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Part II Remote Sensing and Geographic Information Systems (GIS) Applications for Sustainable Development

Validation of SeaWiFS-derived Ocean Color Data and Using for Study Distribution and Seasonal Variation of Chlorophyll-a Concentration in the Vietnam Waters Tran Van Dien1 , Dan Ling Tang2 , Hiroshi Kawamura2 1

Hai Phong Institute of Oceanology 246 Danang Street, Hai Phong City, Vietnan 2

Center for Atmospheric and Oceanic Studies Tohoku University, Japan

Abstract This study implemented the validation of SeaWiFS derived Chl-a in Vietnam waters using 58 insitu measurements by SEAFDEC cruise in May 1999. Monthly mean SeaWiFS-derived Chl-a in May 1999 was used for match-up analysis with insitu measurement. Result of regression found that highly correlated between insitu and SeaWiFS-derived Chl-a. Correlative equation is y = 0.6322x - 0.3209, the root square of fitness is R2 = 0.64. Some scatter points are in very turbid water close to river mouths. When removed these data points, the higher in correlation was received (y = 0.9045x + 0.0119, R2 = 0.72). Ocean Color 4 band algorithm (OC4) for derived Chl-a from SeaWiFS is quite standard for coastal ocean with low suspended sediment concentration in Vietnam waters. Chl-a distribution in Gulf of Tonkin was higher in winter, lower in summer. At sea area around Mekong river mouth, Chl-a concentration was also rather high. From July to September, in the southern Vietnam Central area, there was a phenomena with high Chl-a concentration was created a strip at nearshore of Khanh Hoa province, stretches accordance with Binh Thuan - Ninh Thuan shore. This phenomena is related to summer monsoon upwelling in Vietnam Central. Keywords: Validation, Variation, SeaWiFS-derived Chlorophyll-a

1. Introduction Vietnam has large continental shelf with long coastline above 3200km lying between latitude 7-22oN and longitude of 103-112oE. North part of Vietnam water is Gulf of Tonkin, center part is East Sea (a part of South China Sea), and south part is Gulf of Thailand (Fig.1). Red Rive and Mekong River are two main rivers systems discharge into Vietnam waters. The difference of climate, complex in river discharge, monsoon and other oceanographic properties of this sea region have made the complex in physical, chemical and ecological characteristics and dynamic of Vietnam waters. Due to

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Dien, T.V.; Tang, D.L. and Kawamura, H.

large area coverage, there is difficulty to carry out the field survey for whole Vietnam waters area. Almost studies were concentrated to some small area and near the coast, bay and lagoon. There are a few studies on physical chemical and biological processes have done for whole Vietnam waters. Especially, the Southeast Asian Fisheries Development Center Cruise (SEAFDEC) made a survey for west coast water of South China Sea in may 1999 (Suchint and Puntip1999; Penjan et al., 1999; An and Du, 1999; Anh, 1999). Remote sensing with capability provides synoptic view of the earth environment from global to local scale. It is critical need for using such powerful technique such as remote sensed data for monitoring ocean properties and processes. The Coastal Zone Color Scanner (CZCS) on board the Nimbus-7 satellite was launched in 1978 as a “proof-of-concept” instrument to demonstrate the feasibility of satellite ocean color remote sensing in monitoring Chl-a distributions, which are related to phytoplankton activities in the global ocean. CZCS operated until 1986 and contributed greatly to understanding of marine environment and biological, biochemical, and physical process in the ocean (MüllerKarger, 1989; Abbott and Chelton, 1991; Mclain, 1993; Barale and Schlittenhardt, 1993; Mitchell, 1994; Tang et al., 1998). After decade break in ocean color monitoring, the OCTS sensor provided the only source of global, high spatial resolution ocean color data from November 1996 to June 1997 (Kawamura et al., 1998). Since then many researches have done on calibration and validation OCTS ocean color data and for oceanic researches (Kishino et al., 1998; Shimada et al., 1998, Yokouchi et al., 2000; Tang et al., 2002). The Seaviewing Wide Field-of-view Sensor (SeaWiFS) has been operational since September 18th, 1997 and has been providing global estimates of oceanic chlorophylla and other bio-optical quantities to the international research community since then. These data are critical for understanding the temporal variability of marine

Part II Remote Sensing and Geographic Information Systems (GIS) Applications for Sustainable Development ecosystems, especially with regard to events such as El Nino and La Nina, and the role of oceanic photosynthesis and primary productivity in the Earth's carbon budget and climate (Falkowski et al., 1998). OCTS and SeaWiFS data was useful for study phytoplankton bloom in detection and monitoring in Arabian Sea (Tang et al., 2002). The data are also proving invaluable for fisheries and coastal zone management (Vance et al., 1998). Global calibration and validation of SeaWiFS satellite data has implemented since operational with sensor calibration and atmospheric correction and validation by data of marine optical buoy (Hooker and McClain, 2000). Some algorithms have developed for data processing such as SeaWiFS Ocean Color 2 band algorithm (OC2) and Ocean Color 4 band algorithm (OC4) (John E. O’Reilly et al., 2000; Gohin et al., 2002). Some others empirical and neural network algorithms were also developed for estimation and validation of Chl-a from SeaWiFS data (O'Reilly, 1998; Habbane et al., 1998; Keiner and Brown, 1999; Kahru and Mitchell, 1999). SeaWiFS Data Analysis System (SeaDAS) software has developed for SeaWiFS data processing (Baith et al., 2001). There were some oceanic remote sensing studies in Vietnam water and South China Sea. Tang (1998) has analyzed annual and spatial patterns of CZCS derived pigment concentration on the continental shelf of China that including Vietnam Water of South China Sea. The research has found that high concentration belt of about 50km wide exits along the coastline of China, including the north part of the Gulf of Tonkin. Winter phytoplankton bloom was observed from CZCS satellite data in the Luzon strait in South China Sea (Tang et al., 1999). Kuo N.J. et al. (2000) was observed summer upwelling happened in 1997 along the west coast of South China Sea from AVHRR derived SST in Center of Vietnam coastal area. There is not any studies have done on validation SeaWiFS derived Chl-a for this sea region. Local validation of SeaWiFS derived Chl-a product is essential for oceanographic studies in Vietnam waters. This study examined the monthly composite of Chl-a derived from SeaWiFS satellite data in May 1999 using insitu Chl-a data measurement from SEAFDEC cruise carried out during this month. Then these monthly images was used for study seasonal variation of Chl-a in Vietnam waters.

of South China Sea, Vietnam water (Suchint an Pintip, 1999). Environmental monitoring data with 4 times per year measurement, normally in February, May, August, November. Data was collected at 5 monitoring stations since 1995 along the western coast of Gulf of Tonkin (Fig.1). The following parameters were measured: temperature, salinity, turbidity, total suspended solid (TSS), Chl-a was measured since 2000. Since 2001, two additional stations were established. One is near the boundary between Vietnam and China, other in the center of the Gulf of Tonkin. These parameters were measured at surface and bottom layer at low and high tide. The time for collecting the sample was at neap tide. Water temperature was measured by specific purpose mercury thermometer with accuracy of 0.1 oC. TSS was measured use method fitter and dry with accuracy of 0.1 mg/l. Method for measure Chl-a are use spectrum extract device color measurement in acetone 90% at wavelength of 664, 647, 630 and 750ηm. SEAFDEC cruise No.57/3-99 was carried our from 30th April to 29th May 1999 in South China Sea, Vietnam waters for integrated survey in physical condition for fishery resources investigation in the Interdepartmental Collaborative Research Program on the marine resources in the South China Sea. Total 58 station was measured Chl-a, water temperature, salinity and others chemical parameter (Fig1). Chl-a was analyzed by the Luminescence spec photometer (Suchint and Puntip, 1999; An and Du, 1999).

2. Data and Methodology 2.1. Insitu observation Insitu observation data were used from some sources, environmental monitoring data and especially SEAFDEC cruise survey data carried in May 1999 provided valuable data for Chl-a in western coast region

Figure1. Study area and insitu sampling points

Dien, T.V.; Tang, D.L. and Kawamura, H.

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Part II Remote Sensing and Geographic Information Systems (GIS) Applications for Sustainable Development

SeaWiFS satellite data was received by the National Development Agency Japan (NASDA). L2 SeaWiFS image 1x1km spatial resolution were processed by ocean color 4-band algorithm (OC4) through SeaWiFS Data Analysis System (SeaDAS) (Baith el al., 2000), which were developed by The SeaWiFS Project Calibration and Validation Team. The OC4 algorithm is an empirical algorithm based on more than 2800 biooptical insitu measurements of Chlorophyll-a from all over the world. These data were used for producing monthly composite SeaWiFS-derived Chl-a images using SeaDAS software function to 4km resolution images. Monthly mean SeaWiFS derived Chl-a of 29 scenes in May 1999 was used for match-up analysis with insitu data.

Remove the data at the points in turbid water near Red River mouth and Mekong River mouth in May 1999. Scatter plot these data with insitu data (Fig.4), the correlation got higher in fitness numbers, R2 = 0.719 with the relation by function y = 0.9045x + 0.0119. After remove the points in coastal turbid water, the relation between insitu data and SeaWiFS-derived Chl-a was better linear correlation, higher root square of fitness number. Insitu Chl-a

2.2. Satellite data

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2.4. Seasonal images comparison Monthly images from January 1999 to December 2000 were selected for comparison the distribution and seasonal variation of Chl-a in Vietnam waters. Monthly images in February, May, August and November show specifically for 4 season in the each year (Fig.5, Fig.6).

3. Results 3.1. Correlation of insitu Chl-a and SeaWiFSderived Chl-a Digital value of Chl-a at sampling points was registered with insitu Chl-a measurement by SEAFDEC cruise. The result has shown that SeaWiFS Chl-a and insitu Cha was highly correlated (Fig.2). Fig.3 is scatter plot of the SeaWiFS derived Chl-a and insitu Ch-a show that the fitness is well in scatter logarithm plot. The relation was presented by equation y = 0.6322x - 0.3209, the root square of fitness is R2 = 0.6384.

0.1

0.01 0.01

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1 10 SeaWiFS-derived Chl-a

Figure 3. Scatter plot the relation between logarithm insitu Chl-a (mg/m3) and logarithm monthly composite SeaWiFS derived Chl-a in May 1999. Well fitness is shown in logarithm scale plot. Insitu Chl-a

2.3. Match-up generation The digital value of monthly mean SeaWiFS derived Chl-a product in May 1999 at the same location of insitu sampling station of SEAFDEC cruise was picked for match-up analysis with insitu data. Pick up every values of Chl-a caculated on 5x5 pixels (20x20km2) from these insitu stations.

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R = 0.719

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C h l- a ( m g /m 3 ) 4

S e a W iF S C h l- a ( m g /m 3 )

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I n s itu C h l- a ( m g /m 3 ) 3 2.5 2

0.01 0.01

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1 SeaWiFS-derived Chl-a

1.5 1 0.5 0 1

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9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49 51 53 55 57

Figure 2. Correlation between insitu Chl-a and monthly composite SeaWiFS-derived Chl-a in May 1999

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Figure 4. Correlation between SeaWiFS-derived Chl-a and insitu measurement data without coastal very turbid sampling points. Higher correlation root-square fitness number in comparing with whole data plot.

Part II Remote Sensing and Geographic Information Systems (GIS) Applications for Sustainable Development

3.2. Distribution and seasonal variation of SeaWiFS-derived Chl-a concentration SeaWiFS Chl-a mean monthly product in May 1999 was mapped within coordinate of 5-23oN and 102-114oE to show the distribution of Ch-a in Vietnam waters (Fig.5). This image shows that the Chl-a value very high at near Red River and Mekong River mouths (1020mg/m3). This value is higher than measurement by environmental monitoring system in northeast coast of the Gulf of Tonkin (3-4mg/ m3). High concentration of Chl-a in the northern part of the Gulf of Tonkin Chl-a (0.5-0.8 mg/m3), it was well agree with that mentioned by Tang (1998) and in An (1999). Chl-a concentration along the western coast area Gulf of Tonkin (0.40.7mg/m3), and southern coast area (0.8-1.2mg/m3), are higher in the central coast region (0.2-0.4mg/m3). In South China Sea area, Chl-a concentration was quite low (lest than 0.1mg/m3). This low Chl-a region mentioned also in An (1999). From analysis insitu data, Suchint (1999) mentioned that Chl-a at surface layer at near-shore is higher that offshore, especially high at near major city along the coast. From July to September, at the southern Vietnam Central area, there was a phenomena with high Chl-a concentration was created a strip at nearshore of Khanh Hoa province, stretches accordance with Binh Thuan Ninh Thuan shore.

4. Discussions 4.1. Distribution of SeaWiFS-derived Chl-a Distribution of SeaWiFS-derived Chl-a concentration in Vietnam water was varied according to geographic zone and oceanographic condition. SeaWiFS-derived Chl-a concentration was high in near coast and decreasing offshore-ward. Chl-a concentration in the Gulf of Tonkin and Gulf of Thailand is higher in the South China Sea due to nutrient discharge from Red River and Mekong River. Near river mouth of Red River and Mekong River, SeaWiFS derived Chl-a concentration was very high due to the effect of very turbid water. This value is higher than measurement by environmental monitoring system in northeast coast of the Gulf of Tonkin (3.0-4.0mg/m3). The distribution of SeaWiFS derived Chl-a concentration is well agree with field measurement analysis in An and Du (1999), and Suchint (1999).

4.2. Correlation of insitu Chl-a and SeaWiFSderived Chl-a Correlation of insitu Chl-a data and SeaWiFS derived Chl-a in May 1999 are well fit. Some scattered points on scatter plot of insitu Chl-a and SeaWiFS derived

Chl-a in May 1999 are closed to Red River and Mekong River mouths (point No.3, 38, 49). This phenomena due to the very high suspended sediment concentration in the water. Hooker et al. (1992) mentioned that in the Case I water, specifications called for uncertainties less than ±5% in retrieved water-leaving radiance and less than ±35% in Chl-a concentration over the range of 0.05-50 mg/m3. However, generally fails to deliver such fidelity in turbid or shallow coastal water - Case II waters (Hu et al., 2000). When removed the points in very turbid water, got the higher correlated root-square fitness number (R2 = 0.72). This fitness number is similar with Mohd and Ahmad (2000) when examine SeaWiFS satellite data for ocean color determination in fishery application in Malaysia. It was found that a good relationship between the sea truth measurement of 4 sampling points and satellite data for estimating chlorophyll concentration value (R2 = 0.75). O'Reilly et al. (1998) was commended that improved performance was obtained using the ocean chlorophyll 4 algorithm (OC4), a four-band (443, 490, 510, 555 ηm), maximum band ratio formulation. This maximum band ratio is a new approach in empirical ocean color algorithms and has the potential advantage of maintaining the highest possible satellite sensor signal/noise ratio over a 3orders-of-magnitude range in chlorophyll concentration. Kahru and Mitchell (1999) proposed a new empirical chlorophyll algorithm for SeaWiFS. The CAL-P6 algorithm uses a sixth-order polynomial of the ratio of normalized water leaving radiance (L-WN) at 490 ηm and 555 ηm and is based on 348 measurements of LWN, and chlorophyll-a in the California Current. The validation of the SeaWiFS-derived chlorophyll-a values with 27 concurrent insitu measurements showed high correlation (R2 = 0.93 in the log-log space) but significant overestimation by SeaWiFS at high chlorophyll-a concentration. 4.3. Relation between climate, oceanic hydrology, geographical position and coastal morphology with distribution and seasonal variation of Chlorophyll-a In winter, due to Northeast monsoon with low temperature air flows from North, air temperature over Gulf of Tonkin was lower than in Gulf of Thailand and East Sea create a good condition for phytoplankton grown. So that chl-a is higher in the north Gulf of Tonkin and creates a strip about 50km along west coat of the Gulf (Fig.5, Fig.6). In the summer, due to southeast monsoon flow, the northeast - southwest direction coast of Ninh Thuan Binh Thuan provinces creates a wind stress current along the coast. At the Cam Ranh, the coast change to north - south direction, wind pressure was thrust the water from lower layer up to the surface create an upwelling phenomena. In this upwelling area, low temperature and nutrient upward from low layer to the surface layer create a good condition for growing of phytoplankton. SeaWiFS images from June to

Dien, T.V.; Tang, D.L. and Kawamura, H.

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Part II Remote Sensing and Geographic Information Systems (GIS) Applications for Sustainable Development September 1999 has shown clearly the phenomena, a strip of high Chl-a was generated at near-shore of Khanh Hoa and stretches along the Binh Thuan - Ninh Thuan coastal direction (Fig.5). In 2000, this phenomena was not shown clearly, there is a strip at near-shore of Ninh Thuan - Khanh Hoa in June, a small area with very high Chl-a concentration at offshore above 250km from Binh Dinh coat in July and decay in August (Fig. 6).

5. Conclusions Ocean Color 4 band algorithm (OC4) for derived Chl-a from SeaWiFS satellite data is quite standard for coastal ocean with low suspended sediment concentration in Vietnam water. In case of very high turbid water, SeaWiFS derived Chl-a is higher than insitu measurement Chl-a. Validated SeaWiFS derived Chl-a will contribute much to oceanographic studies in Vietnam water and South China Sea region. Distribution and seasonal variation of Chl-a depend on the climate, geographical zone, oceanographic condition, and coastal morphology. From July to September, due to southeast monsoon, there was a phenomena summer monsoon upwelling in Vietnam Central.

Acknowledgments This study was implemented in the framework of project “Water circulation and material transport in coastal and marginal sea of East and Southeast Asia” in the Multi-cooperation Program on Coastal Oceanography supported by Japanese Society for Promotion of Science (JSPS). The SeaWiFS satellite data was provided by I-LAC Asian Water project. Insitu data were provided by Interdepartmental Collaborative Research Program of SEAFDEC.

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Part II Remote Sensing and Geographic Information Systems (GIS) Applications for Sustainable Development

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May 1999 August 1999 November 1999 Figure 5. Distribution and seasonal variation of SeaWiFS-derived Chl-a in Vietnam waters in 1999

February 2000

May 2000 August 2000 November 2000 Figure 6. Distribution and seasonal variation of SeaWiFS-derived Chl-a in Vietnam waters in 2000

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