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Investigation of different coastal processes in Indonesian waters using SeaWiFS data
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Deep-Sea Research II 51 (2004) 85–97
Investigation of different coastal processes in Indonesian waters using SeaWiFS data Nani Hendiartia,*, Herbert Siegelb, Thomas Ohdeb a
Agency for the Assessment and Application of Technology, M.H. Thamrin 8, Jakarta 10340, Indonesia b Baltic Sea Research Institute, Seestrasse 15, Rostock-Warnemuende D-18119, Germany Received 1 August 2002; received in revised form 28 April 2003; accepted 14 October 2003
Abstract SeaWiFS data were applied to investigate coastal processes in Indonesian waters around the most populated island of Java. Coastal processes due to wind forcing were studied the first time using SeaWiFS-derived chlorophyll and TSM concentrations in combination with AVHRR-derived SST in the period from September 1997 to December 2001. Upwelling events were studied along the southern coast of Java during the southeast monsoon (June to September). Satellite-derived chlorophyll concentrations higher than 0:8 mg=m3 and sea-surface temperatures lower than 28 C are indications of upwelling. Upwelling events influence the distribution and growth of phytoplankton and provide by that good feeding condition for zooplankton, larvae, juvenile and adult of pelagic fish. Coastal discharge into the western Java Sea contains organic and inorganic materials originating from different sources. Diffuse impacts, particularly from fish farms and aquaculture, as well as coastal erosion influence large coastal areas during the rainy season (December to March), and to a lesser extent during the dry season. Strong Citarum river discharge was observed during the transition phase from the rainy to the dry season (March and April), when the maximum amount of transported material reaches the sea. The river plume is evident from chlorophyll concentrations higher than 2:5 mg=m3 ; and suspended particulate matter concentrations of more than 8 mg=dm3 : The Sunda Strait is seasonally influenced by water transport from the Java Sea and from the Indian Ocean. The satellite data show that water transport from the Java Sea occurs during the pre-dominantly easterly winds period (June to September). This is characterized by warm water (SST higher than 29:5 C) and chlorophyll concentrations higher than 0:5 mg=m3 : This water transport influences the fish abundance in the Sunda Strait. High fish catches coincide with the presence of Java Sea water, while the surface currents lead to the migration of pelagic fish. Conversely, during the dominant westerly winds period, oceanic waters from the Indian Ocean with low chlorophyll concentrations influence the Sunda Strait water. r 2004 Elsevier Ltd. All rights reserved.
1. Introduction *Corresponding author. Baltic Sea Research Institute, Seestrasse 15, D-18119 Rostock, Germany. Tel.: +49-3815197378; fax: +49-381-5197440. E-mail addresses: [email protected] (N. Hendiarti), [email protected] (H. Siegel), [email protected] (T. Ohde).
The Indonesian archipelago is located in the tropics between two oceans (Pacific and Indian) and two continents (Asia and Australia), and it plays a unique role in the regional and global climate system. The variation of seasonal solar
0967-0645/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr2.2003.10.003
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heating over the continents of Asia and Australia drives the monsoons, which change wind direction twice a year (Tomascik et al., 1997; Webster et al., 1998). The southeast (SE) monsoon, which occurs between June and September, is influenced by high air pressure over Australia and low pressure over Asia. The wind blows from the southeast (Australian continent) in the southern hemisphere and turns to southwest in the northern hemisphere. The northwest (NW) monsoon, which develops between December and March, is forced by high atmospheric pressure over Asia and low pressure over Australia. The wind blows from northeast (Asian continent and Pacific Ocean) in the northern hemisphere and turns to northwest in the southern hemisphere. The NW monsoon transports moist air, causing precipitation across the archipelago. Numerous authors have investigated upwelling and through flow phenomena in Indonesian waters based on in situ observations and model simulations. Southeasterly winds generate an Ekman offshore transport of surface water along the coasts of south Java, where this offshore transport induces upwelling (Wyrtki, 1987; Susanto et al., 2001). Warm water from the western Pacific Ocean is transported to the Indian Ocean through the Makassar Strait, the Lombok Strait, the Ombai Strait and the Timor passage. This the so-called Indonesian through flow is caused by sea-level differences between these two regions (Murray and Arief, 1988; Meyer, 1996; Gordon et al., 1999; Potemra and Lukas, 1999; Hautala et al., 2001). The peak of the through flow occurs in between June and July and the minimum in February (Schott and McCreary, 2001; Fieux et al., 1996). The upwelling center with low sea-surface temperature (SST) migrates westward and towards the equator during the southeast monsoon (Susanto et al., 2001). The occurrence of upwelling along the coast of southeast Java observed from SST images coincides with the most productive period in terms of fish catch (Hendiarti et al., 1996). Biological features produced by ocean phytoplankton are important for Indonesian waters, since there is no seasonal heating of the water that obliterates the temperature contrasts in the imagery. Our previous study showed that Sea-
Fig. 1. Map of investigation areas including bottom topography.
viewing Wide Field-of-view Sensor (SeaWiFS) data can be successfully applied to distinguish different water types in and around the Sunda Strait using variations in reflectance of the visible spectral range (Hendiarti et al., 2002). In this study, SeaWiFS data were used the first time in combination with SST images to study the influence of coastal processes on phytoplankton development and occurrence of pelagic fish. The areas investigated are located in the southwestern part of Indonesian waters, as shown in Fig. 1, and included the western Java Sea, the Sunda Strait and the Indian Ocean off the southern coasts of Java.
2. Datasets and methods The investigations were performed using SeaWiFS and Advanced Very High Resolution Radiometer (AVHRR) data. SeaWiFS local area coverage (LAC), which were received by the satellite ground receiving station at the National University of Singapore (CRISP) of 1 km spatial resolution, and global area coverage (GAC) data of 5 km spatial resolution were provided by the SeaWiFS project and distributed by Goddard Space Flight Center—NASA. These data were processed using the SeaWiFS Data Analysis System (SeaDAS) provided by the National Aeronautics and Space Administration.
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SST images were derived from the thermal infrared channels of the AVHRR of the NOAA (National Oceanographic and Atmospheric Administration) weather satellites. AVHRR LAC data of 1 km resolution were provided by the ground station at the Agency for the Assessment and Application of Technology (BPPT) in Jakarta, Indonesia. The SST images were produced using an improved algorithm of multi-channel approach of McMillin and Crosby (1984). The SST accuracy is 70:8 C using night time AVHRR data (Farahidy et al., 1996). MESSR—Marine Observation Satellite (MOS) data with a 50 m spatial resolution were provided by the National Space Development Agency of Japan and used to observe river discharge. Wind data were provided by the NOAA—CIRES Climate Diagnostics Centre. Fish catch data of pelagic species were taken from Banyuwangi and Labuhan ports. In situ chlorophyll a concentrations were measured for validation during cruises of R/V Baruna Jaya in August 2000, October–November 2000 and July 2001 using a Shimadzu UV-1600 spectrophotometer and calculated according to Lorenzen (1967). The SeaWiFS data evaluation was performed on the basis of the different SeaDAS versions. The different chlorophyll algorithms are described by Hooker et al. (1992) and O’Reilly et al. (1998). The validation of SeaWiFS-derived chlorophyll in Indonesian waters was realized in two steps. The first step was the check of various SeaDAS atmospheric correction algorithms. An overestimation of atmospheric effects in the correction method caused negative values of water-leaving radiances. Secondly, in situ chlorophyll measurements were compared with SeaWiFS-derived chlorophyll. Seven atmospheric correction algorithms implemented in the SeaDAS versions 3.2 and 4 were tested. These algorithms are: (1) Gordon/ Wang aerosol, (2) multi-scattering for channels 765=865 nm with near-infrared (NIR) iteration, (3) multi-scattering for channels 670=865 nm model, (4) multi-scattering for channels 670=865 nm with NIR iteration, (5) multi-scattering for channels 765=865 nm model, (6) multi-scattering with fixed
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model, and (7) single-scattering white aerosols. The best atmospheric correction for Indonesian waters is the multi-scattering algorithm using 670=865 nm with NIR iteration. This algorithm produced fewer negative values of water-leaving radiances for the channels of 443, 490, 510 and 555 nm (see Fig. 2A), and gave a higher correlation coefficient for the derived chlorophyll concentrations with in situ measurements R2 ¼ 0:87 (Fig. 2B). Differences in chlorophyll concentrations between satellite derived and in situ surface values were explained as follows. In the Sunda Strait, three different water types were distinguished during the cruise in July 2001: inner water, mixed water from the Java Sea and Indian Ocean and outer water. The investigations in upwelling areas and in the Indian Ocean were performed using in situ measurements during two cruises in August and in October–November 2000. The differences in chlorophyll concentrations observed in the mixed region of the Sunda Strait ð0:2 mg=m3 Þ were slightly higher compared to the differences in other regions, upwelling area ð0:18 mg=m3 Þ pure ocean waters ð0:04 mg=m3 Þ: These satellitederived concentrations are higher than in situ surface values since the radiance received by satellite sensor contains contributions from subsurface chlorophyll depending on the signal depth. Subsurface chlorophyll maximum may increase the concentrations derived from SeaWiFS data. To consider this condition, a weighted surface concentration was calculated on the basis of concentration and depth of subsurface chlorophyll maximum, and of the vertical attenuation coefficient. The weighting procedure increased the precision of the SeaWiFS derived chlorophyll concentrations by about 2% in the Sunda Strait and 6% in the upwelling regions (Fig. 3). The concentrations of total suspended matter (TSM) were obtained using a single band algorithm, based on the reflectance distribution in the 555 nm SeaWiFS channel. This TSM algorithm was developed for North Sea and delivered a limited accuracy not better than 15–20% (Hesselmans et al., 2000). Nevertheless, the derived concentration patterns in Banten Bay of Indonesian
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Fig. 2. Distribution of negative values for water-leaving radiances produced by various atmospheric correction algorithms of SeaDAS 3.2 and 4 (A) and matched-up analyses between chlorophyll a concentrations in situ and SeaWiFS derived by 670/865 NIR (B).
waters are similar to the in situ concentrations (Hesselmans, 2001).
3. Application of SeaWiFS data in Indonesian waters 3.1. Upwelling processes and phytoplankton development During the SE monsoon, the Ekman offshore transport along southeast Java coast induces upwelling, transporting nutrient-rich water from deeper layers into the euphotic zone where phytoplankton development starts due to increasing light level. The utilization of SeaWiFS data improves previous observations based on SST images. Upwelling events are indicated by low SST values and high chlorophyll concentrations. In images from the 2 August 2001 upwelling events can be easily observed, where the chlorophyll front coincided with the temperature boundaries in the same region (Fig. 4A and B). In situ measurements of SST and chlorophyll concentration at 1 m depths were performed at the upwelling station
(DS at 10 S and 113 300 E) during global research network system cruises in March and September 1995 to investigate the diurnal cycle. One-day observations were carried out using a CTD every 2 h: Fig. 4C shows that the September was characterized by low SST (26:5 C) and high chlorophyll concentrations (0:6–1 mg=m3 ), and March by high SST (28:5–29 C) and low chlorophyll (0:1 mg=m3 ). During the upwelling event in September, the depth of the thermocline decreased from 60–80 to 10–25 m; resulting in high concentrations of chlorophyll in the euphotic zone (Hendiarti et al., 1996). Chlorophyll concentrations in the ocean waters do not fluctuate as widely as in coastal waters (Fig. 5 left). Chlorophyll concentrations higher than 0:8 mg=m3 were found due to upwelling events during the SE monsoon. In normal years, without any influence of the El Nin˜o-Southern Oscillation climate phenomenon, the upwelling period starts in June and increases in the intensity until September (see Fig. 5 right). The peak event was observed in September with a mean value of derived chlorophyll concentrations of about 2 mg=m3 :
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Fig. 3. Match-up analysis of in situ and satellite-derived chlorophyll a concentrations in the Sunda Strait and Indian Ocean (using standard procedure (A,C) and after correction (B,D)).
The development of upwelling events varies depending on the strengths of the monsoon. Fig. 6A shows annual chlorophyll concentrations from SeaWiFS GAC data during the El Nin˜o in 1997 in comparison to other years from 1998 to 2001. In November 1997 a very strong upwelling event covered a large area from south Java to the equator around mid of west Sumatra (see Fig. 6B). This was significantly different compared with the condition of typical years as shown for November 1999 where no upwelling existed in this region (Fig. 6C). In 1998, a year after El Nin˜o, the intensity of upwelling was lower than that of typical years, as depicted from the years 1999–2001 (see Fig. 6A). The extent of upwelling area developed in similar way (data not shown). The condition where Java–Sumatra upwelling extends in time and space during the El Nin˜o period has been described by Susanto et al. (2001) on the basis of SST and sea-surface height anomaly data.
During the El Nin˜o, less Indonesian through flow occurs via Makassar Strait (Gordon et al., 1999). This condition reduces the influence of the through flow in Lombok Strait. Hence, the upwelling event is more intense because it is not disturbed by this through flow. These upwelling phenomena are important for fisheries, since the local nutrient enrichment leads to large quantities in the entire food chain up to the fish. The relationship between upwelling events and catch rates of pelagic fish will be discussed in Section 3.4. 3.2. Coastal discharge into the western Java Sea Coastal discharge is an important topic in the application of remote sensing data of different spatial resolution in the western Java Sea with the big islands of Java, Sumatra and Kalimantan (Borneo) and numerous rivers and large mangrove
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Fig. 4. SST and chlorophyll a concentration derived from AVHRR and SeaWiFS data from 2 August 2001 (A,B), and in situ daily cycle of water temperatures and chlorophyll concentrations at the surface layer observed from March and September 1995 (C).
regions. The discharged waters contain high concentration of organic and inorganic material. In East Sumatra, diffuse impacts from fish farms, aquaculture and mangrove regions affect large areas during the rainy season (December to March), as shown in Fig. 7B. Discharged material, driven by northwesterly winds, is transported offshore. High concentrations of chlorophyll of more than 3 mg=m3 ; increase the water turbidity. This may influence the coral reef ecosystems in
Kepulauan Seribu (north of Jakarta). During the dry season (July to August) a smaller amount of discharged material is transported along the coastal areas of East Sumatra (Fig. 7A). North of Jakarta, the discharged water can be differentiated by a strongly decreased reflectance in the short visible wavelength range, due to high absorption by phytoplankton and yellow substances, and increased reflectance at long wavelengths due to dominant light scattering by
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Fig. 5. Comparison of annual chlorophyll concentrations between coastal upwelling and Indian Ocean waters (left), and the differences in derived concentrations between these two regions (right).
Fig. 6. Annual chlorophyll concentrations during the El Nin˜o in 1997 in comparison to other years from 1998 to 2001: (A) chlorophyll images in November 1997; (B) and in November 1999; (C) Black represents clouds.
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Fig. 7. Composite chlorophyll images during the dry season (A) and rainy season (B).
Fig. 8. River discharge observed from SeaWiFS chlorophyll in August 2000 (A) and April 2000 (B), SeaWiFS TSM in August 2000 (C), and MOS data in August 1991 (D).
suspended sediments (Hendiarti et al., 2002). The Citarum River plume contains high concentrations of chlorophyll ð> 2 mg=m3 Þ and suspended particulate matter ð> 15 mg=dm3 Þ; as shown in Fig. 8A–D. A more detailed structure of the river plume is shown in Fig. 8D in a scene of MESSRMOS channel 1 ð5102590 nmÞ with a 50 m resolution. The Citarum River discharge, driven by easterly winds, is transported towards the Jakarta Bay. The temporal variations of phytoplankton due to the river discharge were investigated using annual chlorophyll concentrations near the Citar-
um River mouth and compared with the concentrations in the Java Sea. Fig. 9 (left) shows high concentrations caused by strong transport of fresh water and discharges during the transition phase between the rainy and the dry season (March and April), the period of maximum sediment transport into the sea. During the peak event, chlorophyll concentrations near the river mouth were between 2.5 and 3:5 mg=m3 (Fig. 9 right). The concentrations near the coast were about 10 mg=m3 : During the dry season (June to October), the through flow carries nutrient-rich water from the eastern Indonesian seas to the Java Sea. Hence, the
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Fig. 9. Comparison of annual chlorophyll concentrations between the waters from near the Citarum River mouth and the Java Sea (left), and the differences in derived concentrations between these two regions (right).
concentrations in the Java Sea are slightly higher. The cooling effect due to the through flow in the Java Sea has been observed by Tomascik et al. (1997) using SST data. 3.3. Through flow and phytoplankton distribution Water characteristics of the Sunda Strait vary spatially and temporally due to the influence of water transport from the western Java Sea and from the Indian Ocean. The present investigations were focused on the occurrence and duration of water transport from the Java Sea into the Sunda Strait caused by the monsoonal winds. The presence of Java Sea water in Sunda Strait, which is influenced by the discharge water, causes specific environmental conditions. This incoming water can be investigated using differences in water color in terms of spectral reflectances. The spectral reflectance is mainly caused by the backscattering of the water molecular and particulate matter and spectrally modified by the absorption of water, phytoplankton and yellow substances. This is used in the Red–Green–Blue (RGB) image from SeaWiFS channels of 555, 510 and 443 nm: The image shows different colors in distinct areas (Fig. 10A). Java Sea water (white color) is characterized by high reflectance in the entire visible wavelength range due to higher scattering by suspended material (see Fig. 10B). Water from Jakarta Bay
represented by the yellow color has similar conditions as the Java Sea because of the limited influence of river discharges in September. Water transport from the Java Sea also can be identified by SST and chlorophyll images, as well as in situ measurements. Fig. 11A and B shows SST values of higher than 29 C and chlorophyll concentrations of higher than 0:5 mg=m3 ; which occurred in the northeastern part of Sunda Strait in the images from July 2001. During the same period, surface salinity was lower than in the southwestern Sunda Strait (Fig. 11C). Surface currents southwestwards directed into the Indian Ocean were measured by ADCP at 5–10 m water depths (Fig. 11D). These observations indicate that the water transport from the Java Sea into the Sunda Strait, especially in upper layer, occurred in July and was characterized by warm and nutrientrich water with less salinity. The duration of the through flow from the Java Sea into the Sunda Strait was studied using annual SeaWiFS-derived chlorophyll concentrations and radiances. Fig. 12 shows that in the Sunda Strait chlorophyll concentrations higher than 0:5 mg=m3 were retrieved during the SE monsoon (June to September). Chlorophyll concentrations on the order of 0:4 mg=m3 were observed during March and April due to the large amount of discharged material from the southern Sumatra coasts. The analysis of SeaWiFS radiance of channel 555 nm
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Fig. 10. SeaWiFS composite image (A) and mean reflectance of different water types (B) derived from SeaWiFS data from the 19 September 1999.
Fig. 11. SST image on 12 July 2001: (A) chlorophyll image on July 2001; (B) water salinity at 1 m depth; (C) current profile at 5–10 m depths; (D) measurements were performed during the field campaign from 10–18 July 2001.
also showed a similar result. The Java Sea water characterized by higher radiances than ocean water (see also Fig. 10B) was monitored in the northeastern Sunda Strait during the entire SE monsoon (June to September), but in the southwestern Sunda Strait only in September.
A study of the through flow can help the fishery community since the surface currents may lead to a migration of pelagic fish. Further investigations concerning the relationships between Java Sea water and pelagic fish catches are presented below.
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Fig. 12. Comparison of annual chlorophyll concentrations between Sunda Strait and Indian Ocean waters (left), and the differences in derived concentrations between these two regions (right).
3.4. Occurrence of pelagic fish The occurrence of pelagic fish was investigated in relation to upwelling and through flow using chlorophyll and SST images during different monsoon phases. All cloud-free images of SeaWiFS LAC and AVHRR LAC data from September 1999 up to August 2001 were used to derive monthly mean values of chlorophyll a concentrations and SST. Fish catch per unit effort (CPUE) for Banyuwangi port, the most important port for landing fish from the upwelling area, were computed from fish catch statistics for the years 1991–1994. Tongkol (Auxis thazard), kembung (Restrelliger spp.), layang (Decapterus spp.), selar (Selaroides leptolepis), lemuru (sardinella longiceps), anchovy (Stelophorus spp.), and skipjack tuna (Katsuwonus pelamis) are the major pelagic fish species landed in this port. The total catch for each species is more than 12; 000 tons per year. Fish catch data were divided into quarterly periods: 1 (January– March), 2 (April–June), 3 (July–September) and 4 (October–December). High pelagic fish catchments were correlated with high chlorophyll concentration and low SST during the upwelling period of quarter 3 (Fig. 13). These upwelling regions are suitable for fish because they provided good feeding conditions for larvae, juveniles and adult of pelagic fish. Larvae and juveniles feed on plankton. Tuna (Thunnus
sp.) is a pelagic predatory fish feeding on small pelagic fish. The development of secondary production provides an attractive habitat for tuna species. Preys of skipjack tuna in the Western Pacific consist mainly of the zooplanktivorous oceanic anchovy (Hampton and Bailey, 1993). Therefore, high abundance of tuna may occur close to the high-productive areas of upwelling. During the El Nin˜o period, abnormal upwelling events increase the pelagic fish abundance in the upwelling regions. The correlation between seasonal through flow and fish catch data was checked in the Sunda Strait. Pelagic fish catch data were collected from Labuhan port for the period between August 1999 and August 2001. Total fish catch per month were calculated from daily catches of pelagic fish mainly from fishing areas near to the Labuhan port. The major pelagic fish species landed at this port are tembang (Sardinella fimbriata), kembung (Rastrelliger spp.), selar (Selaroides leptolepis), layang (Decapterus spp.), lemuru (Sardinella longiceps), tongkol (Auxis thazard) and tenggiri (Scomberomorus spp.). Fig. 14 shows that during the SE monsoon (May to September), high pelagic fish catches coincide with the presence of Java Sea waters, which are characterized by chlorophyll concentrations of more than 0:5 mg=m3 and SST of more than 29:5 C in the region of the Sunda Strait. Pelagic fish migrates to the Sunda Strait driven by the surface currents. Hence, the
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Fig. 13. Relationships between chlorophyll concentrations, SST values and fish catch data in upwelling region.
Fig. 14. Relationship between chlorophyll concentrations, SST values and fish catch data in die Sunda Strait.
dominant fish are small pelagic species from the Java Sea and only small amount of oceanic pelagic fish are present. The fish catches decrease during the NW monsoon when ocean water is transported into the Sunda Strait (December to March) but also influences by decreasing fishing activities due to higher wind speeds of 7–10 m=s: High SST values in January might occur due to the effect of coastal discharge.
4. Conclusions A suitable multi-scattering atmospheric correction algorithm for SeaWiFS data for Indonesian water was applied to investigate monsoonal
dependence and spatial extent of upwelling along the southeast Java coast, transport of Java Sea water into the Sunda Strait, and coastal discharge in the western Java Sea. The occurrence of upwelling and the transport of Java Sea water influence phytoplankton development and pelagic fish abundance. In typical years, upwelling and through flow phenomena occur during the southeast monsoon (June to September). Chlorophyll concentrations of higher than 0:8 mg=m3 and SST values of lower than 28 C characterize the upwelling events, while concentrations of about 0:5 mg=m3 and SST of higher than 29:5 C characterize the through flow in the Sunda Strait. A correlation between the occurrence of upwelling and through flow and fish catches was
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investigated. High concentrations of derived chlorophyll correspond to high pelagic fish catches. During the rainy season (December to March) large amount of discharged waters occur in the coastal regions of East Sumatra due to the high diffuse impacts from fish farms, aquaculture and mangrove coasts. High diffuse inflow, driven by westerly winds, was observed from chlorophyll images with concentrations of 3–10 mg=m3 : Strong Citarum River discharge occurs during March and April and contains high concentration of suspended particulate matter of larger than 8 mg=dm3 and chlorophyll of higher than 2:5 mg=m3 :
Acknowledgements SeaWiFS data were provided by the Goddard Distributed Active Archive Center under the auspices of the NASA. The authors sincerely thank Prof. Bodo v. Bodungen, the Director of Baltic Sea Research Institute for his support. We sincerely also thank R. Andiastuti and K. Amri for the help in processing the SST data. Finally, we thank the reviewers for the very useful notes. Scholarship from Deutscher Akademischer Austauschdienst (DAAD) is gratefully acknowledged.
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Investigation of different coastal processes in Indonesian waters using SeaWiFS data Nani Hendiartia,*, Herbert Siegelb, Thomas Ohdeb a
Agency for the Assessment and Application of Technology, M.H. Thamrin 8, Jakarta 10340, Indonesia b Baltic Sea Research Institute, Seestrasse 15, Rostock-Warnemuende D-18119, Germany Received 1 August 2002; received in revised form 28 April 2003; accepted 14 October 2003
Abstract SeaWiFS data were applied to investigate coastal processes in Indonesian waters around the most populated island of Java. Coastal processes due to wind forcing were studied the first time using SeaWiFS-derived chlorophyll and TSM concentrations in combination with AVHRR-derived SST in the period from September 1997 to December 2001. Upwelling events were studied along the southern coast of Java during the southeast monsoon (June to September). Satellite-derived chlorophyll concentrations higher than 0:8 mg=m3 and sea-surface temperatures lower than 28 C are indications of upwelling. Upwelling events influence the distribution and growth of phytoplankton and provide by that good feeding condition for zooplankton, larvae, juvenile and adult of pelagic fish. Coastal discharge into the western Java Sea contains organic and inorganic materials originating from different sources. Diffuse impacts, particularly from fish farms and aquaculture, as well as coastal erosion influence large coastal areas during the rainy season (December to March), and to a lesser extent during the dry season. Strong Citarum river discharge was observed during the transition phase from the rainy to the dry season (March and April), when the maximum amount of transported material reaches the sea. The river plume is evident from chlorophyll concentrations higher than 2:5 mg=m3 ; and suspended particulate matter concentrations of more than 8 mg=dm3 : The Sunda Strait is seasonally influenced by water transport from the Java Sea and from the Indian Ocean. The satellite data show that water transport from the Java Sea occurs during the pre-dominantly easterly winds period (June to September). This is characterized by warm water (SST higher than 29:5 C) and chlorophyll concentrations higher than 0:5 mg=m3 : This water transport influences the fish abundance in the Sunda Strait. High fish catches coincide with the presence of Java Sea water, while the surface currents lead to the migration of pelagic fish. Conversely, during the dominant westerly winds period, oceanic waters from the Indian Ocean with low chlorophyll concentrations influence the Sunda Strait water. r 2004 Elsevier Ltd. All rights reserved.
1. Introduction *Corresponding author. Baltic Sea Research Institute, Seestrasse 15, D-18119 Rostock, Germany. Tel.: +49-3815197378; fax: +49-381-5197440. E-mail addresses: [email protected] (N. Hendiarti), [email protected] (H. Siegel), [email protected] (T. Ohde).
The Indonesian archipelago is located in the tropics between two oceans (Pacific and Indian) and two continents (Asia and Australia), and it plays a unique role in the regional and global climate system. The variation of seasonal solar
0967-0645/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr2.2003.10.003
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heating over the continents of Asia and Australia drives the monsoons, which change wind direction twice a year (Tomascik et al., 1997; Webster et al., 1998). The southeast (SE) monsoon, which occurs between June and September, is influenced by high air pressure over Australia and low pressure over Asia. The wind blows from the southeast (Australian continent) in the southern hemisphere and turns to southwest in the northern hemisphere. The northwest (NW) monsoon, which develops between December and March, is forced by high atmospheric pressure over Asia and low pressure over Australia. The wind blows from northeast (Asian continent and Pacific Ocean) in the northern hemisphere and turns to northwest in the southern hemisphere. The NW monsoon transports moist air, causing precipitation across the archipelago. Numerous authors have investigated upwelling and through flow phenomena in Indonesian waters based on in situ observations and model simulations. Southeasterly winds generate an Ekman offshore transport of surface water along the coasts of south Java, where this offshore transport induces upwelling (Wyrtki, 1987; Susanto et al., 2001). Warm water from the western Pacific Ocean is transported to the Indian Ocean through the Makassar Strait, the Lombok Strait, the Ombai Strait and the Timor passage. This the so-called Indonesian through flow is caused by sea-level differences between these two regions (Murray and Arief, 1988; Meyer, 1996; Gordon et al., 1999; Potemra and Lukas, 1999; Hautala et al., 2001). The peak of the through flow occurs in between June and July and the minimum in February (Schott and McCreary, 2001; Fieux et al., 1996). The upwelling center with low sea-surface temperature (SST) migrates westward and towards the equator during the southeast monsoon (Susanto et al., 2001). The occurrence of upwelling along the coast of southeast Java observed from SST images coincides with the most productive period in terms of fish catch (Hendiarti et al., 1996). Biological features produced by ocean phytoplankton are important for Indonesian waters, since there is no seasonal heating of the water that obliterates the temperature contrasts in the imagery. Our previous study showed that Sea-
Fig. 1. Map of investigation areas including bottom topography.
viewing Wide Field-of-view Sensor (SeaWiFS) data can be successfully applied to distinguish different water types in and around the Sunda Strait using variations in reflectance of the visible spectral range (Hendiarti et al., 2002). In this study, SeaWiFS data were used the first time in combination with SST images to study the influence of coastal processes on phytoplankton development and occurrence of pelagic fish. The areas investigated are located in the southwestern part of Indonesian waters, as shown in Fig. 1, and included the western Java Sea, the Sunda Strait and the Indian Ocean off the southern coasts of Java.
2. Datasets and methods The investigations were performed using SeaWiFS and Advanced Very High Resolution Radiometer (AVHRR) data. SeaWiFS local area coverage (LAC), which were received by the satellite ground receiving station at the National University of Singapore (CRISP) of 1 km spatial resolution, and global area coverage (GAC) data of 5 km spatial resolution were provided by the SeaWiFS project and distributed by Goddard Space Flight Center—NASA. These data were processed using the SeaWiFS Data Analysis System (SeaDAS) provided by the National Aeronautics and Space Administration.
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SST images were derived from the thermal infrared channels of the AVHRR of the NOAA (National Oceanographic and Atmospheric Administration) weather satellites. AVHRR LAC data of 1 km resolution were provided by the ground station at the Agency for the Assessment and Application of Technology (BPPT) in Jakarta, Indonesia. The SST images were produced using an improved algorithm of multi-channel approach of McMillin and Crosby (1984). The SST accuracy is 70:8 C using night time AVHRR data (Farahidy et al., 1996). MESSR—Marine Observation Satellite (MOS) data with a 50 m spatial resolution were provided by the National Space Development Agency of Japan and used to observe river discharge. Wind data were provided by the NOAA—CIRES Climate Diagnostics Centre. Fish catch data of pelagic species were taken from Banyuwangi and Labuhan ports. In situ chlorophyll a concentrations were measured for validation during cruises of R/V Baruna Jaya in August 2000, October–November 2000 and July 2001 using a Shimadzu UV-1600 spectrophotometer and calculated according to Lorenzen (1967). The SeaWiFS data evaluation was performed on the basis of the different SeaDAS versions. The different chlorophyll algorithms are described by Hooker et al. (1992) and O’Reilly et al. (1998). The validation of SeaWiFS-derived chlorophyll in Indonesian waters was realized in two steps. The first step was the check of various SeaDAS atmospheric correction algorithms. An overestimation of atmospheric effects in the correction method caused negative values of water-leaving radiances. Secondly, in situ chlorophyll measurements were compared with SeaWiFS-derived chlorophyll. Seven atmospheric correction algorithms implemented in the SeaDAS versions 3.2 and 4 were tested. These algorithms are: (1) Gordon/ Wang aerosol, (2) multi-scattering for channels 765=865 nm with near-infrared (NIR) iteration, (3) multi-scattering for channels 670=865 nm model, (4) multi-scattering for channels 670=865 nm with NIR iteration, (5) multi-scattering for channels 765=865 nm model, (6) multi-scattering with fixed
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model, and (7) single-scattering white aerosols. The best atmospheric correction for Indonesian waters is the multi-scattering algorithm using 670=865 nm with NIR iteration. This algorithm produced fewer negative values of water-leaving radiances for the channels of 443, 490, 510 and 555 nm (see Fig. 2A), and gave a higher correlation coefficient for the derived chlorophyll concentrations with in situ measurements R2 ¼ 0:87 (Fig. 2B). Differences in chlorophyll concentrations between satellite derived and in situ surface values were explained as follows. In the Sunda Strait, three different water types were distinguished during the cruise in July 2001: inner water, mixed water from the Java Sea and Indian Ocean and outer water. The investigations in upwelling areas and in the Indian Ocean were performed using in situ measurements during two cruises in August and in October–November 2000. The differences in chlorophyll concentrations observed in the mixed region of the Sunda Strait ð0:2 mg=m3 Þ were slightly higher compared to the differences in other regions, upwelling area ð0:18 mg=m3 Þ pure ocean waters ð0:04 mg=m3 Þ: These satellitederived concentrations are higher than in situ surface values since the radiance received by satellite sensor contains contributions from subsurface chlorophyll depending on the signal depth. Subsurface chlorophyll maximum may increase the concentrations derived from SeaWiFS data. To consider this condition, a weighted surface concentration was calculated on the basis of concentration and depth of subsurface chlorophyll maximum, and of the vertical attenuation coefficient. The weighting procedure increased the precision of the SeaWiFS derived chlorophyll concentrations by about 2% in the Sunda Strait and 6% in the upwelling regions (Fig. 3). The concentrations of total suspended matter (TSM) were obtained using a single band algorithm, based on the reflectance distribution in the 555 nm SeaWiFS channel. This TSM algorithm was developed for North Sea and delivered a limited accuracy not better than 15–20% (Hesselmans et al., 2000). Nevertheless, the derived concentration patterns in Banten Bay of Indonesian
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Fig. 2. Distribution of negative values for water-leaving radiances produced by various atmospheric correction algorithms of SeaDAS 3.2 and 4 (A) and matched-up analyses between chlorophyll a concentrations in situ and SeaWiFS derived by 670/865 NIR (B).
waters are similar to the in situ concentrations (Hesselmans, 2001).
3. Application of SeaWiFS data in Indonesian waters 3.1. Upwelling processes and phytoplankton development During the SE monsoon, the Ekman offshore transport along southeast Java coast induces upwelling, transporting nutrient-rich water from deeper layers into the euphotic zone where phytoplankton development starts due to increasing light level. The utilization of SeaWiFS data improves previous observations based on SST images. Upwelling events are indicated by low SST values and high chlorophyll concentrations. In images from the 2 August 2001 upwelling events can be easily observed, where the chlorophyll front coincided with the temperature boundaries in the same region (Fig. 4A and B). In situ measurements of SST and chlorophyll concentration at 1 m depths were performed at the upwelling station
(DS at 10 S and 113 300 E) during global research network system cruises in March and September 1995 to investigate the diurnal cycle. One-day observations were carried out using a CTD every 2 h: Fig. 4C shows that the September was characterized by low SST (26:5 C) and high chlorophyll concentrations (0:6–1 mg=m3 ), and March by high SST (28:5–29 C) and low chlorophyll (0:1 mg=m3 ). During the upwelling event in September, the depth of the thermocline decreased from 60–80 to 10–25 m; resulting in high concentrations of chlorophyll in the euphotic zone (Hendiarti et al., 1996). Chlorophyll concentrations in the ocean waters do not fluctuate as widely as in coastal waters (Fig. 5 left). Chlorophyll concentrations higher than 0:8 mg=m3 were found due to upwelling events during the SE monsoon. In normal years, without any influence of the El Nin˜o-Southern Oscillation climate phenomenon, the upwelling period starts in June and increases in the intensity until September (see Fig. 5 right). The peak event was observed in September with a mean value of derived chlorophyll concentrations of about 2 mg=m3 :
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Fig. 3. Match-up analysis of in situ and satellite-derived chlorophyll a concentrations in the Sunda Strait and Indian Ocean (using standard procedure (A,C) and after correction (B,D)).
The development of upwelling events varies depending on the strengths of the monsoon. Fig. 6A shows annual chlorophyll concentrations from SeaWiFS GAC data during the El Nin˜o in 1997 in comparison to other years from 1998 to 2001. In November 1997 a very strong upwelling event covered a large area from south Java to the equator around mid of west Sumatra (see Fig. 6B). This was significantly different compared with the condition of typical years as shown for November 1999 where no upwelling existed in this region (Fig. 6C). In 1998, a year after El Nin˜o, the intensity of upwelling was lower than that of typical years, as depicted from the years 1999–2001 (see Fig. 6A). The extent of upwelling area developed in similar way (data not shown). The condition where Java–Sumatra upwelling extends in time and space during the El Nin˜o period has been described by Susanto et al. (2001) on the basis of SST and sea-surface height anomaly data.
During the El Nin˜o, less Indonesian through flow occurs via Makassar Strait (Gordon et al., 1999). This condition reduces the influence of the through flow in Lombok Strait. Hence, the upwelling event is more intense because it is not disturbed by this through flow. These upwelling phenomena are important for fisheries, since the local nutrient enrichment leads to large quantities in the entire food chain up to the fish. The relationship between upwelling events and catch rates of pelagic fish will be discussed in Section 3.4. 3.2. Coastal discharge into the western Java Sea Coastal discharge is an important topic in the application of remote sensing data of different spatial resolution in the western Java Sea with the big islands of Java, Sumatra and Kalimantan (Borneo) and numerous rivers and large mangrove
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Fig. 4. SST and chlorophyll a concentration derived from AVHRR and SeaWiFS data from 2 August 2001 (A,B), and in situ daily cycle of water temperatures and chlorophyll concentrations at the surface layer observed from March and September 1995 (C).
regions. The discharged waters contain high concentration of organic and inorganic material. In East Sumatra, diffuse impacts from fish farms, aquaculture and mangrove regions affect large areas during the rainy season (December to March), as shown in Fig. 7B. Discharged material, driven by northwesterly winds, is transported offshore. High concentrations of chlorophyll of more than 3 mg=m3 ; increase the water turbidity. This may influence the coral reef ecosystems in
Kepulauan Seribu (north of Jakarta). During the dry season (July to August) a smaller amount of discharged material is transported along the coastal areas of East Sumatra (Fig. 7A). North of Jakarta, the discharged water can be differentiated by a strongly decreased reflectance in the short visible wavelength range, due to high absorption by phytoplankton and yellow substances, and increased reflectance at long wavelengths due to dominant light scattering by
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Fig. 5. Comparison of annual chlorophyll concentrations between coastal upwelling and Indian Ocean waters (left), and the differences in derived concentrations between these two regions (right).
Fig. 6. Annual chlorophyll concentrations during the El Nin˜o in 1997 in comparison to other years from 1998 to 2001: (A) chlorophyll images in November 1997; (B) and in November 1999; (C) Black represents clouds.
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Fig. 7. Composite chlorophyll images during the dry season (A) and rainy season (B).
Fig. 8. River discharge observed from SeaWiFS chlorophyll in August 2000 (A) and April 2000 (B), SeaWiFS TSM in August 2000 (C), and MOS data in August 1991 (D).
suspended sediments (Hendiarti et al., 2002). The Citarum River plume contains high concentrations of chlorophyll ð> 2 mg=m3 Þ and suspended particulate matter ð> 15 mg=dm3 Þ; as shown in Fig. 8A–D. A more detailed structure of the river plume is shown in Fig. 8D in a scene of MESSRMOS channel 1 ð5102590 nmÞ with a 50 m resolution. The Citarum River discharge, driven by easterly winds, is transported towards the Jakarta Bay. The temporal variations of phytoplankton due to the river discharge were investigated using annual chlorophyll concentrations near the Citar-
um River mouth and compared with the concentrations in the Java Sea. Fig. 9 (left) shows high concentrations caused by strong transport of fresh water and discharges during the transition phase between the rainy and the dry season (March and April), the period of maximum sediment transport into the sea. During the peak event, chlorophyll concentrations near the river mouth were between 2.5 and 3:5 mg=m3 (Fig. 9 right). The concentrations near the coast were about 10 mg=m3 : During the dry season (June to October), the through flow carries nutrient-rich water from the eastern Indonesian seas to the Java Sea. Hence, the
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Fig. 9. Comparison of annual chlorophyll concentrations between the waters from near the Citarum River mouth and the Java Sea (left), and the differences in derived concentrations between these two regions (right).
concentrations in the Java Sea are slightly higher. The cooling effect due to the through flow in the Java Sea has been observed by Tomascik et al. (1997) using SST data. 3.3. Through flow and phytoplankton distribution Water characteristics of the Sunda Strait vary spatially and temporally due to the influence of water transport from the western Java Sea and from the Indian Ocean. The present investigations were focused on the occurrence and duration of water transport from the Java Sea into the Sunda Strait caused by the monsoonal winds. The presence of Java Sea water in Sunda Strait, which is influenced by the discharge water, causes specific environmental conditions. This incoming water can be investigated using differences in water color in terms of spectral reflectances. The spectral reflectance is mainly caused by the backscattering of the water molecular and particulate matter and spectrally modified by the absorption of water, phytoplankton and yellow substances. This is used in the Red–Green–Blue (RGB) image from SeaWiFS channels of 555, 510 and 443 nm: The image shows different colors in distinct areas (Fig. 10A). Java Sea water (white color) is characterized by high reflectance in the entire visible wavelength range due to higher scattering by suspended material (see Fig. 10B). Water from Jakarta Bay
represented by the yellow color has similar conditions as the Java Sea because of the limited influence of river discharges in September. Water transport from the Java Sea also can be identified by SST and chlorophyll images, as well as in situ measurements. Fig. 11A and B shows SST values of higher than 29 C and chlorophyll concentrations of higher than 0:5 mg=m3 ; which occurred in the northeastern part of Sunda Strait in the images from July 2001. During the same period, surface salinity was lower than in the southwestern Sunda Strait (Fig. 11C). Surface currents southwestwards directed into the Indian Ocean were measured by ADCP at 5–10 m water depths (Fig. 11D). These observations indicate that the water transport from the Java Sea into the Sunda Strait, especially in upper layer, occurred in July and was characterized by warm and nutrientrich water with less salinity. The duration of the through flow from the Java Sea into the Sunda Strait was studied using annual SeaWiFS-derived chlorophyll concentrations and radiances. Fig. 12 shows that in the Sunda Strait chlorophyll concentrations higher than 0:5 mg=m3 were retrieved during the SE monsoon (June to September). Chlorophyll concentrations on the order of 0:4 mg=m3 were observed during March and April due to the large amount of discharged material from the southern Sumatra coasts. The analysis of SeaWiFS radiance of channel 555 nm
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Fig. 10. SeaWiFS composite image (A) and mean reflectance of different water types (B) derived from SeaWiFS data from the 19 September 1999.
Fig. 11. SST image on 12 July 2001: (A) chlorophyll image on July 2001; (B) water salinity at 1 m depth; (C) current profile at 5–10 m depths; (D) measurements were performed during the field campaign from 10–18 July 2001.
also showed a similar result. The Java Sea water characterized by higher radiances than ocean water (see also Fig. 10B) was monitored in the northeastern Sunda Strait during the entire SE monsoon (June to September), but in the southwestern Sunda Strait only in September.
A study of the through flow can help the fishery community since the surface currents may lead to a migration of pelagic fish. Further investigations concerning the relationships between Java Sea water and pelagic fish catches are presented below.
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Fig. 12. Comparison of annual chlorophyll concentrations between Sunda Strait and Indian Ocean waters (left), and the differences in derived concentrations between these two regions (right).
3.4. Occurrence of pelagic fish The occurrence of pelagic fish was investigated in relation to upwelling and through flow using chlorophyll and SST images during different monsoon phases. All cloud-free images of SeaWiFS LAC and AVHRR LAC data from September 1999 up to August 2001 were used to derive monthly mean values of chlorophyll a concentrations and SST. Fish catch per unit effort (CPUE) for Banyuwangi port, the most important port for landing fish from the upwelling area, were computed from fish catch statistics for the years 1991–1994. Tongkol (Auxis thazard), kembung (Restrelliger spp.), layang (Decapterus spp.), selar (Selaroides leptolepis), lemuru (sardinella longiceps), anchovy (Stelophorus spp.), and skipjack tuna (Katsuwonus pelamis) are the major pelagic fish species landed in this port. The total catch for each species is more than 12; 000 tons per year. Fish catch data were divided into quarterly periods: 1 (January– March), 2 (April–June), 3 (July–September) and 4 (October–December). High pelagic fish catchments were correlated with high chlorophyll concentration and low SST during the upwelling period of quarter 3 (Fig. 13). These upwelling regions are suitable for fish because they provided good feeding conditions for larvae, juveniles and adult of pelagic fish. Larvae and juveniles feed on plankton. Tuna (Thunnus
sp.) is a pelagic predatory fish feeding on small pelagic fish. The development of secondary production provides an attractive habitat for tuna species. Preys of skipjack tuna in the Western Pacific consist mainly of the zooplanktivorous oceanic anchovy (Hampton and Bailey, 1993). Therefore, high abundance of tuna may occur close to the high-productive areas of upwelling. During the El Nin˜o period, abnormal upwelling events increase the pelagic fish abundance in the upwelling regions. The correlation between seasonal through flow and fish catch data was checked in the Sunda Strait. Pelagic fish catch data were collected from Labuhan port for the period between August 1999 and August 2001. Total fish catch per month were calculated from daily catches of pelagic fish mainly from fishing areas near to the Labuhan port. The major pelagic fish species landed at this port are tembang (Sardinella fimbriata), kembung (Rastrelliger spp.), selar (Selaroides leptolepis), layang (Decapterus spp.), lemuru (Sardinella longiceps), tongkol (Auxis thazard) and tenggiri (Scomberomorus spp.). Fig. 14 shows that during the SE monsoon (May to September), high pelagic fish catches coincide with the presence of Java Sea waters, which are characterized by chlorophyll concentrations of more than 0:5 mg=m3 and SST of more than 29:5 C in the region of the Sunda Strait. Pelagic fish migrates to the Sunda Strait driven by the surface currents. Hence, the
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Fig. 13. Relationships between chlorophyll concentrations, SST values and fish catch data in upwelling region.
Fig. 14. Relationship between chlorophyll concentrations, SST values and fish catch data in die Sunda Strait.
dominant fish are small pelagic species from the Java Sea and only small amount of oceanic pelagic fish are present. The fish catches decrease during the NW monsoon when ocean water is transported into the Sunda Strait (December to March) but also influences by decreasing fishing activities due to higher wind speeds of 7–10 m=s: High SST values in January might occur due to the effect of coastal discharge.
4. Conclusions A suitable multi-scattering atmospheric correction algorithm for SeaWiFS data for Indonesian water was applied to investigate monsoonal
dependence and spatial extent of upwelling along the southeast Java coast, transport of Java Sea water into the Sunda Strait, and coastal discharge in the western Java Sea. The occurrence of upwelling and the transport of Java Sea water influence phytoplankton development and pelagic fish abundance. In typical years, upwelling and through flow phenomena occur during the southeast monsoon (June to September). Chlorophyll concentrations of higher than 0:8 mg=m3 and SST values of lower than 28 C characterize the upwelling events, while concentrations of about 0:5 mg=m3 and SST of higher than 29:5 C characterize the through flow in the Sunda Strait. A correlation between the occurrence of upwelling and through flow and fish catches was
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investigated. High concentrations of derived chlorophyll correspond to high pelagic fish catches. During the rainy season (December to March) large amount of discharged waters occur in the coastal regions of East Sumatra due to the high diffuse impacts from fish farms, aquaculture and mangrove coasts. High diffuse inflow, driven by westerly winds, was observed from chlorophyll images with concentrations of 3–10 mg=m3 : Strong Citarum River discharge occurs during March and April and contains high concentration of suspended particulate matter of larger than 8 mg=dm3 and chlorophyll of higher than 2:5 mg=m3 :
Acknowledgements SeaWiFS data were provided by the Goddard Distributed Active Archive Center under the auspices of the NASA. The authors sincerely thank Prof. Bodo v. Bodungen, the Director of Baltic Sea Research Institute for his support. We sincerely also thank R. Andiastuti and K. Amri for the help in processing the SST data. Finally, we thank the reviewers for the very useful notes. Scholarship from Deutscher Akademischer Austauschdienst (DAAD) is gratefully acknowledged.
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