Detection of declining phytoplankton chlorophyll concentration levels south of the Arabian Peninsula from MODIS satellite data

Phytoplankton are microscopic organisms that live in the sunlit layer of oceans and water bodies. Marine phytoplankton play a key role in the ecology of the marine ecosystem, affects its diversity and drives its functioning as well as setting the upper limits for fishery yields. Eventually, any significant changes in their patterns of spatial distribution and abundance can have significant impact on the entire ecosystem. The primary objectives of this study are to investigate the inter-annual variations of phytoplankton chlorophyll concentration levels south of the Arabian Peninsula using MODIS data in the period 2003-2013. The sea area south of the Arabian Peninsula is characterized by high primary productivity and strong phytoplankton blooms at different times of the year. The data over the study area comes from Moderate Resolution Imaging Spectroradiometer (MODIS) aboard NASA Aqua satellite. MODIS data are processed to ground reflectance and then further processed into chlorophyll concentration using chlorophyll estimation algorithms. The study area was subdivided from the global products into three regions (Sea of Oman, Arabian Sea and the Gulf of Aden.) The average annual chlorophyll concentrations were calculated from the MODIS product for each area. The preliminary hypothesis is that inter-annual phytoplankton chlorophyll concentrations should oscillate annually with no particular direction, if there is no significant change in the average annual concentration levels. The preliminary results suggest declining concentration levels in the whole area in contrast to the initial assumptions. MODIS estimated annual concentrations ranged between 0.97 to 2.2 mg/m³ for the sea of Oman, 0.411 to 0.88 mg/m³ for the Arabian Sea and between 0.38 to 0.86 mg/m³ for the Gulf of Aden. The estimated values depict an overall pattern of annual concentrations decline during the period 2003-2013 of approximately 7% for the sea of Oman, 1% for the Arabian sea and 4% for the Gulf of Aden. This result confirms previous study of global decline of phytoplankton chlorophyll levels. To investigate this pattern further, MODIS annual average sea surface temperatures (SST) for the study area were analyzed along with the chlorophyll concentration levels. The SST observations are showing a slight rate rise of less than 1% for the Sea of Oman, however, it is not strong enough to relate the phytoplankton chlorophyll decline to SST rise in this area. The other parts of the study area are displaying flat SST levels during the study period with no particular direction. While only eleven annual observations are not enough to be a statistically valid sample to draw conclusions, this pattern of decline is of major concern. This work will be further improved by the analysis of more than 30 years worth of satellite data and the incorporation of other factors such as the water salinity, and water and wind currents in another study.