The Use of SeaWiFS Satellite Data For Ocean Colour Determination In Fisheries Application
Figure 2 presents a relationship between the normalised water leaving radiance ( nLw) at 443 nm and chlorophyll . When the chlorophyll concentration increase, there is more absorption in the blue and the nLw at 443 nm should decrease.
Figure 2: Relation between chlorophyll concentration and normalised water leaving radiance at 443 nm ( blue wavelength) observed by Unipertama 7 cruise.
Figure 3 allows a comparison of chlorophyll determined from in - situ with the chlorophyll concentration as estimate from the SeaWiFs data. Results show that the chlorophyll concentration derived from satellite image is generally larger than that derived from in-situ measurement at some stations. The CAL-P6 algorithm uses a sixth-order polynomial of the ratio of radiances at 490 nm and 555 nm proposed by Kahru and Mitchell (1999) also show a significant overestimation by SeaWiFS at high chlorophyll concentration.
The details about comparison of the value are shown in table 2. The root-mean-square error (RMSE) of chlorophyll concentration is about 0.050 mgm
-3. Validation of the SeaWiFS-derived chlorophyll values with 7 concurrent in - situ measurements showed high correlation r
2 = 0.75.
Table 2. Sea truth measurement and the calculated amout of chlorophyll concentration using satellite data.
| Station |
Chlorophyll concentration from in-situ measurement |
Chlorophyll concentration estimated by satellite |
| 1 |
0.146 |
0.080 |
| 2 |
0.138 |
0.083 |
| 3 |
0.117 |
0.104 |
| 5 |
0.075 |
0.114 |
| 6 |
0.089 |
0.116 |
| 7 |
0.113 |
0.116 |
| 8 |
0.063 |
0.118 |
Figure 3: A good relationship between the sea truth measurement and satellite data for estimating chlorophyll concentration
value( r2 = 0.75).
Conclusions
Site specific algorithm have been derived from the nLw at 443, 490, 555 using modified Frouin algorithm . The regression coefficient for A and B are 0.22 and 0.89 respectively. Although the results are encouraging , further in - situ measurements need to be done to compare the variability of the chlorophyll distribution before and after the monsoon seasons. The avaibility to relate chlorophyll concentration to optical characteristic is seriously impaired by the relatively small quantity of coincident in - situ measurement during satellite overpass.
In general, chlorophyll with high concentration was distributed around the coast line compared to area far from the coast. However from the study some of the area which is far from the coast has a high chlorophyll concentration. This phenomena happen due to a strong current that move the chlorophyll patchines from coastal area. Moreover the effect of upwelling is more marked when upwelling occurs on coastlines with a continental shelf. When upwelling occurs along a coastline, the upwelled water which carry the floating chlorophyll tend to move away from the coast. The monsoon winds have a significant effect on the current circulation in the South China Sea. According to Nasir et al. 1996, from July to September 1996, the net current flow near the coast of Kuala Terengganu is toward the southwest direction.
The movement of the current can be detected clearly when observe the map of chlorophyll distribution using satellite image( Figure 4). The result of the study reveals that the chlorophyll concentration pattern of 0.104 - 0.129 mgm
-3 can be used as a tracer for the current movement.
Figure 4: Chlorophyll concentration distribution estimated using SeaWiFS satellite data.
Acknowledgements
The authors would like to thank the SeaWiFS Project ( Code 970.2 ) and Distributed Active Archive Center (Code 902 ) at the Goddard Space Flight Center,Greenbelt MD 20771 for the production and distribution of these data respectively. These activities are sponsored by NASA's Mission to Planet Earth program. Thanks are also due to CRISP of NUS for supplying LAC data. The contribution from technical staff of Universiti Teknologi Malysia and Universiti Putra Malaysia Terengganu are duely acknowledge.
References
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