Kon-Kee Liu

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.

Winter upwelling off Luzon in the northeastern South China Sea

A major upwelling region centered about 100 km offshore between 16° and 19°N off northwest Luzon in the South China Sea is revealed in the distributions of temperature, salinity, and dissolved oxygen concentration during a cruise in December 1990. Upward movement of water low in dissolved oxygen can be identified to a depth of 300 m. Climatological distributions of temperature and upper ocean heat content and tracer distribution obtained from a recent numerical experiment lend support to the existence of this upwelling region. Results from a general circulation model of the South China Sea further show that upwelling off Luzon occurs between October and January, in agreement with the climatological data. Current structures obtained from a simple two-layer model and the general circulation model both suggest that this upwelling is not driven by local winds but contains a large, remotely forced component arising from the basin circulation. The upwelling is sustained by offshore Ekman drift from above and a converging northward undercurrent from below. The would-be downwind coastal jet in the surface layer is largely offset by currents associated with the winter circulation gyre. Upwelled water with high nutrient content spreads southwestward in January and could be an important nutrient source for the surface water in the interior South China Sea in spring.

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.

Organic carbon and nitrogen contents and their isotopic compositions in surficial sediments from the East China Sea shelf and the southern Okinawa Trough

Abstract The southern Okinawa Trough has been noted as an important depocenter for the particulate organic matter from the East China Sea shelf. In order to determine the major sources of sedimentary organic matter in the southern Okinawa Trough, we analyzed surficial sediments from the East China Sea shelf and the southern Okinawa Trough for organic carbon and nitrogen contents and their isotopic compositions in the decarbonated fraction. Distributions of total organic carbon content, δ13Corg and nitrogen content all show a similar spatial pattern, resembling the distribution of fine-grained sediments. The coastal belt of elevated organic carbon content extends southward from the Changjiang mouth and veers offshore towards the southern Okinawa Trough just north of Taiwan, implying a pathway for channeling fine-grained sediments from the inner shelf to the depocenter. This distribution pattern is consistent with the shelf circulation. The isotope compositions (δ13Corg, δ15N) of sediments from the southern Okinawa Trough fall between those of riverine particulate organic matter and the mid-outer shelf sediments, but overlap with those of the inner shelf sediments. In addition, the previously reported δ13Corg values of sinking particles collected by sediment traps in the southern Okinawa Trough are also close to those of the inner shelf sediments, especially during high-flux conditions. Therefore, the isotopic evidence strongly supports the notion that a major fraction of the sedimentary organic matter in the southern Okinawa Trough may originate from the inner shelf.

Determination of dissolved oxygen in seawater by direct spectrophotometry of total iodine

Abstract A modified procedure has been proposed for the colorimetric determination of dissolved oxygen in seawater to improve its precision and accuracy. When a pickled sample is acidified, iodine liberated in the iodometric reaction is measured by direct spectrophotometry at 456 nm. Loss of molecular iodine by volatilization is eliminated by transferring the sample to a flow cuvette without contact with air. The method was calibrated for oxygen by spiking known amounts of potassium iodate. Precision was found at better than 0.2% r.s.d. (full scale). Evaluation of accuracy was made by comparison with calculated oxygen solubilities, which shows a relative bias of no more than 0.5% for oxic waters. The analytical throughput was much faster than that of the standard titration procedure.

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.

A unique seasonal pattern in phytoplankton biomass in low‐latitude waters in the South China Sea

elevated to 0.3 mg/m 3 , 35 mg/m 2 and 300 mg-C/m 2 /d, respectively, in the winter but stayed low, at 0.1 mg/m 3 , 15 mg/m 2 and 110 mg-C/m 2 /d as commonly found in other low latitude waters, in the rest of the year. Concomitantly, soluble reactive phosphate and nitrate+nitrite in the mixed layer also became readily detectable in the winter. The elevationofphytoplanktonbiomasscoincidedapproximately with the lowest sea surface temperature and the highest wind speed in the year. Only the combined effect of convective overturn by surface cooling and wind-induced mixing could have enhanced vertical mixing sufficiently to make the nutrients in the upper nutricline available for photosynthetic activities and accounted for the higher biomass in the winter. Citation: Tseng, C.-M., G. T. F. Wong, I.-I. Lin, C.-R. Wu, and K.-K. Liu (2005), A unique seasonal pattern in phytoplankton biomass in low-latitude waters in the South China Sea, Geophys. Res. Lett., 32, L08608, doi:10.1029/2004GL022111.

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.

Continental Margin Exchanges

Biogeochemical processes principally occur in the upper 200 metres of the sea and are often associated with continental margins. Although the continental margins, with waters shallower than 200 m, occupy a mere 7% of the ocean surface and even less than 0.5% of the ocean volume, they still play a major role in oceanic biogeochemical cycling. Significantly higher rates of organic productivity occur, in fact, in the coastal oceans than in the open oceans because of rapid turnover and the higher supply of nutrients from upwelling and riverine inputs. Also, 8 to 30 times more organic carbon and 4 to 15 times more calcium carbonate per unit area accumulate in the coastal oceans than in the open oceans. Similarly, gas exchange fluxes of carbon and nitrogen are considerably higher in coastal waters than in the open oceans per unit area. As a result, it has been reported that around 14% of total global ocean production, along with 80–90% of new production and as much as up to 50% of denitrification takes place in the coastal oceans. The burial sites of 80% of the organic carbon derived from both oceanic processes and terrestrial sources, in excess of 50% of present day global carbonate deposition, are also located in the coastal oceans. The unburied portion of organic carbon may be respired on the shelf, thus forming a potential natural source of atmospheric carbon dioxide. However, how much is actually respired is unknown since much of this carbon is highly inert and only mixes conservatively with seawater (Smith and Mackenzie 1987; Mantoura et al. 1991; Wollast 1998).