초록 ▼

○통합 시계열 해양관측 시스템 (EAST-I)구축을 통해 Time-series Monitoring 을 확립(총 21회 동해 전역을 대상으로 한 관측 조사 수행 : 국제공동 7회)하고, 다양한 최첨단의 관측 방법(자동 수직 이동 실시간 관측 장치, 연안 HF 레이다, 실시간 복합 해양 부이, 수중 계류 관측)을 동원하여 입체적인 관측을 수행하였다. 또한 국제학술지 특집호 발간(Journal of Marine Systems V.78(2))과 국제 학술호의 개최, CREAM/PICES Advisory Panel 선도 등 국제 협력 네트워

○통합 시계열 해양관측 시스템 (EAST-I)구축을 통해 Time-series Monitoring 을 확립(총 21회 동해 전역을 대상으로 한 관측 조사 수행 : 국제공동 7회)하고, 다양한 최첨단의 관측 방법(자동 수직 이동 실시간 관측 장치, 연안 HF 레이다, 실시간 복합 해양 부이, 수중 계류 관측)을 동원하여 입체적인 관측을 수행하였다. 또한 국제학술지 특집호 발간(Journal of Marine Systems V.78(2))과 국제 학술호의 개최, CREAM/PICES Advisory Panel 선도 등 국제 협력 네트워크를 구축하여 국제적인 동해 연구를 주도하였다.
○수중 계류선과 해양부이 등 시계열 모니터링 시스템 구축과 장기 자료 분석을 통하여 동해의 해양생태계와 열·염순환과 연관된 해수유동과 물성의 다양한 시·공간규모 변동을 파악하고, 기후변화에 따른 해수순환 예측과 생태계 접합 모델 기반 확립을 위한 3차원 순환모델을 개발하였다.
○ 동해는 탄소에 대해 작은 대양이라는 별명에 어울리는 거동을 보여주었다. 인류기원 이산화탄소 심해 유입은 지구온난화의 영향으로 위축되었다. 반면 표층 해수의 이산화탄소 흡수와 이로 비롯된 산성화는 전형적인 대양에 비해 2배가량 빠르게 진행되고 있었다. 컨베이어 벨트의 감속과 심층수 형성 모드의 변동에 따라 저층 무산소화가 주목을 받고 있다. 일본분지에서는 저층수 형성이 일어나므로 문제가 없지만 울릉분지에 대해서는 보다 강화된 감시가 필요하다. 울릉분지 바닥 표면에는 금속산화물이 특징적으로 존재하는데 아표층 엽록소 최대층의 발달과 결부된 송출 생산 플러스와 유기물 분해가 무산소화의 열쇠를 쥐고 있다고 판단된다. 한편 심층의 용존유기탄소 농도는 태평양과 대서양보다 훨씬 높게 나타났다. 그리고 황산염환원은 동해 표층퇴적물에서 유기물 재순환에 중요한 역할을 수행하고 있음이 밝혀졌다. 규산염은 중층수괴의 추적자로 쓰일 수 있는 가능성을 보여주었다.
○ 동해의 저차 생태계 상호작용은 수층의 물리·화화적 변동에 의해 반응하였으며, 일차생산력은 표면혼합층 변동에 따른 아표층의 높은 신생산력에 의해 조절되었다. 동·식물플랑크톤 군집구조는 동해 수리학적 특성의 변동에 따른 영양염 구조에의해 간 뚜렷한 차이를 나타냈으며, 동해의 먹이망 내 에너지 흐름은 pelagic 과 benthic 의 두 pathway 를 통하여 저차영양단계에서 최상위영양단계인 어류에게 전달되는 것을 확인 할 수 있었다.

Abstract ▼

Ⅳ. Results
1. Integrated Time-Series monitoring
A. Establishment
(1) Establishment, improvement and consolidation of time-series network
● Baseline/Monthly monitoring
● Industry-University and international cooperation
(2) Development of high-tech observation equipment
● E-RAP

Ⅳ. Results
1. Integrated Time-Series monitoring
A. Establishment
(1) Establishment, improvement and consolidation of time-series network
● Baseline/Monthly monitoring
● Industry-University and international cooperation
(2) Development of high-tech observation equipment
● E-RAP (EAST-I Real-time Automatic Profiler)
● High Frequency Radar
● UBIM(Ulleung Basin Integrated Mooring)
B. International cooperative research
(1) International cooperative research
(2) Host a international worshop and PICES summer school
(3) Lead a CREAMS/PICES Advospru Panel
C. Development and standardization of observational/analytical technique
(1) Supporting Real-time satellite Data (SST, Ocean Color, Wind, Altimetry)
(2) Publish Cruise/Technical report
(3) Publish the Korean translation of "Guide to Best Practices for Ocean CO2 measurements"
(4) Development of pH meter
2. Upper Circulation and Pelagic Ecosystem
A. Structures and chanracteristics of upper circulation in the East Sea
(1) Spatial and temporal structure of eddy kinetic energy in the East Sea
The spatio-temporal structure of the upper circulation of the East Sea and factors affecting the upper circulation variability are investigated with Eddy Kinetic Energy(EKE) calculated using the merged satellite altimetry data obtained from two satellites over 15 years (1993~2007).
The spatial pattern of the EKE is characterized by a low EKE region (<30(cm/s)²) to the north, and a high EKE region (>150 (cm/s)²) to the south of the subpolar front in the Ulleung and Yamato Basins. The monthly mean EKE is the lowest in spring, March to May, and highest in September, lingering into November, Exhibiting the seasonal variation. The basin-averaged EKE over 15years has an increasing trend with a slope of 0.21((cm/s)²/15yr) in the annual time evolution.
To investigate the relationship between the frontal variability and the long-term trend of the EKE, the difference of the mean sea surface height is calculated between the northern (SSH_N) and the southern (SSH_S) area averaged over respective 2° latitudinal bands from 38°N representing the subpolar front. The 2° latitudinal band was chosen considering the width of the subpolar front of about 200km. The temporal evolution of △h shows not only high frequency signals and low frequency fluctuations but also an increasing trend of about 5.0×10^-5 (cm/15yr), implying that the horizontal instability due to an intensified along-frontal jet and associated amplification of the frontal meandering may contribute to the increasing trend of the EKE in the East Sea.
(2)Coastal upwelling
An anomalous surface cooling event was detected at an ocean buoy station (ESROB: East Sea Real-time Ocean Buoy) from 8 to 20 August 2007. During the period, temperature at 2m depth decreased by 14 °C within 3 days due to a coastal upwelling. An analysis of long-term monitoring data acquired by ESROB confirms that surface water temperature during the event was colder by 13 °C than 10-year mean temperature, and upwelling favorable southerly winds during the event were the strongest (>5m/s, max.:llm/s) and most persistent ( ~10 days) over the 10-year period. Although the occurrence of the strong and persistent southerly winds was reported to be linked to a large scale atmospheric circulation (Park and Kim , 2010), other possible factors affecting the upwelling and ocean's internal structure during the period of the anomalous upwelling have been largely unknown. According to the ESROB data, northwardalongshore currents, which are stronger than the 10-year mean value, were dominant at 20-60m (strongest at 40m) prior to the upwelling. Vertical shear of alongshore currents between surface and 40m was negative, while the shear between 40m and 70m was positive. The northward alongshore currents and observed current shear suggest that subsurface isotherms slope upward towards the coast, which would then become more vulnerable to surfacing when southerly winds blow. Hence, the background circulation worked in concert with the upwelling favorable winds to produce the abnormal upwelling in summer 2007.
(3) Interaction between NIW(Near-inertial Internal Wave) and warm eddy
Internal waves influence the movement of pollutant in the ocean, marine ecosystem, and climatic change by contributing to the mixing-driven upwelling in the interior ocean. The internal waves have most of their energy in near-inertial frequency band called near-inertial internal waves (NIWs). This study provides, for the first time, an observational evidence of the NIW reflection in the upper thermostad of an anticyclonic mesoscale eddy based on long-term current meter mooring data.
Downward phase propagation, hence upward energy propagation, of NIWs in the upper thermostad was captured by the mooring, when it was placed in the middle of an anticyclonic mesoscale eddy. Scaling analysis and ray tracing indicate that the observed reflection occurs because the effect of the vertical shear of subinertial horizontal currents is larger than the buoyancy effect in controlling the propagation of NIWs. During the upward NIW reflection periods, the observation hints the shortening of the vertical wavelength, which would then cause an energy cascade toward small scales, wave breaking, and enhanced turbulence within the thermostad layer of the eddy. Highly-resolved CTD and ADCP data taken in other period show that calculated Richardson number across the anticyclonic mesoscale eddy is below 0.25 in the upper thermostad layer.
These observations provide significant motivation to more systematically observe and closely examine the role of mesoscale eddies in NIW propagation and related mixing.
This study investigates aninteraction between the mesoscale eddies and near-inertial internal waves (NIWs) using data from current meter mooring, which captured a passage of an anticyclonic eddy for longer than one month. Analyses of data reveal that (1) an in-phase NIW propagations within the thermostad of the eddy, (2) an amplification of NIWs at the base of the eddy, and (3) an increase in spectral energy at high frequency during the eddying period, suggestive of the dissipation of the NIWs inside the eddy ultimately contributing to turbulent mixing.
Furthermore, this study indicates that the NIWs are trapped at the critical layer where the relative vorticity goes to zero near the base of the eddy as mentioned in
(2)in the above rather than propagates down below the eddy to enhance the NIW energy at deeper depth levels as reported in the previous study. As mentioned in
(3), the amplified NIWs are ultimately dissipated to become higher frequency internal waves, and breaking of those waves may contribute to the vertical mixing near the base of the eddy and play a role in supplying nutrients to the upper layer resulting in changes in marine biota. Observational efforts should put forth to elucidate this possibility.
(4)Spring bloom in the Ulleung Basin in 2010 observed by time series measurement of biogeochemical parameters
To examine the temporal evolution of biogeochemical and physical properties and to understand the causal mechanism of spring phytoplankton bloom in the Ulleung Basin, an ocean buoy station equipped with fluorometers, PAR sensors, temperature profilers, and current meters was deployed near the center of the basin at a depth of about 2000 m for 2.5 months from February to May 2010. The mooring was placed near the frontal boundary formed by the East Korean Warm Current, which flowed eastward after separating from the coast and turned to the north south of Ulleungdo. Recorded temperature data alternated between warm and cold episodes, reflecting the frontal movement. The observed low-frequency variability of the chlorophyll concentration during the entire observation period appears to be related with the warming and cooling of the surface layer due to the frontal movement. The amplitude of the chlorophyll variation, however, is much smaller prior to the bloom period than those during the bloom period. The chlorophyll concentration during the bloom period ranges from 0.6 ug/l to 3.7 ug/l with fluctuating time scales of 7 ~ 20 days. Semidiurnal variation and subinertial fluctuations with a period of 2~3 days of the chlorophyll concentration are also obvious.
Apart from the high- and low-frequency variations of the chlorophyll concentration, the measured chlorophyll at 30m clearly shows the spring bloom from April 8 until the end of the mooring deployment, when the mean chlorophyll concentration doubled the mean value during the pre-bloom period. The mean mixed layer depth also decreased by about 10 m as compared to that prior to the bloom period. According to this observation, the spring bloom in the Ulleung Basin was triggered by the shoaling of deep isotherms and mixed layer depth because of the cold water intrusion in April. The shoaling of deep isotherms implies an enhanced supply of nutrients to the euphotic zone. The cold water intrusion also lowered temperature at 80 m. The cold water is thought to be the East Sea Intermediate Water considering its temperature range of 1.0~5.0°C.
The dissolved oxygen concentration measured at 30 m is highly correlated with the chlorophyll concentration at the same depth level over a broad frequency range, indicating that the dissolved oxygen concentration is mainly determined by the biological process.
(5)Characteristics and variability of the Kuroshio in the East China Sea
The Korea-USA joint moored observation was performed along a section for two years to quantify the Kuroshio in the East China Sea (ECS) and to investigate the temporal variability of the ECS Kuroshio. Major results are as follows.
Mean currents across the ECS Kuroshio section consist of the northeastward flowing Kuroshio, southwestward flowing recirculation, and also southwestward flowing undercurrent beneath the Kuroshio. Mean volume transports of each flow are 24.0 Sv (northeastward), -4.9 Sv, and -0.5 Sv for the Kuroshio, recirculation, and undercurrent, respectively. Accordingto long-term transport time series estimated from satellite altimeter data and calibrated by the directly observed transports, the 12-year mean Kuroshio and Ryukyu Current transports are 18.7±0.2 (standard error) Sv and 5.4+0.4 Sv, respectively.
The Kuroshio volume transport shows a distinct intraseasonal variation near 60-day period, which mainly arises from the strengthening and weakening of the Kuroshio transport, not due to changes in the Kuroshio axis. On this timescale the co-variability of the Kuroshio and the recirculation shows an out-of-phase relationship. The near 60-day variation of the ECS Kuroshio transport is poorly correlated with the transport variation in the upstream region east of Taiwan. Instead, the 60-day variation is thought to be caused by water exchange through Kerama Gap between Taiwan and Ryukyu Islands. Apart from the intraseasonal variation, the ECS Kuroshio transport is shown to be related to the PDO (Pacific Decadal
Oscillation) index with a correlation coefficient of 0.76. The PDO-related Kuroshio transport variation amounts to about 4.0 Sv.
Regarding the relationship between the ECS Kuroshio transport and the volume transport of the Tsushima Current through the Korea Strait, when the Kuroshio transport is large (positive PDO index), the Kuroshio flow is inertially-dominated and passes across bottom topography to exit to the Tokara Strait and the Tsushima Current transport becomes smaller. While, the Kuroshio is frictionally-dominated when its transport is small (negative PDO index), then it follows bottom topography to induce a larger volume transport through the Korea Strait.
(6)Variability of surface currents between Ulleungdo and Dokdo
Surface geostrophic currents are calculated from sea level difference between Ulleungdo and Dokdo using data from tide-gauge station at Ulleungdo and bottom pressure measurement for 40 months near Dokdo. Together with sea surface height (SSH) derived from satellites, the data sets allow to investigate the temporal variability of surface currents in the Ulleung Interplain Gap (UIG) between Ulleungdo and Dokdo, which is a unique conduit for deep water exchange between the northern and southwestern East Sea.
Calculated 28-day low-passed geostrophic currents from the sea level difference are significantly correlated with directly measured upper currents in the middle of the UIG with a mean correlation coefficient of 0.74. Comparisons between merged SSH productand directly observed sea levels at Ulleungdo and Dokdo show correlation coefficients of 0.90 and 0.95, respectively, with their RMS errors of 3.83 cm at Ulleungdo and 5.13 cm at Dokdo.
Mean surface currents in the UIG between 2001 —2007 based on the satellite-derived SSH difference are directed southwards with mean value of -3.64+0.39 (mean error) cm/s. Standard deviation of currents is 7.49 cm/s, indicating a large variation of currents ranging from -20 cm/s to 15 cm/s. Current spectra peak at 80 days,120 days, and 300 days, and the frequency of the occurrence of the southward currents has an increasing tendency since 2004. Monthly mean currents are strong from September to January, and show minima in winter (March) and summer (July and August). Annualmean currents indicate a large interannual variation of surface currents in the UIG, with the strongest annual mean southward currents in 2004. Southward currents occurred in the UIG when the central location of the Ulleung Warm Eddy (UWE) was placed southof Ulleungdo. While, northward currents occurred in the UIG under the following situation, 1) when the UWE is located north of Ulleungdo or south of Dokdo, or 2) when the Dok Cold Eddy migrates towards the east coast of Korea, or 3) the Offshore Branch extends towards Dokdo.
B. Community structure and productivity of low trophic level in pelagic ecosystem in the East/Japan Sea
To understand the spacial and temporal variabilities of bacterial abundance (BA), production (BP) and community compositions in theEast Sea, seven investigations were made in 2006-2010. Maximum BA and BP were mostly found at surface or subsurface (10-50 m). BA at coastal sites (0.7-4.3x10⁹ cells 1^-1) was generally higher than that at offshore sites (0.7-2.8x10⁹ cells 1^-1). BP at coastal sites (0.2-4.9x10⁹ cells 1^-1 d^-1) was also generally higher than that at offshore sites (0.1-2.3X10⁹ cells 1^-1 d^-1). BA and BP gradually decreased with depth below 100 m.
Analyses of bacterial community structure using PCR (Polymerase Chain Reaction)-DGGE (Denaturing Gradient Gel Electrophoresis (-sequencing for 16S rRNA gene showed that Gamma-, Alpha-Proteobacteria and Bacteroidetes dominated in the epipelagic and mesopelagic zones of the East Sea. As for Archaea, Euryarchaeota was detected in the epipelagial, and three groups of archaea (Euryarchaeota,Crenarchaeota and Thaumarchaeota) in the mesopelagial.
Quantitative analyses for major bacterial groups using CARD-FISH (Catalyzed Reporter Deposition-Fluorescence In Situ Hybridization) revealed that Gamma-, Alpha-Proteobacteria or Bacteroidetesdominated in seawaters depending on investigated season or sampling depth. Alpha-Proteobacteria (mean of 36.9%) and Bacteroidetes (mean of 16.8%) mostly dominated in the epipelagic and mesopelagic zones, respectively. In this study, eleven new species constituting 6 new genera in Alpha-Proteobacteria and 5 new species in Alpha-, Gamma-Proteobacteria and Actinobacteriawere subjected to phylogenetic, physiological and biochemical analyses, 10 of them have been already approved as new taxa.
Our study on the compositions of airborne prokaryotes over open seas was the first modern study, unveilling that airborne bacteria could be as diverse as surface water bacteria. The occurrences of prokaryotic sequences originated from marine and terrestrial sources in marine aerosols indicated that aerial transport of prokaryotes might be possible. In addition, the diversities of algal viruses and cyanophages specific to primary producers (algae and cyanobacteria, respectively) were for the first time investigated in surface waters of the East Sea, suggesting that aerial transport of those viruses by marine aerosols seems to be quite possible. In this study, a collecting method using impingers for marine viruses were set up. PEI issuggested to be an efficient chemical in collecting and concentrating marine viruses in the impinger-collecting method.
A relationship between the ratio of bacterial production to primary production and sea-surface temperature was similar between the previous and current investigations in the East Sea. This observation suggests that no pronounced changes occur in respect with that physio-chemical variabilities of water column drives similarly the interaction of bacteria with phytoplankton in the East Sea as before.
C. Spatial variation of summer phytoplankton community along a meridional transect
in the East/Japan Sea
The East/Japan Sea (EJS) located in the northwestern Pacific Ocean, a semi-closed marginal sea, is characterized by dynamic hydrologic features including northern cold waters, southern warm waters, subpolar fronts, and eddies. Despite the prevalence of the nutrient-poor surface water mass introduced by the Tsushima Warm Current (TWC), primary production in the EJS remains relatively high compared to the adjacent seas. Spatial variation of phytoplankton community structure was investigated in the EJS in July 2009 using HPLC pigment analysis. Phytoplankton was size-fractionated and numerated to examine the composition of picophytoplankton (<3 pm in size) using flow cytometry. Total chlorophyll a concentrations in the euphotic layer (>50 m) in the northern cold waters of the EJS were approximately two-fold higher than those in the southern warm waters. The relative contribution of each phytoplankton group to the total chlorophyll a was different depending on water mass properties. In the southern part (warm waters), diatoms, dinoflagellates and cyanobacteria dominated the phytoplankton community. In the subpolar fronts, prasinophytes, cryptophytes and cyanobacteriadominated the phytoplankton community. In the northern part (cold waters), diatoms, cryptophytes, and prasinophytes dominated the phytoplankton community. Cyanobacteria were also a dominant group in the small-scale warm eddy and surface waters of subpolar fronts but the dominance of this group was replaced by prasinophytes, cryptophytes and prymnesiophytes in the northern cold waters. The biomass of phytoplankton in the southern part was high in the surface layer. But vertical profile of each phytoplankton group in the northern part showed subsurface maximum layer. Chlorophyll a concentrations of picoplankton contributed up to ~60% of the total chlorophyll a in the EJS. One of the picoplankton groups (Synechococcus, Prochlorococcus, andpicoeukaryotes) measured by flow cytometry, picoeukaryotes occupied more than 90% of total picoplankton abundances at most the studied stations of the EJS. The abundances of Prochlorococcus and Synechococcus, typical of the oligotrophic oceans due to the adaptability in the low nutrient condition, in the EJS were one to two orders of magnitude lower than those in the Pacific Ocean and East China Sea. Difference in the distribution of phytoplankton and picoplankton in the EJS ecosystem was likely to controlnutrient supply into the euphotic zone due to degree of the upper water stratification during summer.
D. Spatial and temporal distribution characteristics of zooplankton community in the East/Japan Sea
Zooplankton samples were collected in the East Sea on eight cruises from July 2006 to May 2010. Mean abundances and species number of zooplankton ranged 82.4 to 996.8 ind. m^-3 and 25 to 118 species, respectively. In spring, the temperate coastal species, Paracalanus parvus s. l. and Calanus sinicus, were dominant in the Korea Strait and the cold water species, Pseudocalanus newmani and Scolecithricella minor,appeared frequently in the region at 37°N. Whereas, the cold water species, such as Neocalanus cristatus, N. plumchrus and Metridia pacifica, predominated in the region above 39°N. In summer, the temperate coastal species and warm water species co-occurred broadly below 39°N, and the cold water species predominated in the coastal area from 37.5°N to 38°N. The composition of zooplankton in autumn was similar to that in summer. In the coastal area from 37.5°N to 38°N, abundance
of the cold water species decreased in autumn. The composition and distribution of zooplankton in the East Sea mainly varied with currents and water masses. Based on the data of zooplankton composition and abundance, the study area in the East Sea can be divided into three regions: the Korea Strait region, Ulleung Basin Region and the region above 39°N.
3. Air-Sea Interaction, Mixed Layer and Ecosystem
A. Variation of primary productivity by the nitrate vertical distribution and subsurface mixed layer in the East/Japan Sea
Primary, new, and regenerated production were measured to elucidate the high primary productivity in the East/Japan Sea (EJS) with the East China Sea (ECS) and the Western Pacific Ocean (WPO), by an uptake experiment using ¹³C labeled
carbon and ¹⁵N labeled nitrogen with physical and chemical parameters in summer 2008. In order to examine the seasonal variation of primary production in EJS, we conducted additional survey in fall 2008 and spring in 2009. In this study, primary productivity (PP) in summer greatly increased along the northward trajectory to the EJS (0.37-0.96 g C nT2d_1) through the ECS (0.28-0.28 g C m^-2d^-1) from the WPO (0.17-0.26 g C m^-2d^-1). Such a high PP in summer in the EJS was comparable with those in fall and spring (0.61-0.94 and 0.38-0.40 g C m^-2d^-1c, respectively). In summer, new productivity (NP) in the EJS was higher (p = 0.005) than those in the WPO and the ECS. On the other hand, regenerated productivity was not significant difference in the study area (p =0.065). PP showed significant positive correlation with NP and f-ratio (r²= 0.94 and 0.73, respectively). Nitrate uptake rate in the mixed layer was strongly correlated with the nitrate upward flux (r² = 0.80). Therefore, the variation of NP is determined by the nitrate vertical distribution and the physical structure of the water column in summer. In the EJS, vertical distribution of PP showed that the highest PP in summer was at the subsurface layer near nitracline while those in fall and spring were the highest at the surface layer. In conclusion, the high summer PP results from NP which is supported by diffusive nitrate upward flux in the water column.
B. Long-term variation of heat content in the East Sea
A decadal variability was extracted as the most dominant mode of upper-ocean heat content variability in the East Sea aside from the seasonal cycle. The decadal
variability is strong to the west of northern Honshu, west of the Tsugaru Strait, and west of southern Hokkaido. Temperature anomalies in 50-125 m levels exhibit a large contribution to the decadal variability particularly in the eastern part of the East Sea. The decadal variability seems to be related to the inflow through the Korea Strait, the Tsushima Warm Current. The decadal variability also exhibits an increasing trend, which indicates that the regions showing large decadal variations experienced warming in the decadal time scale. The warming in the East Sea is not locally isolated but is related to the warming in the Northwestern Pacific.
C. Phytoplankton community related to water mass properties along a meridional
transect in the East/Japan Sea
There were the intra- and inter-regional variations in biomass and community composition of phytoplankton in the East/Japan Sea (EJS) and the Korea Straitdue to complex hydrological environments. The integrated chlorophyll a concentrations in the euphotic layer were approximately two-fold higher in the Korea Strait (mean: 31.6 ± 9.1 ng m^-2) than in the EJS (mean: 17.8 + 8.9 ng m^-2), while the phytoplankton community composition was more diverse in the EJS in May 2007. Total chlorophyll a concentrations in the euphotic layer in the northern cold waters of the EJS were approximately two-fold higher than those in the southern warm waters because of vertical water column stability. Chemotaxonomic analysis of phytoplankton, using a CHEMTAX package calculation, was used to estimate the contribution of phytoplankton groups to total chlorophyll a. Cyanobacteria was a dominant group in the surface layer (<20 m) in the southern warm waters including the subpolar frontal zone but the contribution of this group to the total chlorophyll a was replaced by diatoms and cryptophytes in the northern cold waters. The contribution of cyanobacteria decreased with depth and diatoms and prymesiophytes were dominated in the subsurface layer (>20 m). On the other hand, diatoms and dinoflagellates were predominantin the Korea Strait under relatively high nutrient condition. These results suggest that the difference of phytoplankton community structure between the EJS and the Korea Strait is mainly due to the nutrient and hydrographic conditions (e.g. surface-water structure).
4. Role of Straits in Circulation and Ecosystem Variability
A. Volume transport through straits
It is newly found that bottom cold water in the Korea Strait originating from the northern region of the East Sea appears not only in summer and autumn but also in winter. Intensive observations in the Korea Strait revealed two distinct cores of northeastward currents in the upper layer of the western and eastern channels. Mean volume transport through the Korea Strait is calculated as 2.5±0.5 Sv from four-year direct and indirect measurements. For the first time, simultaneous time series of volume transports are available in the Korea and Tsugaru Straits during the winter of 1999-2000. Outflow through the Tsugaru Strait accounts for about 70% of inflow through the Korea Strait for this period.
B. Long-term variation of the Korea Strait Bottom Cold Water
The interannual variability of the Korea Strait Bottom Cold Water (KSBCW) was investigated and the strength of the KSBCW fluctuates yearly with a majorspectral peak around 3 years. The interannual KSBCW variability is closely linked with the temperature variability in the southwestern region of the East Sea at about 50-100 m depth. The source of the KSBCW is traced at deeper level from north to south, confined more toward the east coast of Korea. The interannual KSBCW variability is also related to the southward wind stress along the east coast of Korea. It appears that strong cooling/warming of upper water temperature induced by the basin.scale wind stress results in the interannual KSBCW variability. This connection is verified by showing a reasonable interannual covariability between the KSBCW and the basin.scale wind stress.
C. Temporal variation of phytoplankton community related to water column structure in the Korea strait
Photo synthetic pigments, nutrients, and hydrographic variables were examined in order to elucidate the spatio-temporal variation of water column structure and its effect on phytoplankton community structure in the western channel ofthe Korea Strait in fall 2006 and spring 2007. High phytoplankton biomass in the spring was associated with high salinity, implying that nutrients were not supplied by coastal waters or the Yangtze-River Diluted water with low salinity. Expansion of the Korea Strait Bottom Cold Water (KSBCW) and a cold eddy observed during the spring season might enhance the nutrient supply from the subsurface layer to the euphotic zone. Chemotaxonomic examination showed that diatoms accounted for 60-70% of total biomass,followed by dinoflagellates. Nutrient supply by physical phenomena such as the expansion of the KSBCW and the occurrence of a cold eddy
appears be the controlling factors of phytoplankton community composition in the Korea Strait. Further study is needed to elucidate the mechanisms by which the KSBCW is expanded, and its role in phytoplankton dynamics.
D. Understanding boundary processes
Sea water transport througth the Korea strait is an important major factor for controlling the material cycles and changing the ecosystem dynamics in the East Sea. In this study, we are going to understand
the effect of water mass transport through the Korea strait in the southern part of the East Sea (including Ulleung Basin, Yamato Basin) using simple box-model. The change of monthly water transport through the strait was confirmed by the comparison of KODC salinity data at 206 and 207 lines between 1975 and 2004, which showed strong seasonal cycle. In the Upper surface layer(0-50 m), the water mass in the strait showed high saliniy in April and low salinity in August. In the Lower surface layer(50-150 m), seasonal variability is much smaller than that in the Upper surface layer. The result of box model agreed well with the seasonality of salinity, which suggest the monthly transport value used in the model was appropriate to explain the variability of salinity in the Korea strait.
E. Plankton community response to physico-chemical forcing.
To understand the response of plankton to the three distinct physio-chemical settings developed in summer in the Ulleung Basin of the East Sea, a multidisciplinary survey was conducted onboard Haeyang2000 in August 2008. Baseline setting of hydrographic condition were the presence of thin (<20 m) Tsushima Surface Water (TSW) on top of the Tsushima Middle Water (TMW). It extended from Korea Strait to 37 °N along the 130 °E and then turned toward offshore and encompassed the relatively saline (T> 26 °C, S>33.7) Ulleung Warm Eddy surface water centered at the 36.5 °N and 131 °E. In the southeastern coast of Korea relatively colder and saline water mass appeared. It was accompanied by higher nutrient and chlorophyll-a concentrations suggesting a coastal upwelling. Most of the surface offshore waters support a low phytoplankton biomass (0.3 mg chl-a m^-3). Much denser phytoplanlton biomass (1-2.3 mg m^-3) was accumulated at the subsurface layer between 20-50 m depth. The subsurface chlorophyll-a maxium (SCM) layer were closely related to the nutricline, suggesting an active growth of phytoplankton at depth. The SCM developed at shallow depth (20-30 m) near the coast and deepened towards offshore (50-60 m). A fucoxanthin/zeaxanthin indicates that diatoms dominate coastal waters while cyanobacteria dominate offshore waters. The community structure and biomass of phytoplankton were closely related to the availability of nitrogen availability. The biomass of zooplankton was higher in coastal region than in the offshore region while the species richness showed opposite trend. The community structure of zooplankton retained a coastal/offshore contrast. This suggests that summer hydrography is a stable structure, lasting long enough to allow a hydrography specific plankton community to evolve.
F. Distribution of dissolved trace elements in the East Sea
Tracer metals exist very small amount in natural condition, but they are very fatal in human helth because of their tendency to accumulate. They also play critical role in the growth of phytoplankton. Thus it is very important to understand the spatial and temporal distribution patter. So far we are focused on our resource to understand the distribution pattern of tracer metal concentration near to coastal region. Thus the understanding on the characteristic on the tracer metal in offshore region was
very poor. In this study, we measured the concentration of dissoved tracer metal in the surface layer of the East Sea onboard R.V Lavrentyev during July 2009. Horizontal distribution of tracer metal was closely related to the water temperature. The concentration of tracer metal increased as temperature decreased in the coastal region of Russia(T<10 °C) but increased as temperature increased in the Ulleung Basin
G. Trace elements within organisms of the East Sea
To understand the content of tracer metal within the biomass exist in the coastal region of the East Sea, we measured the concentration of tracer metal from kelp, sea weed,sea lectus, sargasso from coatal region to Ulleung Islands. The concentration of tracer metal showed large variation depend on the time of harvest, havitat, degree of exposure to source, rank at food chain. So we have to choose specific time and location for long-term monitoring of trace metal concentration
4. Thermohaline Circulation and its Variability
A. Characteristics of currents off Uljin in the East Sea
The characteristics of low-pass filtered currents off Uljin in the East Sea is
analyzed using current meter mooring data obtained during May 2006 - February 2010. Mean flows during the common-length period in summer 2006 are directed southward at all moorings except surface layers. The longest data were obtained at mooring UB2 at a depth of about 1800 m during the entire period. Upper currents with depths shallower than and equal to 200 m at mooring UB2 are mainly northward with large temporal fluctuations, while deep flows at 1000 m and 1600 m are persistently southward. Seasonal variations of upper and deep currents are indistinct. Current fluctuations in the upper layer are poorly correlated with those in the deep layers during the entire period of moored current measurement. Statistically significant correlation, however, was found between the upper and deep currents in 2006-2007 winter when strong northward currents were observed in the upper layer. Weak and northward deep currents occurred during the period instead of dominant southward currents. The mooring UB2 was placed in the western periphery of an anticyclonic mesoscale eddy, and it appears the barotropic component of currents was intensified during the period.
B. Vertical structure of low-frequency currents at EC1
Vertical structure of low-frequency flows in the central Ulleung Interplain Gap of the southwestern East Sea is analyzed based on full-depth current measurement during November 2002 - April 2004. Record-length mean flows are directed toward the Ulleung Basin throughout the entire water column. Upper current variability above the permanent thermocline with a dominant period of about 50 - 60 days is shown to be closely related with the displacement of an anticyclonic warm eddy associated with the westward meander of the Offshore Branch. Fluctuations of deep currents below the permanent thermocline have a dominant period of about 40 days. Coherency between the current near the seabed and shallower depths is statistically significant up to 360 m for a period range between 15 and 100 days, but less significantly correlated with currents in upper 200 m. Data from the densely equipped mooring line reveal that mean and eddy kinetic energies are minima at 1000 m, where isotherm slopes are also relatively flat. Empirical orthogonal function (EOF) analyses suggest that more than 79% of total variances of upper and deep currents can be explained by their respective first EOF mode characterized by nearly depth-independent eigenvectors. Spectral and EOF analyses of observed currents suggest that most of the deep current variability is not directly related with local upper current variability during the observation period.
C. Dokdo Abyssal Current
Deep circulation in the southwestern East Sea through the Ulleung Inter plain Gap (UIG), a possible path way for deep-water exchange, was directly measured for the first time. Five concurrent current meter moorings were positioned to effectively span the UIG between the islands of Ulleungdo to the west and Dokdo to the east. They provided a 495-day time series of deep currents below 1800m depth spanning the full breadth of the East Sea Deep and Bottom Water flowing from the Japan Basin in to the Ulleung Basin. The UIG circulation is found to be mainly a two-way flow with relatively weak
southward flows directed into the Ulleung Basin over about two-thirds of the western UIG. A strong, persistent, and narrow compensating northward outflow occurs in the eastern UIG near Dokdo and is first referred to here as the Dokdo Abyssal Current. The width of the abyssal current is about 20km below 1800m depth. The low-frequency variability of the transports is dominated by fluctuations with a period of about 40days for inflow and outflow transports. The overall mean transport of the deep water below 1800m into the Ulleung Basin over the 16.5 months is about 0.005 Sv, with an uncertainty of 0.025 Sv indicating net transport is negligible below 1800m through the UIG.
D. Long-term variation of deep currents in the Ulleung Interplain Gap
Recent studies clearly show that oceans are undergoing dramatic changes. In recent years, the East Sea has also undergone significant long-term trend of water properties characterized by a rise in deep water temperature and decline of dissolved oxygen. The water property changes have been attributed to the modification of its ventilation system! a slowdown of bottom water formation and an enhancement of upper water formation instead. However, little is known about the actual long-term trend of the deep circulation. The Ulleung Interplain Gap(UIG) in the southwestern East Sea, that is about 90 km wide and oriented northeast-to-southwest, serves as a unique passageway for the exchange of deep waters below 1500 m between the Japan Basin, where deep water formation occurs, and Ulleung Basin (UB). Long-term trends of deep flow and deep water properties in the central UIG are investigated using data from a 15-year (1996-2010) long moored current measurement (station EC1) and full-depth CTD surveys between 1995 and 2010. Deep flows below 400 m (below the permanent thermocline) are mainly directed to the southwest towards the UB.
E. Topographic Rossby wave in the Ulleung Interplain Gap
Moored current measurements were made over steep bottom slope in the eastern Ulleung Interplain Gap in the southwestern East Sea. The observed currents below 300 m show dominant fluctuations at periods around 20 and 40 days. They also show bottom-intensified vertical structure at both periods, which is reasonably consistent with the theory of linear topographic Rossby waves (TRWs). The TRWs found in the Ulleung Interplain Gap are characterized by short wavelengths of 17 - 30 km primarily due to the smallness of the buoyancy frequency below 300 m. A relationship is observed between warm events in the upper layer and current fluctuations in the deep layer near 20-day periods: the latter are significantly enhanced when the former develop. This indicates local coupling between upper and deep layers over the sloping bottom topography. On the other hand, the 40-day deep current fluctuations appear to be unrelated to upper warm events.
F. Characteristic of deep circulation pattern in the Ulleung Basin from 1-D advection diffusion model of potential temperature and silcate
Deep Ulleung Basin is filled with East Sea Proper Water (ESPW) which is characterized by uniform density. However, characteristics of ESPW are investigated only for physical parameters. In this study, we compared and investigated the vertical profile of potential temperature and silicate to test homogeneity of ESPW. By applying 1 dimensional advection diffusion model to potential temperature and silicate, we calculated the z* value which is the ratio of vertical eddy diffusion coefficient to upwelling velocity, of 0.55 km and 1.24 km, respectively. We ascribed the reason why silicate exhibits lager z* value over potential temperature is due to dilution effect by deep water of surface origin from Northern East Sea. The effect of inflow of deep water can be estimated by J/w, and J/w showed maximum around 1400 m depth. We interpreted this depth as the core layer of deep water inflow. Adopting the reported value of upwelling velocity to z* of potential temperature, one can simulated the response time of bottom sequestrated dissolved material. It turns out that the time scale of sequestration is fairly short, about 100 year or so.
G. Discontinuity layer of phosphate:silicate developed at mid-depth in the East Sea
The CREAMS (Circulation Research of the East Asian Marginal Sea) survey in 1999 revealed a sharp mid-depth discontinuity of the phosphate silicate ratio in all basins of the East/Japan Sea. Incidentally,this discontinuity layer corresponds to the
oxygen minimum layer. Directly the discontin uity layer, oxygen concentration is increase. This increase in oxygen concentration is interpreted as a proof of intermediate water formation. Oxygen minimum indicates that the water parcel is old and stable against mixing. So it seems be an efficient barrier to vertical exchange of materials. This means that, once materials enter the lower domain, they rarely return to the upper domain. Therefore, the biogeochemistry of the East/Japan Sea depends heavily on material input through the Korea Strait, and flux is exprected to be sensitive to the climate change. As a result, the East/Japan Sea Ecosystem seems vulnerable to tipping (regime shift), which occurred on a decadal time scale.
H. Understanding dynamic of Conveyor belt in the East Sea
In the summer of 2008(August 4-14), vertical and horizontal distributions of inorganic nutrients and dissolved organic carbon(DOC) were measured in the southwestern East Sea. Concentrations of DOC were determined for the first time in the southwestern East Sea using the high-temperature combustion oxidation (HTCO) method, and results were compared with those measured by another laboratory. Concentrations of DOC ranged from 58 to 104 uM in the upper 200m, showing a typical decresing pattern with depth. Generally, concentrations of DOC were relatively lower, with higher nutrient concentration, in the upper layer of the coastal upwelling zone. Concentrations of DOC ranged from 54 to 64 uM in the deep Ulleung Basin (200-1500m), and were higher than those in the Pacific and Atlantic ocean. In association with rapid vertical ventilation of the euphotic, this difference indicates a larger accumulation of semi-labile DOC in the deep East Sea than in the major oceans. A correlation between apparent oxygen utilizaton (AOU) AND DOC in the deep ocean of the East Sea revealed that only a small portion (<10%) of the sinking DOC, relative to the sinking particulate organic carbon (POC), contributes to microbial degradation. Our results present an important data set of DOC in the East Sea, which plays a critical role in carbon cycle modeling and sequestration.
6. Carbon Cycle and Response to Climate
A. Understanding Biological Pump
Photosynthesis produce organic material from carbon dioxide in the surface layer of ocean and transfer to deep ocean, called biological pump, Photosynthetic process
is controlled by the availability of nutrient and light in the surface layer. Thus primary production in the euphotic layer is closely related to carbon transport to deep layer. The subsurface chlorophyll-a maximum(SCM) in the southwestern part of the East Sea is not developed by the passive accumulation in the pycnocline due to decrease of sinking velocity but by active growth of phytoplankton at depth using nutrient from deep layer. The development of SCM may result in underestimation of primary production by satellite surface chlorophyll data due to the limitation of satellite measurement. SCM is generally developed by depletion of nutrient in the surface layer and supply of nutrient from deep layer during summer. However, there was a development of SCM in May. To understand the spaial and temporal distribution of SCM, we measured vertical chlorophyll profile 8 perpendicular lines along coastal line from the Korea Strait in south and north of Ulleung Island using Haeyang2000. In general, chlorophyll-a concentration was low(<0.3 mg m 3) in the surface layer and high(l-2.3 mg m 3) between 20-50 m depth in most region.
B. Design of drifting sediment trap and measurement of sinking material flux within water column
Photosynthesis of phytoplankton in the surface layer fix ogranic material from carbon dioxide, which is consumed by herbivore or is tranported to deep layer as sinking particle (dead body, fecal pallet). Recently, there were sever modification of material flux through the Korea Strait due to the construction of dam in the Changjiang river, which may influence the primary production in the surface layer of the Ulleung Basin in the East Sea. It is necessary to understand the response of carbon flux from surface layer to deep layer. We designed simple and convinet sediment trap that can be used in the surface ocean. We also measured sinking flux from February 21 2010 to October 10 2010 at 1000 m depth at UBM station of the Ulleung Basin. We are planning to measure total mass flux, POC PON, ¹³C, ¹⁵N, ¹⁴C, and other. There were large amount of sinking material from February to late April and the early July to middle of August. Unexpectedly, there was no collected matrial from early May to the end of June and frome mid August to the end of September. We can see small amount of material in the early October.
C. Understanding boundary processes
(1) Geochemical characteristics and sulfate reduction rates in the pore water of the deep basin sediment in the Ulleung Basin, East Sea
In conjunction with geochemical characteristics, rate of sulfate reduction was investigated at two sediment sites in the continental slope and rise (basin) of the Ulleung Basin in the East Sea. Geochemical sediment analysis revealed that the surface sediments of the basin site (D2) were enriched with manganese oxides (348 ymol cm 3) and iron oxides (133 ymol cm 3), whereas total reduced sulfur (TRS) in the solid phase was nearly depleted. Sulfate reduction rates (SRRs) ranged from 20.96 to 92.87 nmol cm^-3d^-1 at the slope site (M1) and from 0.65 to 22.32 nmol cm^-3d^-1 at the basin site (D2). Depth integrated SRR within the tip 10 cm depth of the slope site (M1;5.25mmol m^-2d^-1) despite high organic content(>2.0^\% dry wt.) in the sediment of both sites. The results indicate that the spatical variations of sulfate reduction are affected by the distribution of manganese oxide and iron oxide-enriched surface sediment of the Ulleung Basin.
(2) Biological production and sulfate redutiokn coupled to coastal upwelling and an anticyclonic eddy in the Ulleung basin, East Sea
We investigated the biomass and production rates of microorganisms associated with coastal upwelling and the anticyclonic Ulleung warm eddy (UWE) of the Ulleung basin in the East Sea. Shipboard observations revealed that depth-integrated phytoplankton biomass and production were higher in the moderately stratified ring of the UWE than in the vertically well-mixed eddy core or in the stratified region outside of the eddy in the Ulleung basin. Similarly, heterotrophic bacterial production was higher in the eddy ring than in the eddy core or outside of the eddy. Bacterial biomass in the highly productive eddy ring showed little difference compared to the bacterial biomass of the core site due to the impact of grazing by heterotrophic protozoa. Satellite imagery and diatom species composition data in conjunction with physico-chemical parameters demonstrated that wind driven coastal upwelling in the southeast of Korea was largely responsible for the phytoplankton bloom and enhanced bacterial production along the UWE. Overall, the results indicated that the UWE entrained highly productive upwelling coastal waters and delivered enhanced microbial biomass and production into the central Ulleung basin. The results further implied that the UWE and the subsequent effect on vertical particulate carbon flux may play a significant role in stimulating benthic respiration and in sequestering organic carbon produced by coastal upwelling down into the deep Ulleung basin.
(3) Measurement of methane concentration within water column
Methane is an important greenhouse in the atmosphere with a global warming potential of 20 compared to CO₂. This means that a methane emission will have 20 times the impact on temperature of a carbon dioxide emission of the same mass.
Methane is generated by fermentation of organic matter in anaerobic water column and sediments. And the Alternative source of methane is emission from methane clathrates. methane release from methane clathrates could cause rapid global warming. To understand use of methane clathrates and global warming by methane, we have to know sink, source and transformation processes of methane in air-water and water-sediment interface. Methane concentration in water column ranged from 1.7~7.9 nM at Line-U August 2008. Methane concentration was increased from sea surface to thermocline. Methane concentration is constant with depth under thermocline.
(4) Nitrogen cycles
To determine the flux or organic mater from the photic zone to the deep ocean in the East Sea, it is important to quantify remineralization and transformation processes of nitrogen in the marine sediments. However, today, there are only a few studies about remineralization and transformation processes of nitrogen in the deep sea sediment of East Sea. Especially, denitrification is an important process that converts biologically available nitrogen to gaseous dinitrogen in marine ecosystems.
Denitrification is a major pathway for N removal in marine environments. Therefore, to improce our understanding of organic matter and nitrogen cycle in water-sediment interface. it is important to know about sediment oxygen demand(SOD), denitrification, nutrient flux and porewater nutrient profile in the deep sea sediment of East Sea. SOD and denitrification rate measured in teh northeast East sea and northwest Pacific ranged from 649~8114 umol-O₂ m^-2 d^-1 and 0.1~27.4 umol-N₂m^-2 d^-1, respecitively. Nitrate(+nitrite) flux measured in the northeast East sea and northwest Pacific ranged from 963~2455 umol-N₂m^-2 d^-1.
D. Anaerobic deep sea
(1) Estimation of basin-specific oxygen utilization rates(OURs) in the East Sea
The oxygen utilization rate (OUR) is one of the crucial parameters for ocean carbon cycling and climate models. However, parameterization of OUR in the East Sea (Sea of Japan) is yet to be established. We estimated the basin-specific OURs in the East Sea and fitted them with exponential functions with depth by using pCFC-12 age and apparent oxygen utilization (AOU) measured in summer 1999. The estimated OURs are higher in the upper water column and decrease with depth, in general. The vertical distributions of the estimated OURs in the Western and Eastern Japan Basins (WJB & EJB) are very similar. The OURs in the Ulleung Basin (UB) varied greatly depending on whether the surface layer (CT200 m) data are included in the OUR estimate or not. Apparently, weaker oxygen consumption occurs in the deep layer of Yamato Basin (YB). The ranges of the OURs between 200 m and 2000 m at WJB, EJB, UB, and YB are 8.15(±0.83), 8.1K+0.68), 5.29(±0.73, and 7.31(±0.06) gmol kg 1 yr \ respectively. Consideration of the wintertime surface water oxygen disequilibrium condition in estimating the OUR will be necessary in the future study.
E. Nutrients and phytoplankton
(1) Characteristics of nutrient distribution and NT ratio in the Ulleung Basin of the East Sea
Nutrients are major component in marine ecosystem and are closely related to phytoplankton and zooplankton dynamics. To understand vertical and horizontal distribution of nutrients and low NT ratio caused by physical mixing of different water mass, we measured nutrients concentration in July 2006, May and October 2007, August 2008, and July 2009 in the East Sea. Vertical distribution of nutrient in the East Sea showed similar pattern that observed in other ocean. Within surface 200m, nitrate, phosphate, silicate ranged 0~20 gM, 0~1.3 jdM and 2~30 gM, respectively. In deep layer (>200 m), the maximum concentration of nitrate, phosphate, and silicate was 25 /iM, 2.1 gM and 90 gM, respectively. Nitrate concentration decreased exponentially as chlorophyll-a concentration increased. From the relationship between NT ratio and chlorophyll-a concentration, phytoplankon didn't utilize phosphate as much as nitrate. It seems that nitrate was depleted in the surface layer of the East Sea.
(2) Hydrographically mediated patterns of photosynthetic pigments in the East Sea: Low NT ratios and cyanobaterial dominance
Nutrients and photosynthetic pigments were measured over a north - south transect of the East/Japan Sea (EJS), which includes various oceanographic environments such as the cold and warm-water masses, subpolar fronts, and eddies in the surface ocean. Sampling was conducted in May 2004, July 2005, and October 2005. The NT ratios were low (b4) for high chlorophyll a areas owing to the active Redfieldian consumption of N and P from waters with a low NT ratio (approximately 13) in the entire EJS. The compositions of photosynthetic pigments were used for identifying phytoplankton groups at a class level using the CHEMTAX program. It is notable that cyanobacteria (40 -60%) dominated the phytoplankton community in the frontal zone in 2004 and formed approximately 10 - 50% of the community over the entire surface layer in 2005. This dominance of cyanobacteria appears to be associated with the seawater conditions of low NT ratio and optimum temperature. As such, the relative amount of prymnesiophytes increased with a decrease in the NT ratio in the upper 200 m of the EJS. In contrast, more preferentially in high NT ratio areas, diatoms (40 - 80%) dominated the phytoplankton community in May 2004 because of the spring bloom, and pelagophytes (20 - 60%) dominated the community in July and October 2005. Therefore, the EJS seems to provide an ideal environment for studying the physical and chemical factors that control the community structures of phytoplankton.
D. Food web structure and function of pelagic and benthic ecosystems in the East/Japan Sea
To identify the food web structure of the deep sea ecosystem and to calculate the relative contribution of pelagic and benthic organic matter sources to each consumer in the East Sea, carbon and nitrogen stable isotope ratios were measured in potential organic matter source, invertebrates, and fishes collected in June and November 2008. The range of isotope signatures between potential organic matter sources allowed clear separation of pelagic and benthic sources. No significant differences in S13C and S15N values of potential food sources were found between sampling seasons. From the cluster analysis based on average S13C and S15N values, consumers differed significantly among feeding groups (modes). Relative contribution of pelagic and benthic organic matter sources to each consumers and trophic levels of consumers were different among consumer species, suggesting that they use different dietary items. Isotopic signatures of most consumers at the deep sea ecosystem showed a separation of pelagic and benthic pathways, indicating the important trophic importance of both pathways. Our results suggest that stable isotope techniques may be a powerful tool to elucidate the trophic structure and to study the long-term changes of biological environments with climate variability in the East Sea.
7. Circulation and Ecosystem Model Coupling
A. Development of the East Sea ocean circulation model
An Ensemble Kalman Filter (EnKF) is applied to a ROMS (Regional Ocean Modeling System)-based East Sea circulation model. The system employs the localization of the background error covariance and it adopts the disturbance of observations and the inflation of the background error covariance to prevent the collapse of ensembles. Sea surface temperature and temperature profiles have been assimilated into the model with 8 ensembles from 1999 to 2000.
The EnKF system reproduces the mean surface circulation more realistically as compared to that without data assimilation, and simulated area-averaged vertical temperature profile also well corresponds to that from other reanalysis product. With two-year long observed temperature at 100 m in the Ulleung Basin, spatial and temporal correlations are higher and RMS errors are reduced for the EnKF system as compared with model results without the data assimilation.
Future work is targeted to improve the present system by using more than 16 ensembles, and tuning the ocean model to simulate the East Sea better.
B. Future projection of sea surface temperature in the East Sea due to global warming
Observations show the East Sea has undergone drastic changes in a past few decades such as a warming of upper and deep waters, and a decrease in dissolved oxygen contents in deep waters. No postulation, however, has been attempted concerning the future oceanographic states of the East Sea as the climate continues to change. Motivated by this gap, we analyzed results from a high-resolution global coupled climate model (MIROC-hi) to examine climate change projections over the East Sea. The future projection is focused on SST change in the East Sea, which is based on A1B scenario suggested by IPCC.
Sea surface temperature (SST) in the East Sea has steadily increased since the early 20th century, which has been accelerated since year 2000. The magnitude of the warming during the 20th century is 0.25°C/100 year which is extremely small as compared to that during the 21st century, 4.1°C/100 year. Moreover, the early-step warming appears near the subpolar frontal region in winter while warming after year 2000 appears not only in the whole basin but also in all seasons, with summer warming being stronger than that of winter.
The dramatic change of warming pattern is caused by difference of the causes of the warming. The localized winter warming is caused by the extending of warm current on the subpolar frontal region while the main cause of summer warming is shallow mixed layer in that season. The warming force through the surface almost equally affects in both seasons, however, warming effectively appears in summer duo to the shallow mixed layer depth.