Gwo-Ching Gong

Monsoon-forced chlorophyll distribution and primary production in the South China Sea: Observations and a numerical study

Although the South China Sea (SCS) exchanges water constantly with the western Philippine Sea, its nutricline is much shallower and its chlorophyll level in surface waters is twice as high. Analysis of CZCS-SeaWiFS data and shipboard data reveals a strong seasonality of chlorophyll in the SCS in three upwelling regions. A three-dimensional numerical model with coupled physics and biogeochemistry is developed to study the effect of monsoonal forcing on nutrient upwelling and phytoplankton growth in the SCS. The model has a horizontal resolution of 0.41 in the domain 2–24.81N and 99–124.61E and 21 layers in the vertical. The circulation is driven by monthly climatological winds. The nitrogen-based ecosystem model has four compartments: dissolved inorganic nitrogen (DIN), phytoplankton, zooplankton and detritus. The chlorophyll-to-phytoplankton ratio depends on light and DIN availability. The biological equations and parameters are taken from previous modeling studies of the Sargasso Sea. The model simulates the nitrate profile, the strong subsurface chlorophyll maximum, and the primary production in the central basin with reasonable success. It also generates intense chlorophyll patches in the monsoon-driven upwelling regions northwest of Luzon and north of the Sunda Shelf in winter and off the east coast of Vietnam in summer. The results are in reasonable agreement with shipboard observations and CZCS-SeaWiFS data. The primary production derived from SeaWiFS data shows a strong peak in winter and weak peak in summer with an annual mean of 354 mg C m � 2 d � 1 for the whole basin. The modeled primary production displays seasonal variation resembling the trend derived from SeaWiFS data, but the magnitude (280 mg C m � 2 d � 1 ) is smaller by 20%. The model also predicts an export fraction of 12% from the primary production in the euphotic zone. r 2002 Elsevier Science Ltd. All rights reserved.

Chemical hydrography and chlorophyll a distribution in the East China Sea in summer: implications in nutrient dynamics

Abstract A survey of the chemical hydrography and Chla distribution in the middle and outer shelves of the East China Sea was carried out in July 1992. Very high standing stock of Chia (30–110 mg m−2) and supersaturation of oxygen (up to 130%) were observed in the plume of the Changjiang Diluted Water, which was enriched in nitrate and silicate but depleted in phosphate. Beneath the plume, a strong depletion in oxygen and an accompanied enrichment in nitrate as well as phosphate were evident. The rich phosphate in the subsurface water may have been crucial in supporting the rapid phytoplankton growth in the overlying water. The Taiwan Current Warm Water (TCWW) occupied most of the shelf to the south and to the east of the plume. The negative relationships between nitrate and temperature suggest that the nutrient-laden Kuroshio Sub-surface Water from the slope was the main source of nutrients for the TCWW. The input of the slope water to the shelf was probably facilitated by countercurrents associated with the Kuroshio upwelling northeast of Taiwan and the mesoscale eddy at the shelf-break east of the Changjiang river mouth. Yet another source of nutrients for the TCWW was revealed by the positive deviations from the mixing trend. These nutrients that may have originated from local remineralization accounted for up to 30% of the total nutrients in the bottom water on the middle shelf. Compared with the Changjiang river plume, the TCWW was relatively poor in Chia, except in the upwelling region off northern Taiwan where vertical mixing was strong. The nutrients in the subsurface layer of the TCWW further away from Taiwan were generally not available for phytoplankton uptake in the euphotic zone in summer. However, the reserved nutrients could have supported a phytoplankton bloom subsequent to strong vertical mixing.

The nitrogen isotopic composition of nitrate in the Kuroshio Water northeast of Taiwan: evidence for nitrogen fixation as a source of isotopically light nitrate

The Kuroshio Water, which provides nutrients to the shelf water of the East China Sea through upwelling at the shelf break northeast of Taiwan, is abundant with nitrogen fixing cyanobacteria. Accordingly, the δ15N values of nitrate at three stations in the sea northeast of Taiwan were analyzed to estimate the flux of biologically fixed nitrogen in the Kuroshio Water by characterization of the isotopic signature of the upwelled nitrate. Prior to sample analysis, we verified using standard KNO3 solution the validity of the sample preparation procedure, which has been modified by reducing the amount of Devardas alloy to lower the reagent blank. However, the concentrations of nitrate in seawaters estimated from the amount of extracted nitrogen were consistently higher than those measured colorimetrically, suggesting interference from dissolved organic nitrogen of which the δ15N value was estimated to be −3.0 ± 0.2%., Correction was made on each sample based on the estimated δ15N for DON. The δ15N values of waters from 500 to 780 m in the sea northeast of Taiwan fell in the range of + 5.5 to + 6.1%c, which was consistent with those observed in deep waters elsewhere. The δ15N values decreased in the overlying Kuroshio Water by 1–3%. in general. In order to achieve isotope balance in the water column, the input of isotopically light nitrogen, presumably from nitrogen fixation, is required. The nitrogen flux from this source was estimated to be 0.5 mg m−2 day−1 N which is only 8% of the estimated new nitrogen to the Kuroshio surface water. On the other hand, this nitrogen may account for 20–40% of the nitrate in the subsurface water (100–400 m) as a result of accumulation and recycling. The nitrate in the source upwelling water had a δ15N value of +4.7 ± 0.6%., implying 20 ± 9% of it originating from nitrogen fixation.

Response of Kuroshio upwelling to the onset of the northeast monsoon in the sea north of Taiwan: Observations and a numerical simulation

A cold water anomaly, which manifests upwelling of the subsurface Kuroshio Water, has been frequently observed at the shelf break of the East China Sea to the north of Taiwan. Its response to the onset of northeast monsoon was observed during August–October 1990. The wind direction reversed in mid-September, indicating the onset of northeast monsoon. Shortly thereafter, the sea surface temperature at the center of the cold eddy showed a pulselike sudden drop, and a significant concentration of nitrate (up to 5 μM) appeared in the surface water, suggesting intensification of upwelling. Subsequently, the upper layer of the Kuroshio Water intruded onto the shelf. The general circulation model of the East China Sea previously developed by Chao was used to simulate the overall response of the East China Sea. The numerical simulation reproduced the intensification of upwelling. It also predicted extensive Kuroshio intrusion along the shelf break farther north of Taiwan as well as the temporary intensification of the northeast branch of the Yangtze River outflow. The energy source of this sudden intensification comes from the potential energy released by the Kuroshio as the isopycnals maintaining the Kuroshio rise in response to the wind change.

Cross-shelf and along-shelf nutrient fluxes derived from flow fields and chemical hydrography observed in the southern East China Sea off northern Taiwan

Cross-shelf exchange of nutrients (N, P and Si) off northeastern Taiwan and along-shelf transports just north of the Taiwan Strait was assessed using chemical hydrography and velocity fields observed in August 1994 (summer) and March 1997 (winter). The summer survey was conducted along the periphery of a triangle. The velocity field was determined by phase-averaged current velocities measured with ship-borne ADCP on two rounds separated by 3.5 cycles of the dominant M2 tide. Nutrient distributions were derived from phase-averaged hydrographic data. Although the inflowing and outflowing volume transports were mismatched by 14% largely due to poorly estimated surface flow that carried little nutrients, the inputs and outputs of nutrients agreed within 2–5%. Such consistency lends support to the adopted approach for estimating nutrient fluxes. The winter survey, which provided observations on two transects, one along-shelf and one cross-shelf, was conducted using two ships separated by 1/2 cycle of the M2 tide. Results from the two ships allowed determination of the phase-averaged flow fields and chemical hydrography. The two surveys indicated that the Kuroshio upwelling provided rather constant nutrient fluxes to the shelf, which were comparable to the total nutrient influxes from slope waters to the Mid and South Atlantic Bight and considerably larger than the riverine sources. Significant amounts of nutrients from the shelf leaked back into the surface layer of the Kuroshio, replenishing its impoverished nutrient reserves. Along-shelf nutrient fluxes from the Taiwan Strait changed drastically between seasons, ranging from less than half to more than double the Kuroshio inputs. In the light of the variability of the shelf environment, more observations are needed to better determine the mean condition of this transport.

Prediction of nitrate concentration from two end member mixing in the southern East China Sea

Abstract Monthly hydrographic surveys were conducted along two transects across the Kuroshio frontal zone in the southern East China Sea from August 1990 to July 1991. The concentration of nitrate in the shelf water was linearly related to temperature. However, the relationship varied with time. This relationship could have resulted from two end member mixing between the nitratedevoid surface water and the nitrate-laden upwelled water. While the temperature of surface water was temporally variable, the temperature-nitrate properties of the upwelled water were almost invariant. The temperature of the surface end member ( T s ) correlated well with the mean air temperature ( T a ) at a weather station in the study area according to the equation: T s = 15.3 + 0.45 × T a The upwelled water, which was brought to the shelf by bottom intrusion of the Kuroshio, had properties similar to those of the Kuroshio subsurface water at σ θ = 25.8 with a mean temperature of 14.8°C and a nitrate concentration of 13.2 μM. With the exception of the data obtained in September 1990, the concentration of nitrate in the shelf water can be predicted from the mean air temperature with a mean error of 0.9 μM from the observed values. The anomaly observed in September 1990 might have been the result of the unsteady condition during the reversal of the direction of the monsoon. There was also a slight deviation from the predicted trend in March 1991 when there was a seaward excursion of the Continent Coastal Water which may have constituted a third end member in the mixing process. In general, this simple mixing model is applicable to the middle and outer shelf north of Taiwan.

Surface distributions of carbon chemistry parameters in the East China Sea in summer 2007

[1] Comprehensive carbon chemistry data including total alkalinity (TA), dissolved inorganic carbon (DIC), pH, fugacity of CO 2 and other pertinent data (i.e., temperature, salinity, and levels of nitrate and chlorophyll a) were measured in surface waters of the East China Sea (ECS) shelf in July 2007. The results show that spatial variations in these parameters closely correspond to the distributions of various water types. The Changjiang Diluted Water (CDW) and Yellow Sea Water (YSW) areas are the two major sinks of atmospheric CO 2 ; the Coastal Upwelling Water (CUW) area is the most important CO 2 source, whereas the Kuroshio Water and Taiwan Current Warm Water areas are weak sources. The entire ECS acted as a sink for atmospheric CO 2 , with a flux of -2.4 to - 4.3 mmol C m -2 d -1 during the study period. Identification of the CUW source area suggests that previous studies might have overestimated CO 2 uptake by the ECS in summer. Our results further suggest that high biological production might be responsible for the strong sink in the CDW area but that high input of TA from the Huanghe River, which led to an elevated TA/DIC ratio, could have resulted in formation of a significant CO 2 sink in the YSW area. The present data set represents the most comprehensive CO 2 survey in the ECS to date and can thus be used as a baseline for monitoring future changes in the CO 2 system arising from the construction of the Three Gorges Dam in the middle stretch of the Changjiang River.

Distribution patterns and phylogeny of marine stramenopiles in the north pacific ocean.

ABSTRACT Marine stramenopiles (MASTs) are a diverse suite of eukaryotic microbes found in marine environments. Several MAST lineages are thought to contain heterotrophic nanoflagellates. However, MASTs remain uncultured and data on distributions and trophic modes are limited. We investigated MASTs in provinces on the west and east sides of the North Pacific Subtropical Gyre, specifically the East China Sea (ECS) and the California Current system (CALC). For each province, DNA was sampled from three zones: coastal, mesotrophic transitional, and more oligotrophic euphotic waters. Along with diatoms, chrysophytes, and other stramenopiles, sequences were recovered from nine MAST lineages in the six ECS and four CALC 18S rRNA gene clone libraries. All but one of these libraries were from surface samples. MAST clusters 1, 3, 7, 8, and 11 were identified in both provinces, with MAST cluster 3 (MAST-3) being found the most frequently. Additionally, MAST-2 was detected in the ECS and MAST-4, -9, and -12 were detected in the CALC. Phylogenetic analysis indicated that some subclades within these lineages differ along latitudinal gradients. MAST-1A, -1B, and -1C and MAST-4 size and abundance estimates obtained using fluorescence in situ hybridization on 79 spring and summer ECS samples showed a negative correlation between size of MAST-1B and MAST-4 cells and temperature. MAST-1A was rarely detected, but MAST-1B and -1C and MAST-4 were abundant in summer and MAST-1C and MAST-4 were more so at the coast, with maximum abundances of 543 and 1,896 cells ml−1, respectively. MAST-4 and Synechococcus abundances were correlated, and experimental work showed that MAST-4 ingests Synechococcus. Together with previous studies, this study helps refine hypotheses on distribution and trophic modes of MAST lineages.

Effect of the Kuroshio intrusion on the chlorophyll distribution in the southern East China Sea during spring 1993

Abstract In order to assess the effect of the Kuroshio intrusion on phytoplankton biomass in the southern East China Sea north of Taiwan, chlorophyll a distributions observed before and after the recession of the seasonal Kuroshio intrusion were compared. Weekly hydrographie surveys and moored current meter data showed that the Kuroshio intruded onto the shelf under the prevailing northeasterly in early April 1993, and it retreated in early May 1993 shortly after the cessation of the northeasterly. The chlorophyll a distribution showed an eminent increase with a concentric distribution pattern, which was obviously related to the re-emergence of the year-round upwelling at the shelf break northeast of Taiwan. Strengthened upwelling was evidenced by a concomitant increase of nitrate concentration in the subsurface water near the shelf break. The mean value of the euphotic zone integrated chlorophyll a concentration in the normal upwelling area increased from 16 to 36 mg m−2 after the Kuroshio withdrawal. The high chlorophyll a concentration in the upwelling region was shown associated with the high primary productivity measured in the following year, May 1994. The euphotic zone integrated chlorophyll a concentration and primary production in the upwelling water were found to be 33 mg m−2 and 1540 mgC m−2 d−1, respectively. The higher chlorophyll and primary production was apparently the results of ample nutrient supply from the upwelling. At the same time, very high primary production was found in the mainland China coastal water with a value of 1900 mgC m−2 d−1 corresponding to a dinofiagellate bloom with population densities exceeding 1 × 104 cells 1t-1. The Kuroshio water, on the other hand, has the lowest primary production value of 420 mgC m−2 d−1.

Estimation of annual primary production in the Kuroshio waters northeast of Taiwan using a photosynthesis-irradiance model

Photosynthesis-irradiance (PB–E) relationships for Kuroshio waters at different depths in different seasons were established using repeated measurements of primary productivity at a station northeast of Taiwan. The PB–E approach is different from the conventional PB–E approach in that a much longer incubation time is required to measure primary productivity. By plotting the daytime averaged, chlorophyll specific primary productivity (PB) against the daytime averaged irradiance (E), the optimal rate of photosynthesis (PBopt) in the mixed layer was found to be 5.87 mgC [mgChl a]-1 h-1 in summer and 2.88 mgC [mgChl a]-1 h-1 in winter. The initial slope of the PB–E curve, however, remained around 0.01 mgC [mgChl a]-1 h-1 [μEinstein m-2 s-1]-1 throughout the year. Below the mixed layer, the PBopt decreased substantially to less than half of that within the mixed layer. A similar decrease was also observed in the initial slope. The euphotic zone integrated primary production as calculated from these PB–E parameters was close to that measured in situ, with relative error less than 6%. Based on the measured PB–E parameters, it was estimated here that the annual primary production was 104 gC m-2 yr-1 (285 mgC m-2 d-1). The established PB–E relationships may serve as a better regional algorithm in the assessment of primary production for Kuroshio waters.