Seasonal and annual variability in chlorophyll-a in the shelf region of the Northern Bay of Bengal using MODIS-Aqua data

Abstract

Chlorophyll-a (Chl-a) concentration is an important issue in ocean ecosystem management and research. This study investigates seasonal and annual variability in Chl-a and its relationship with sea surface temperature (SST) and river discharge in the shelf region of the Northern Bay of Bengal (BoB), as well as validates satellite data against in-situ data. Moderate Resolution Imaging Spectroradiometer (MODIS) Aqua satellite data on Chl-a concentration and SST from 2002–2018 were used in this study. River discharge data were obtained from the Bangladesh Water Development Board (BWDB). The annual Chl-a concentration ranged from 2.08 to 2.94 mg m−3, with an average of 2.43 ± 0.24 mg m−3. The Chl-a concentration was found higher (2.21 ± 0.56 mg m−3) during the northeast monsoon (October–February) and lower (1.81 ± 1.14 mg m−3) during the pre-monsoon season (March–May). The study revealed a declining trend in Chl-a concentration from 2002 to 2018, and the rate of change was −0.0183 mg m−3 year−1. Chl-a concentration showed a weak inverse relationship with SST, both annually and seasonally, especially in the pre-monsoon season. River discharge masked the effect of SST on Chl-a variability during the southwest and northeast monsoon. A reasonable correlation (r = 0.78) was found between the MODIS-Aqua data and in-situ Chl-a observations.

Seasonal and Spatial Variability of Chlorophyll-a in Response to ENSO and Ocean Current in the Maritime Boundary of Bangladesh

“…Plenty of nutrients are deposited along with the sediments to the Bangladesh coast in the northern BoB from the confluence of two major river systems: the Ganges and the Brahmaputra system and the whole Ganga-Brahmaputra-Meghna [23,37,41] during the southwest monsoon [38]. However, a recent study [17] for the years 2012-2017 at the shelf region in the northern BoB found a decreasing trend of Chl-a during the premonsoon due to the lower wind speed and rise in temperature. The same trend was observed for the year 1999-2000 by Nagamani et al [24].…”

“…During the summer monsoon (June-September), increased wind-driven upwelling along the eastern coast of India brings nutrient-rich waters to the surface, leading to increased Chl-a concentrations in the northern BoB region [1,14,16,23]. The season (Jul-Sep) has higher river discharge [17] enriched with higher amounts of nutrients like nitrate, silicon, and phosphate [34] due to river runoff following monsoonal rainfall [1] that causes a substantial increase of Chl-a abundance during this season. [23,37,42].…”

“…Level 3 mapped (SMI) with 4 km resolution monthly, and annual Chl-a data were collected from MODIS-aqua satellite (https://oceancolor.gsfc.nasa.gov) for estimating seasonal and spatial variability of Chl-a considering our study areas from July 2002 to June 2020. The MODIS-aqua Chla datasets were processed and analyzed by using SeaDAS 7.5.3, developed by NASA for processing, displaying, analyzing, and quality control of ocean color data [17]. Four seasons were recognized as premonsoon (March, April, and May), southwest monsoon (June, July, August, and September), postmonsoon (October and November), and northeast monsoon (December, January, and February) [17,37] to investigate the seasonal variability of Chl-a in this study.…”

“…The MODIS-aqua Chla datasets were processed and analyzed by using SeaDAS 7.5.3, developed by NASA for processing, displaying, analyzing, and quality control of ocean color data [17]. Four seasons were recognized as premonsoon (March, April, and May), southwest monsoon (June, July, August, and September), postmonsoon (October and November), and northeast monsoon (December, January, and February) [17,37] to investigate the seasonal variability of Chl-a in this study. Surface and meridional current datasets were retrieved directly from the Asia-Pacific Data Research Center (APDRC) live access service, LAS8.6.13 of NOAA (National Oceanic and Atmospheric Administration) (http://apdrc .soest.hawaii.edu/).…”

“…Several studies have been conducted over the Bay of Bengal (BoB) to investigate Chl-a distribution in response to various physical and biological parameters [14][15][16], such as the annual and seasonal variability of Chl-a in the shelf region of the northern BoB [17], the bathymetric features of BoB [18], the validation of Chl-a in this region [19], biological productivity [20], hydrography, and circulation of BoB [21], particularly in the southwest and western areas of BoB [19,[22][23][24][25][26]. In addition, a few studies [27,28] investigated the phytoplankton distribution in the Arabian Sea, which is close to the BoB.…”