Seasonal influence of freshwater discharge on primary productivity and euphotic depth in the northern Bay of Bengal

The cornerstone of the entire marine food web is the ocean's productivity, which is essential for removing carbon dioxide from the atmosphere and supporting life on land and at sea. To evaluate the spatial distribution and net primary production (NPP) between the coastal region and the open ocean in the Northern Bay of Bengal, the study area was divided into five zones (Z1-Z5). Mangrove forests heavily influence zone Z1(20-22
⁰
N,87-90
⁰
E) and rivers, whereas Z2(21-23
⁰
N,90-92
⁰
E) is characterized by high turbidity from suspended sediment discharge from the Ganges, the Brahmaputra, and the Meghna Rivers. Z4(17-20
⁰
N,87-90
⁰
E) is out in the open ocean, and Z3(18-20
⁰
N,85-87
⁰
E) is in the southwestern part of the Bay of Bengal. In contrast, Z5(18-21
⁰
N,91-95
⁰
E) along the Myanmar-Bangladesh coast has stable coastal upwelling and circulation dynamics, primarily influenced by the Irrawaddy River. This study examines the spatiotemporal variability of NPP, chlorophyll-a distribution, sea surface temperature (SST), sea surface salinity (SST), and Euphotic zone depth (EZD) in the Bay of Bengal during three seasons using remote sensing data from 2005 to 2020. The NPP across zones 1-5 ranges from 5315 to 346.7 mgCm
^-2
day
^-1
, as seen in Figure. Z2 has the highest productivity among all the zones, while Z4 has the lowest. Z1 shows relatively higher productivity across most months, with particularly high NPPs during August to November, exceeding 3273 mgCm
^-2
day
^-1
. Z2 has high productivity levels in February, March, June, and August to October, with 4336 mgCm
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day
^-1
or higher values. Zones 3 and 5 have moderate productivity compared to zones 1 and 2, with monthly NPPs ranging from 4719 (April) to 462 mgCm
^-2
day
^-1
(March) in Z3 and from 1434 (February) to 576 mgCm
^-2
day
^-1
(November) in Z5. Z4 has low productivity levels with minimal monthly variation, with NPP values varying from 795 (September) to 346 mgCm
^-2
day
^-1
(March). NPP in the northern BoB is influenced by various factors such as phytoplankton biomass, light penetration, nutrients, salinity, temperature, surface turbulence, and EZD [1]. The availability of light and nutrients is the main driver of primary productivity in the BoB. In addition, seasonally reversing monsoonal wind, freshwater input, and remote atmospheric forcing, such as Indian Ocean Dipole, play a significant role in shaping the productivity of the BoB. The seasonal distribution of NPP, especially in Zones 2 and 1, is heavily influenced by these biophysical processes. During the Southwest monsoon, the east coast of India experiences favorable winds for upwelling [2]. Our analysis showed that the highest level of NPP in Z3 is likely driven by the southwest monsoon, with a shallower thermocline, relatively intensified vertical mixing, and less rain [3]. Seasonal variability in freshwater discharge also affects SSS distribution in Z1-2. Z2 is the most productive among the five Zones analyzed, driven by a heavy influx of nutrients and mixing due to monsoon winds. EZD is deeper during the summer monsoon and shallower during winter, with a corresponding increase in productivity. Long-term trends in sea surface temperature and productivity were positive in the head bay zones but negative in the open ocean. The long-term changes in SST suggest that it is increasing over time, likely due to rapidly increasing atmospheric CO
2
concentration and climate change [4]. The findings can be used for developing sustainable management of marine resources and overall environmental conditions in response to climate change in the Bay of Bengal.