Wong
George T. F.
Wong
George T. F.
No Thumbnail Available
Search Results
Now showing
1 - 6 of 6
-
ArticleInternal waves influence the thermal and nutrient environment on a shallow coral reef(Wiley, 2019-03-26) Reid, Emma C. ; DeCarlo, Thomas M. ; Cohen, Anne L. ; Wong, George T. F. ; Lentz, Steven J. ; Safaie, Aryan ; Hall, Austin ; Davis, Kristen A.Internal waves can influence water properties in coastal ecosystems through the shoreward transport and mixing of subthermocline water into the nearshore region. In June 2014, a field experiment was conducted at Dongsha Atoll in the northern South China Sea to study the impact of internal waves on a coral reef. Instrumentation included a distributed temperature sensing system, which resolved spatially and temporally continuous temperature measurements over a 4‐km cross‐reef section from the lagoon to 50‐m depth on the fore reef. Our observations show that during summer, internal waves shoaling on the shallow atoll regularly transport cold, nutrient‐rich water shoreward, altering near‐surface water properties on the fore reef. This water is transported shoreward of the reef crest by tides, breaking surface waves and wind‐driven flow, where it significantly alters the water temperature and nutrient concentrations on the reef flat. We find that without internal wave forcing on the fore reef, temperatures on the reef flat could be up to 2.0°C ± 0.2°C warmer. Additionally, we estimate a change in degree heating weeks of 0.7°C‐weeks warmer without internal waves, which significantly increases the probability of a more severe bleaching event occurring at Dongsha Atoll. Furthermore, using nutrient samples collected on the fore reef during the study, we estimated that instantaneous onshore nitrate flux is about four‐fold higher with internal waves than without internal waves. This work highlights the importance of internal waves as a physical mechanism shaping the nearshore environment, and likely supporting resilience of the reef.
-
ArticleClimate modulates internal wave activity in the Northern South China Sea(John Wiley & Sons, 2015-02-10) DeCarlo, Thomas M. ; Karnauskas, Kristopher B. ; Davis, Kristen A. ; Wong, George T. F.Internal waves (IWs) generated in the Luzon Strait propagate into the Northern South China Sea (NSCS), enhancing biological productivity and affecting coral reefs by modulating nutrient concentrations and temperature. Here we use a state-of-the-art ocean data assimilation system to reconstruct water column stratification in the Luzon Strait as a proxy for IW activity in the NSCS and diagnose mechanisms for its variability. Interannual variability of stratification is driven by intrusions of the Kuroshio Current into the Luzon Strait and freshwater fluxes associated with the El Niño–Southern Oscillation. Warming in the upper 100 m of the ocean caused a trend of increasing IW activity since 1900, consistent with global climate model experiments that show stratification in the Luzon Strait increases in response to radiative forcing. IW activity is expected to increase in the NSCS through the 21st century, with implications for mitigating climate change impacts on coastal ecosystems.
-
ArticleValidation of the remotely sensed nighttime sea surface temperature in the shallow waters at the Dongsha Atoll(Terrestrial, Atmospheric and Oceanic Sciences, 2017-06) Pan, Xiaoju ; Wong, George T. F. ; DeCarlo, Thomas M. ; Tai, Jen-Hua ; Cohen, Anne L.Fine scale temperature structures, which are commonly found in the top few meters of shallow water columns, may result in deviations of the remotely sensed night-time sea surface temperatures (SST) by the MODIS-Aqua sensor (SSTsat) from the bulk sea surface temperatures (SSTbulk) that they purport to represent. The discrepancies between SSTsat and SSTbulk recorded by temperature loggers at eight stations with bottom depths of 2 - 20 m around the Dongsha Atoll (DSA) between June 2013 and May 2015 were examined. The SSTsat had an average cool bias error of -0.43 ± 0.59°C. The bias error was larger in the warmer (> 26°C) waters which were presumably more strongly stratified. The root mean square error (RMSE) between SSTsat and SSTbulk, ±0.73°C, was 25% larger than that reported in the open northern South China Sea. An operational calibration algorithm was developed to increase the accuracy in the estimation of SSTbulk from SSTsat. In addition to removing the cool bias error, this algorithm also reduced the RMSE to virtually the same level as that found in the open northern South China Sea. With the application of the algorithm, in June 2015, the average SST in the lagoon of the DSA was raised by about 0.5°C to 31.1 ± 0.4°C, and the area of lagoon with SSTbulk above 31°C, the median value of the physiological temperature threshold of reef organisms, was increased by 69% to about three quarters of the lagoon.
-
ArticleMass coral mortality under local amplification of 2°C ocean warming(Nature Publishing Group, 2017-03-23) DeCarlo, Thomas M. ; Cohen, Anne L. ; Wong, George T. F. ; Davis, Kristen A. ; Lohmann, George P. ; Soong, KeryeaA 2°C increase in global temperature above pre-industrial levels is considered a reasonable target for avoiding the most devastating impacts of anthropogenic climate change. In June 2015, sea surface temperature (SST) of the South China Sea (SCS) increased by 2 °C in response to the developing Pacific El Niño. On its own, this moderate, short-lived warming was unlikely to cause widespread damage to coral reefs in the region, and the coral reef “Bleaching Alert” alarm was not raised. However, on Dongsha Atoll, in the northern SCS, unusually weak winds created low-flow conditions that amplified the 2°C basin-scale anomaly. Water temperatures on the reef flat, normally indistinguishable from open-ocean SST, exceeded 6°C above normal summertime levels. Mass coral bleaching quickly ensued, killing 40% of the resident coral community in an event unprecedented in at least the past 40 years. Our findings highlight the risks of 2°C ocean warming to coral reef ecosystems when global and local processes align to drive intense heating, with devastating consequences.
-
ArticleCommunity production modulates coral reef pH and the sensitivity of ecosystem calcification to ocean acidification(John Wiley & Sons, 2017-01-31) DeCarlo, Thomas M. ; Cohen, Anne L. ; Wong, George T. F. ; Shiah, Fuh-Kwo ; Lentz, Steven J. ; Davis, Kristen A. ; Shamberger, Kathryn E. F. ; Lohmann, George P.Coral reefs are built of calcium carbonate (CaCO3) produced biogenically by a diversity of calcifying plants, animals, and microbes. As the ocean warms and acidifies, there is mounting concern that declining calcification rates could shift coral reef CaCO3 budgets from net accretion to net dissolution. We quantified net ecosystem calcification (NEC) and production (NEP) on Dongsha Atoll, northern South China Sea, over a 2 week period that included a transient bleaching event. Peak daytime pH on the wide, shallow reef flat during the nonbleaching period was ∼8.5, significantly elevated above that of the surrounding open ocean (∼8.0–8.1) as a consequence of daytime NEP (up to 112 mmol C m−2 h−1). Diurnal-averaged NEC was 390 ± 90 mmol CaCO3 m−2 d−1, higher than any other coral reef studied to date despite comparable calcifier cover (25%) and relatively high fleshy algal cover (19%). Coral bleaching linked to elevated temperatures significantly reduced daytime NEP by 29 mmol C m−2 h−1. pH on the reef flat declined by 0.2 units, causing a 40% reduction in NEC in the absence of pH changes in the surrounding open ocean. Our findings highlight the interactive relationship between carbonate chemistry of coral reef ecosystems and ecosystem production and calcification rates, which are in turn impacted by ocean warming. As open-ocean waters bathing coral reefs warm and acidify over the 21st century, the health and composition of reef benthic communities will play a major role in determining on-reef conditions that will in turn dictate the ecosystem response to climate change.
-
ThesisDissolved inorganic and particulate iodine in the oceans(Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 1976-02) Wong, George T. F.Analytical methods have been developed for the determination of iodate, iodide and particulate iodine in sea water. Iodate is converted to tri-iodide and the absorbance of tri-iodide at 353 nm is measured. The precision of this method is ca. ±3%. Iodide is first separated from most other anions by an AG 1-x8 anion exchange column and then precipitated as palladous iodide with elemental palladium as the carrier. The precipitate is analyzed by neutron activation analysis. The precision of the method is ±5% and the reagent blank is 0.005 uM. Marine suspended matter is collected by passing sea water under pressure through a 0.6 u (37 mm diameter) Nuclepore filter. The iodine content of the particles is determined by neutron activation analysis. The method has excellent reproducibility and the filter blank is ca. 3 ng. Iodate is depleted in the surface waters of the Equatorial Atlantic. The depletion is more pronounced than in the Argentine Basin and possibly reflects the higher productivity in the equatorial area. Superimposed on this feature, a thin lens of water, of a few tens of meters thick and with high iodate concentrations, can be traced across the Atlantic. Along the equator, this lens occurs at 80 m at 33˚W and rises upwards to 55 m at 10˚W and it coincides with a core of highly saline water which is characteristic of the Equatorial Undercurrent. Longitudinal sections reflect the complexity of the equatorial current system. At least three cores of water with high iodate concentrations may be identified. These waters may be transported to the equatorial region from the highly productive areas along the north-western and western African coasts and the Amazon plume. In anoxic basins, the concentration of iodide increases rapidly in the mixing zone from 0.02 uM to 0.44 uM in the Cariaco Trench and from 0.01 uM to 0.23 uM in the Black Sea. The iodate concentration, meanwhile. decreases to zero. A maximum in the total iodine to salinity ratio is observed just above the oxygen-sulfide interface (15 to 17 nmoles/g); it is suggestive of particle dissolution in a strong pycnocline. Below the interface, the total iodine to salinity ratio is constant at 12.3 nmoles/g in the anoxic zone of the Cariaco Trench, whereas, in the Black Sea, it increases with depth from 10.0 to 19.4 nmoles/g and suggests a possible flux of iodide from the sediments. By considering the distribution of iodate and iodide in oxic and anoxic basins and our present analytical capability, the lower limit of the pE of the oceans is estimated to be 10.7. Thermodynamic considerations further suggest that the iodide-iodate couple is a poor indica tor for the pE of the oceans with a limited usable range of 10.0 to 10.7. In the Gulf of Maine during the winter of 1974 to 1975. the effect of winter mixing was conspicuous. Uniform concentrations of iodide and iodate were observed in the mixed layer above the sill. The absence of a depletion of iodate and the low iodide concentration (0.04 uM) in the surface waters reflect the low biological activity in this region during winters. Profiles of particulate iodine are characterized by high concentrations in the euphotic zone (>5 ng/kg), and lower concentrations (< 2 ng/kg) at greater depths. Occasionally, high concentrations have also been observed in the nepheloid layer. The iodine-containing particles are probably biogenic. A section in the Western Atlantic from 75°N to 55˚S shows evidence of the transport of particles along isopycnals and the re-suspension of surface sediments to considerable distance from the bottom. The standing crops in the top 200 m may be qualitatively correlated with the primary productivity. Thermodynamic considerations show that iodide is a metastable form at the pH of sea water. Laboratory studies fail to show the oxidation of iodide at measurable rates. Elemental iodine is unstable in sea water and undergoes hydrolysis to form hypoiodous acid in seconds. Hypoiodous acid is also unstable and has a life time of minutes to hours. It may react with organic compounds to form iodinated derivatives or it may be reduced to iodide by a reducing agent. The disproportionation of hypoiodite to form iodate seems to be a slower process. A possible chemical cycle for iodine in the marine environment is proposed.