest MAPE of 27.31% and RMSE of 0.29 m−1, the new dual ratio algorithm (NDRA) performed the best in estimating Kd(490) in the target area, among the six algorithms. Further, four main types of Kd(490) diurnal variation were found from buoy data, showing different variabilities compared to the area closer to the shore. One typical diurnal variation pattern showed that Kd(490) decreased at flood tide and increased at ebb tide, which was confirmed by GOCI images through the use of NDRA. Hydrometeorological factors influencing the diurnal variations of Kd(490) were also studied. In addition to verifying the predominant impact of tide, we found that the dominant effect of tide and wind on the water column is intensifying sediment resuspension, and the change of sediment transport produced by them are secondary to it.

Invasive Spartina alterniflora changes the Yangtze Estuary salt marsh from CH4 sink to source

Expansion of Spartina alterniflora salt marshes in the East China Sea coast is changing the ecosystem and thus, uncertainty has hampered methane (CH4) flux accounts in these areas. To analyse seasonal and diurnal patterns of the CH4 fluxes and their relationship with environmental factors, four plots were established in a salt marsh of the Nanhui coast in the southern fringe of the Yangtze River estuary, differing in sediment salinity and vegetation history and including one bare mudflat. Monthly studies from March 2017 to January 2018 using a chamber technique showed that CH4 fluxes from the plots ranged from −1.7 to 72.2 mg m−2 h−1. The mature Spartina sites showed higher CH4 emission, peaking in the summer. In the mudflat CH4 consumption was observed in January and the summer. The seasonal CH4 fluxes showed positive correlation (p < 0.05) with temperature, and plant development (height of vegetation), and negative correlation (p < 0.05) with water salinity. Various diurnal cycles in the CH4 fluxes were observed at different seasons. Average CH4 emissions were higher during the daytime than at night, however, without significant difference. Thus, the CH4 fluxes started to rise at noon, and the maximum CH4 flux was observed after the ebb tide (at 18:00) during nighttime. The diurnal variation in CH4 fluxes showed a significant correlation with season but not with temperature.

Numerical Study of the Three Gorges Dam Influences on Chlorophyll-a in the Changjiang Estuary and the Adjacent East China Sea

The impoundment of the Three Gorges Dam (TGD) has altered downstream hydrological characteristics and sediment discharge, and it has caused ecological impacts, such as changes in chlorophyll-a (Chl-a) in estuaries and coastal oceans. To investigate the TGD’s influence on Chl-a’s interannual and seasonal variations in the Changjiang Estuary and the adjacent coastal East China Sea, a physical-biogeochemical model was developed with numerical experiments covering a decade, including TGD’s pre-period (pre-TGD, 1999–2003) and post-period (post-TGD, 2004–2008). The modeling results demonstrate an annual increase in the regional average Chl-a from pre- to post-TGD, with the largest increase reaching up to 20.8% in spring in the nearshore region beyond the Changjiang mouth. The spatial variations in Chl-a are high, with the largest variation being observed around the 20–40 m isobaths. The increase in Chl-a is influenced by improved light and nutrient conditions, which is attributed to dam construction and fertilization by human activities. A decline in nitrogen-phosphorus fertilizer usage ratio along the Changjiang River watershed after the TGD’s impoundment is another factor that influences the Chl-a’s variation. The modeling results also show severe NO3 and PO4 imbalances with a larger N/P ratio during the post-TGD period, especially in regions with large Chl-a increases. Moreover, the greater increase in the usage of phosphorus fertilizer than nitrogen fertilizer influences Chl-a’s variation and has potential promotion effects on the outbreak of harmful algal bloom events.