Temporal and spatial variability of phytoplankton pigment concentrations in the Indian Ocean, derived from the CZCS time series images

Variabilidad temporal y espacial de las concentraciones de pigmentos fitoplanctónicos en el Océano Índico, a partir de imágenes CZCS

Abul B.M. Alauddin Talukder

*School of Biological Science Flinders, University of South Australia, GPO Box 5100 Adelaide, Australia. E-mail: talu0001@flinders.edu.au

Recibido en septiembre de 2003;
aceptado en enero de 2005.

Abstract

A total of 93 monthly global composite remotely sensed ocean color images from the Coastal Zone Color Scanner (CZCS) on board the Nimbus-7 satellite were extracted for the Indian Ocean region (35°N-55°S; 30-120°E) to examine the seasonal variations in phytoplankton pigment concentrations, resulting from large-scale changes in physical oceanographic processes. The CZCS data sets were analyzed with the PC-SEAPAK software, and revealed large phytoplankton blooms in the northwest Arabian Sea and off the Somali coast. The blooms were triggered by wind-driven upwelling during the southwest monsoonal months of August and September. In the northern Arabian Sea, phytoplankton blooms, detected from January to March, appeared to be associated with nutrient enhancement resulting from winter convective mixing. In the Bay of Bengal, higher pigment concentrations were confined to the coastal regions but varied only marginally between seasons both in the coastal and offshore regions. Phytoplankton pigment concentrations were consistently low in the open Indian Ocean. Analysis of pigment concentrations extracted from the monthly-accumulated images revealed that the Arabian Sea sustained a greater biomass of phytoplankton compared with any other region of the Indian Ocean. Overall, the coastal regions of the Indian Ocean are richer in phytoplankton pigment than the open Indian Ocean. The number of images in individual areas was highly variable throughout the region due to varying cloud cover.

Key words: phytoplankton pigment concentration, monsoon, Indian Ocean, Coastal Zone Color Scanner.

Resumen

Se obtuvieron 93 imágenes mensuales compuetas del sensor remoto de color del mar Coastal Zone Color Scanner (CZCS), a bordo del satélite Nimbus-7, para la región del Océano Índico (35°N-55°S; 30-120°E), con el fin de determinar las variaciones estacionales de las concentraciones de pigmentos fitoplanctónicos producidas por cambios a gran escala en los procesos oceanográficos físicos. Los datos del CZCS fueron analizados usando la paquetería PC-SEAPAK y mostraron florecimientos extensos de fitoplancton en el noroeste del Mar Arábigo y frente a la costa de Somalia. Los florecimientos resultaron de surgencias inducidas por el viento durante los meses del monzón del suroeste de agosto y septiembre. En el norte del Mar Arábigo, los florecimientos fitoplanctónicos, detectados entre enero y marzo, parecen estar asociados con el enriquecimiento de nutrientes debido a la mezcla convectiva invernal. En la Bahía de Bengala, las mayores concentraciones de pigmentos se limitaron a las regiones costeras, variando sólo marginalmente entre estaciones del año tanto en las áreas costeras como mar adentro. Las concentraciones de pigmentos fitoplanctónicos fueron consistentemente bajas en el Océano Índico abierto. El análisis de las concentraciones de pigmentos obtenidas de las imágenes acumuladas mensualmente mostró que el Mar Arábigo sostiene una biomasa mayor de fitoplancton en comparación con otras regiones del Océano Índico. En general, las regiones costeras del Océano Índico son más ricas en pigmentos fitoplanctónicos que las oceánicas. El número de imágenes de áreas específicas fue altamente variable en toda la región debido a la cambiante nubosidad.

Introduction

In the marine environment, pigments provides an index of phytoplankton biomass irrespective of species composition and size (Esais, 1980; Yentsch, 1983; McClain et al., 1990). The amount of back-scattered solar radiation reradiated from just beneath the sea surface is the best measure of this index (Hovis et al., 1980). The Coastal Zone Color Scanner (CZCS) on board the Nimbus-7 satellite, launched on 23 October 1978 (Hovis et al., 1980) and operated until June 1986, accumulated more than 68,000 images (Feldman et al., 1989). Remote sensing of ocean color is an important tool for recording regional and global scale phytoplankton pigment concentrations (Gordon et al., 1980; Hovis et al., 1980; Yentsch, 1983; Gordon and Morel, 1983), marine primary production (Platt and Sathyendranath, 1988; Sathyendranath et al., 1991) and surface patterns of the oceans (Barnard et al., 1997).

The monsoonal wind system of the Indian Ocean causes semi-annual reversal of the surface currents in the Bay of Bengal and Arabian Sea. Wyrtki (1973) stated that the Indian Ocean region is a classic example of atmospheric forcing of upper ocean circulation. In January and February, the winds bring cool dense air to India from the Asian land mass; this flow of air is the northeast (NE) monsoon. The southwest (SW) monsoon blowing more or less parallel to the Somali and Arabian coasts commences in May/June, reaches a peak in July/August, and fades away in September/October. Both monsoons change the oceanography of the Arabian Sea (Wyrtki, 1971; Krey and Babenered, 1976). The distribution of chlorophyll a in the Indian Ocean region was reported (Qasim, 1978) using data from the International Indian Ocean Expedition. Global ocean seasonal pigment concentrations have been analyzed by Yoder et al. (1993) and Banse and English (1994). Yet the whole Indian Ocean remains one of the least studied areas in the world oceans with relatively few reports available on the distribution of phytoplankton pigment concentrations. The CZCS pigment concentration data of the Arabian Sea have been used by Banse and McClain (1986), Banse (1987), and Banse and English (2000).

The purpose of this study was to investigate the temporal variability of the phytoplankton pigment concentrations in the Indian Ocean, using ocean color imagery. Image count data during 93 months were also considered for the interpretation of pigment pattern distribution.

Study area

The area of the Indian Ocean (fig. 1), including the Red Sea and the Persian Gulf, is about 49 x 10⁶ km². The Indian Peninsula divides the Indian Ocean into the Bay of Bengal and the Arabian Sea, which is extended to the north by the Gulf of Oman and the Persian Gulf and to the west by the Gulf of Aden and the Red Sea. The Bay of Bengal, of about 2.2 x 10⁶ km² and lying between the Indian Peninsula and Myanmar, is characterized by complex current systems. It is bordered on the north by the deltaic regions of the Ganges and Brahmaputra river systems and experiences surface stratification due to the influx of fresh water from these river systems. According to UNESCO (1988), the combined annual discharge from the Ganges, Brahmaputra, Irrawady and Godavari rivers exceeds 1.5 x 10¹² m³.

Methods

The CZCS global data sets were obtained from NASA's Goddard Space Flight Centre, as level 3 global CZCS processing data and processed on dedicated computer facilities at the Institute of Hydrospheric and Atmospheric Sciences, Nagoya University, Japan. Time series data sets of pigment concentrations from October 1978 to June 1986 (93 months) were analyzed by an image analysis system, PC-SEAPAK software (Darzi et al., 1989). Monthly global composite images were used in this study and subsets of data for the Indian Ocean region (35°N-55°S; 30°-120°E) were extracted from the global CZCS data using the Pstimg program. The extracted monthly images of phytoplankton pigment concentration and image count data were accumulated on disk, and the software programs Meanf and Addf (Darzi et al., 1989) were used to calculate the mean monthly pigment and image counts. Forty positions at different latitudes and longitudes (see fig. 1; table 1) were selected over the 12 interpolated images of January through December (fig. 2) using the Read program to calculate the pigment concentrations. Stations 1-20 and 21-40 were selected in the coastal region and open ocean region, respectively, to estimate pigment concentrations. Stations in the Arabian Sea are numbered 1-5 and 21-24, those in the Bay of Bengal are numbered 6-10 and 26-29, and those in open ocean regions are numbered 25, 31-33 and 35-39 (fig. 1; table 1).

Results and discussion

The 93-month time series of CZCS pigment concentration data (fig. 2) shows large-scale differences in physical ocean-ographic processes in the Indian Ocean. Large-scale phytoplankton blooms are apparent in the northwest Arabian Sea and off the Somali coast. These were apparently triggered by wind-driven upwelling during the SW monsoonal months of August and September, which also show that the biological effects of upwelling were greatly pronounced all along the Somali coast and northwest Arabian Sea. In the northern Arabian Sea, phytoplankton blooms, detected from January to March (NE monsoon), are associated with nutrient enhancement resulting from winter convective mixing. During the NE monsoon (November/December to March/April) cool dry continental air is brought into the northern Arabian Sea by the prevailing NE trade winds that intensify evaporation, leading to surface cooling (Prasanna-Kumar and Prasad, 1996) and the subsequent upward transport of nutrients from the thermocline region. Nutrient injection into the upper layers of the water column triggers the primary productivity in the Arabian Sea (Madhupratap et al., 2000). During the winter monsoon the surface pigments exhibited a large range, from 0.904 to 1.26 mg m^-3 in the coastal region and from 0.07 to 0.138 mg m^-3 on the east coast of India. Banse and McClain (1986) identified large-scale phytoplankton blooms in the winter months of 1979/80 in the northern Arabian Sea. Higher pigment concentrations were confined to the coastal regions and varied marginally over seasons, both in the coastal and offshore region of the Bay of Bengal. Phytoplankton pigment concentrations were low in the Bay of Bengal during the NE monsoon, but increased during the SW monsoon. Chauhan et al. (2001) reported that open ocean waters of the Bay of the Bengal are oligotrophic, with chlorophyll a concentrations less than 0.03 mg m^-3. The authors also reported a large-scale river plume extending into the open ocean in the months of January to March 2000. A region of upwelling has been detected on the south coast of Sri Lanka and the west coast of India from August to October. During this time surface currents move southward carrying high nutrients from the west coast of India to the south coast of Sri Lanka. Wind-driven upwelling and a series of cyclonic storms transport nutrients to the well-mixed zone and into the upper layer of the water column. From January to March the south coast of Sri Lanka experiences cooling and densification, leading to convective mixing and an injection of nutrients into the surface water layer. With the onset of the NE monsoon the surface currents reverse their direction of flow northwards, carrying low nutrients from the equatorial region, and produce the low pigment concentration on the south coast of Sri Lanka.

An absence of large-scale upwelling was evident on the western Australian continental shelf, with low pigment concentrations because of the Leeuwin Current, which carries warmer water and flows strongly during the winter (see CSIRO at http://www.per.marine.csiro.au). However, pigment concentrations increased in May and reached a maximum concentration of 0.5 mg m^-3 by July and August, indicative of upwelling on the western Australian coast.

Monthly time series of the 93-month image count data (fig. 4) show that the number of images in individual months is variable due to varying cloud cover and the objectives of the Nimbus-7 satellite. A comparison of monthly mean image count data in coastal regions and in the open ocean (fig. 5) shows that the number of images in the coastal region is higher than in the open ocean region, and that the NE monsoon is relatively cloud-free compared with the SW monsoon. Image count data for the Arabian Sea, Bay of Bengal and equatorial region of the Indian Ocean (fig. 6) indicate that the Arabian Sea has the highest image count, followed by the Bay of Bengal, and then the equatorial region of the Indian Ocean. Cloud cover over the Arabian Sea, however, is largely confined to the period from May to August. In the Bay of Bengal, on the other hand, irrespective of the monsoon season, cloud cover is greater throughout the year. Annual cloud cover ranges from 4.1 to 5.1 oktas over the Bay of Bengal and from 1.5 to 3.7 oktas over the Arabian Sea (Indian Meteorological Department, 1976).

The low pigment concentrations observed in the Bay of Bengal may result from light limitation due to cloud cover. The physical aspects of the Somali upwelling described by Schott (1983) indicate that this upwelling is usually characterized by a two-gyre system. The CZCS images of August and September indicated that gyre systems increased pigment concentrations along the coast. Through Ekman pumping, extensive upwelling brings up nutrients along the Somali and Arabian coast (Swallow, 1984; Brock et al., 1990; Bauer et al., 1991). In the NW Arabian Sea, pigment concentrations <0.39 mg m^-3 were observed during the NE monsoon, in contrast to values of 6.61 mg m^-3 during the SW monsoon. The Somali current flows toward the SW during the NE monsoon and toward the NE during the SW monsoon.

Diatom blooms are common in the Arabian Sea (Nair et al., 1992) and their composition has been reported by Sawant and Madhupratan (1996). A dense, green bloom of the marine prymnesiophycean, Phaeocystis globosa, was observed in the central Arabian Sea (Madhupratap et al., 2000) during the summer monsoon. Qasim (1977) and Banse (1984) observed biological productivity associated with the annual monsoon cycle, and showed that the Arabian Sea was the most biologically productive region of the Indian Ocean. The biological productivity of the NW Arabian Sea has been attributed to the presence of high concentrations of nitrate, phosphate and silicate at shallow depths within the euphotic zone (Ryther and Menzel, 1965; Ryther et al., 1966; Qasim, 1977, 1982). The maximum chlorophyll a concentrations and highest particulate matter flux out of the photic zone occur during the SW monsoon (Babenerd and Krey, 1974; Banse, 1987; Nair et al. , 1989). Phytoplankton pigment concentrations were consistently low in the open Indian Ocean due to the down-welling and anti-clockwise gyre. The CZCS image shows very low pigment concentration (0.150-0.406 mg m^-3) in the months of May and June as a result of low salinity water, which accumulates in the northeastern Bay of Bengal, being carried into the southern Arabian Sea (Donguy and Meyers, 1996). In the Bay of Bengal phytoplankton pigment concentrations ranged from 0.150 to 0.406 mg m^-3 from November to April.

Phytoplankton pigments were very variable due to upwelling and winter convective mixing in the northwestern Arabian Sea and low in the open ocean where none of these effects occurred.

Acknowledgements

Funding for this research was provided by the Japanese Ministry of Education, Sports and Culture, and by an award from the Flinders University of South Australia. This work was supported by an Australian Research Council grant to Jim G. Mitchell. The author gratefully acknowledges Toshiro Saino (Nagoya University, Japan) for his valuable comments and suggestions for use of the PC-SEAPAK software, and thanks Scoresby Shepherd (SARDI, Australia) and three anonymous reviewers for valuable and encouraging comments on the manuscript.

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