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Chen T.-Y.,Research Center for Environmental Changes | Tai J.-H.,Research Center for Environmental Changes | Ko C.-Y.,Research Center for Environmental Changes | Hsieh C.-h.,Institute of Oceanography | And 3 more authors.
Environmental Microbiology | Year: 2016

This study demonstrated the potential effects of internal waves (IWs) on heterotrophic bacterial activities for the first time. Nine anchored studies were conducted from 2009-2012 in the South China Sea areas with different physical conditions, i.e. areas subjected to elevation IWs, to depression IWs, and to weak/no IWs. The latter two areas were treated as the Control sites. Field survey results indicated that within the euphotic zone, the minima of the depth-averaged bacterial production (IBP; ∼1.0 mgC m-3 d-1) and growth rate (IBμ; ∼0.1 d-1) at all sites were similar. Except for one case, the maxima of IBP (6-12 mgC m-3 d-1) and IBμ (0.55-1.13 d-1) of the elevation IWs areas were ∼fivefolds higher than those of the Control sites (IBP 1.7-2.1 mgC m-3 d-1; IBμ 0.13-0.24 d-1). Replicate surveys conducted at the north-western area of the Dongsha atoll during spring-to-neap (NW1 survey) and neap-to-spring (NW2 survey) tide periods showed a great contrast to each other. Low variation and averages of IBμ in NW1 survey were similar to those of the Control sites, while those in NW2 were similar to the other elevation IWs sites with larger variation and higher averages of IBμ. This finding suggests that bacterial activities may be a function of the lunar fortnightly (14-day) cycle. Enrichment experiments suggested more directly that the limiting inorganic nutrients introduced by the elevation waves (EIWs) may contribute a higher IBμ within the euphotic zone. © 2016 The Authors. Environmental Microbiology published by Society for Applied Microbiology and John Wiley & Sons Ltd.

Qu T.,University of Hawaii at Manoa | Hsin Y.-C.,Research Center for Environmental Changes
Journal of Geophysical Research C: Oceans | Year: 2015

Analyses of outputs from an eddy-resolving ocean general circulation model show that there are at least two groups of subthermocline eddies near the Philippine coast: one originates from the southeast, and the other from the east. The dominant period and principal depth of the former (latter) group of eddies are about 55 days (67 days) and 600 m (350 m), respectively. The propagation speed (∼0.12 m s-1) and diameter (∼3°) of the two groups of eddies are similar. We suggest that the westward propagating eddies are generated by interactions between meridional movement of the westward-flowing North Equatorial Current, the eastward-flowing North Equatorial Undercurrent, and their interactions with topography. Besides, the analysis indicates that, in comparison with the northwestward propagating subthermocline eddies, the westward propagating ones play a more important role in modulating the subsurface circulation near the Philippine coast. © 2015. American Geophysical Union. All Rights Reserved.

Zhang Y.,Xiamen University | Xie X.,Xiamen University | Jiao N.,Xiamen University | Hsiao S.S.-Y.,Research Center for Environmental Changes | And 2 more authors.
Biogeosciences | Year: 2014

Coupled nitrification-denitrification plays a critical role in the removal of excess nitrogen, which is chiefly caused by humans, to mitigate estuary and coastal eutrophication. Despite its obvious importance, limited information about the relationships between nitrifying and denitrifying microbial communities in estuaries, and their controlling factors have been documented. We investigated the nitrifying and denitrifying microbial communities in the estuary of turbid subtropical Yangtze River (YRE), the largest river in Asia, by analyzing the ammonia monooxygenase gene amoA, including archaeal and bacterial amoA, and the dissimilatory nitrite reductase gene nirS using clone libraries and quantitative PCR (qPCR). The diversity indices and rarefaction analysis revealed a quite low diversity for both β2-proteobacterial and archaeal amoA genes, but qPCR data showed significantly higher amoA gene copy numbers for archaea than β2-proteobacteria. Compared with the amoA gene, a significantly higher level of diversity but lower gene copy numbers were found for the nirS gene. Nitrification and denitrification rates based on 15N incubation experiments supported gene abundance data as denitrification rates were below detection limit, suggesting lower denitrification than nitrification potential. In general, the abundances of the amoA and nirS genes were significantly higher in the bottom samples than the surface ones, and in the high-turbidity river mouth, were significantly higher in the particle-associated (> 3 μm) than the free-living (0.2 ~ 3 μm) communities. Notably, positive correlations between the amoA and nirS gene abundances suggested potential gene-based coupling between nitrification and denitrification, especially for the particle-associated assemblages. Statistical analysis of correlations between the community structure, gene abundances and environmental variables further revealed that dissolved oxygen and total suspended material might be the key factors controlling community spatial structure and regulating nitrification and denitrification potentials in the YRE ecosystem. © Author(s) 2014.

Hsin Y.-C.,Research Center for Environmental Changes
Journal of Geophysical Research C: Oceans | Year: 2015

Based on the analyses of 59 year (1950-2008) surface geostrophic velocities, the multidecadal changes of Kuroshio from the eastern Luzon to the southern Japan are investigated. Result shows that the upstream Kuroshio from the east of Luzon to southern East China Sea suffers much more obvious multidecadal changes. Except for the decade of 1980, the Kuroshio east of Luzon possesses a multidecadal tendency opposite to that east of Taiwan. Besides 1980s, the multidecadal change of Kuroshio bordering Taiwan is mainly governed by the eddy activity off the eastern Taiwan, while the wind stress curl plays a major role in the Kuroshio in the east of Luzon and in the Luzon Strait. The wind-stress-induced Ekman transport plays a secondary role in regulating the Kuroshio east of Luzon. In addition, the multidecadal fluctuation of Kuroshio east of Luzon also modulates the westward intrusion in the Luzon Strait. Instead, the Ekman transport dominated the whole upstream Kuroshio area from the eastern Luzon to the vicinity of Taiwan during the exceptional decade of 1980. Associated changes of water properties in the northern South China Sea and southern East China Sea are also ascribed to the multidecadal changes of surface Kuroshio in the upstream area. © 2015. American Geophysical Union.

Wong G.T.F.,Research Center for Environmental Changes | Pan X.,Research Center for Environmental Changes | Li K.-Y.,Research Center for Environmental Changes | Shiah F.-K.,Research Center for Environmental Changes | And 2 more authors.
Deep-Sea Research Part II: Topical Studies in Oceanography | Year: 2015

The hydrographic characteristics and the distributions of nitrate+nitrite, or (N+N), and soluble reactive phosphate, or SRP, in the Northern South China Sea Shelf-sea (NoSoCS) were determined in four transects across the shelf in the summer of 2010 and in two transects in the winter of 2011. The NoSoCS may be sub-divided into the inner shelf, the middle shelf and the outer shelf at water depths of <40 m, 40-90 m and 90-120 m, respectively. The water in the inner shelf is colder and its concentrations in the nutrients and chlorophyll-a (Chl_a) are higher in both seasons while the water in the outer shelf is warmer and its concentrations are lower. With depth, since the mixed layer depth in the NoSoCS in the winter (~70 m) and in the summer (~40 m) are both shallower than the shelf break depth (~120 m), the colder and relatively nutrient-rich upper thermocline-upper nutricline water in the open South China Sea (SCS) can freely extend into the NoSoCS to become its bottom water. This is a distinguishing characteristic of the NoSoCS as, unlike many of the more extensively studied temperate shelf-seas, vertical mixing within the NoSoCS, rather than shelf-edge processes such as upwelling, is the primary mechanism for bringing the nutrients from the sub-surface in the adjoining northern SCS to its mixed layer for supporting primary production. Four processes that may contribute to this vertical mixing in the NoSoCS include: shelf-wide winter surface cooling and convective mixing; the actions of internal waves which probably occur in both seasons along the entire outer shelf; wind and topographically induced coastal upwelling in the summer off Shantou at the northwestern corner of the NoSoCS; and winter formation of bottom water at the outer portion of the inner shelf and the inner portion of the middle shelf. The density of this bottom water formed in the winter of 2011 was equivalent to the density at ~120 m in the open SCS so that it could cascade across the shelf and contribute to not only vertical mixing but also in cross shelf mixing and the ventilation of the upper thermocline and nutricline of the open SCS. An enrichment in (N+N) over SRP, where (N+N)/SRP exceeded the Redfield ratio of 16 and [N+N]ex>0, was found when salinity dropped below about 33. In these fresher waters, which were found exclusively in the inner shelf during this study, SRP potentially could become the limiting nutrient. © 2015 Elsevier Ltd.

Freychet N.,Research Center for Environmental Changes | Hsu H.-H.,Research Center for Environmental Changes | Chou C.,Research Center for Environmental Changes | Wu C.-H.,Research Center for Environmental Changes
Journal of Climate | Year: 2015

Change in extreme events in climate projections is a major concern. If the frequency of dry events is expected to increase in a warmer climate (thus, the overall number of wet days will decrease), heavy and extreme precipitation are also expected to increase because of a shift of the precipitation spectrum. However, the forecasts exhibit numerous uncertainties. This study focuses on the Asian region, separated into the following three subregions: the East Asian region, the Indian region, and western North Pacific region, where the summer monsoon can bring heavy rainfall. Particularly emphasized herein is the reliability of the projection, using data from a large ensemble of 30 models from phase 5 of the Coupled Model Intercomparison Project. The scattering of the ensemble enables obtaining an optimal estimate of the uncertainties, and it is used to compute the correlation between projected changes of extreme events and circulation changes. The results show clear spatial and temporal variations in the confidence of changes, with results being more reliable during the wet season (i.e., the summer monsoon). The ensemble predicts changes in atmospheric circulation with favorable confidence, especially in the low-level moisture flux convergence (MFC). However, the correlation between this mean change and the modification of extreme events is nonsignificant. Also analyzed herein are the correlation and change of MFC exclusively during these events. The horizontal MFC exerts a nonnegligible influence on the change in the intensity of extremes. However, it is mostly the change in vertical circulation and moisture advection that is correlated with the change in frequency and intensity of extreme events. © 2015 American Meteorological Society.

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