Seasonal influence of freshwater discharge on spatio-temporal variations in primary productivity, sea surface temperature, and euphotic zone depth in the northern Bay of Bengal
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Abstract: Ocean productivity is the foundation of the marine food web, which continuously removes atmospheric carbon dioxide and supports life at sea and on land. Spatio-temporal variability of net primary productivity (NPP), sea surface temperature (SST), sea surface salinity (SSS), mixed layer depth (MLD), and euphotic zone depth (EZD) in the northern Bay of Bengal (BoB) during the three monsoon seasons were examined in this study based on remote sensing data for the period 2005 to 2020. To compare the NPP distribution between the coastal zones and open BoB, the study area was divided into five zones (Z1-Z5). The results suggest that the most productive Z2 and Z1 are located at the head bay area and are directly influenced by freshwater discharge together with riverine sediment and nutrient loads. Across Z1-Z5, the NPP ranges from 5315.38 mg/(m2·d) to 346.7 mg/(m2·d) (in terms of carbon, since then the same). The highest monthly average NPP of 5315.38 mg/(m2·d) in February and 5039.36 mg/(m2·d) in June were observed from Z2, while the lowest monthly average of 346.72 mg/(m2·d) was observed in March from Z4, which is an oceanic zone. EZD values vary from 6-154 m for the study area, and it has an inverse correlation with NPP concentration. EZD is deeper during the summer season and shallower during the wintertime, with a corresponding increase in productivity. Throughout the year, monthly SST shows slight fluctuation for the entire study area, and statistical analysis showed a significant correlation among NPP, and EZD, overall positive between NPP and MLD, whereas no significant correlation among SSS, and SST for the northern Bay of Bengal. Long-term trends in SST and productivity were significantly positive in head bay zones but negatively productive in the open ocean. The findings in this study on the distribution of NPP, SST, SSS, MLD, and EZD and their seasonal variability in five different zones of BoB can be used to further improve the management of marine resources and overall environmental conditional in response to climate changes in the BoB as they are of utmost relevance to the fisheries for the three bordering countries.
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Key words:
- Chlorophyll a /
- sea surface temperature /
- euphotic zone depth /
- primary productivity /
- Ganges-Brahmaputra /
- ocean color /
- Bay of Bengal /
- monsoon
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Figure 1. Map of the Bay of Bengal and the five zones identified for studying the NPP, SST, SSS MLD, and EZD variability (coastline data retrieved from
https://www.naturalearthdata.com and Bathymetric data fromhttps://www.gebco.net ). "Z" represents the abbreviation for "zone".
Figure 2. Prevailing northeast and southwest monsoon in the Bay of Bengal and the Arabian Sea. The arrow indicates the direction of wind flow.
Figure 3. Freshwater discharge (m3/s) from the seven major rivers joining the BoB (Sandeep and Pant, 2019). Monthly discharge rates of seven rivers (Brahmaputra, Ganges, Irrawaddy, Godavari, Mahanadi, Krishna, and Cauvery); major sources of sediments & nutrients in the BoB.
Figure 9. Monthly-averaged sea surface temperature (SST) distribution for January, June, October, and December, based on data from 2005 to 2020.
Figure 12. Monthly distribution of euphotic zone. a-l presents months from January to December.
Table 1. Seasonal NPP average and variability
Zone NPP average NPP variability Northeast monsoon Pre-monsoon Southwest monsoon Northeast monsoon Pre-monsoon Southwest monsoon 1 2566.30 1484.98 2074.39 2696.37 1500.52 1637.74 2 4033.14 3844.31 4379.77 1582.55 4049.19 3523.57 3 854.64 2022.13 827.24 694.11 1037.27 1129.52 4 388.73 400.02 563.99 64.87 134.33 184.57 5 634.32 969.46 919.49 612.74 1259.97 770.60 
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Table 2. Seasonal SSS average and variability.
Zone SSS average SSS variability Northeast monsoon Pre-monsoon Southwest monsoon Northeast monsoon Pre-monsoon Southwest monsoon 1 25.842 32.144 27.024 2.279 0.480 2.670 2 20.710 32.144 19.807 4.820 3.899 6.671 3 28.779 32.144 30.856 0.987 0.332 1.020 4 30.214 32.144 31.258 1.073 0.282 0.976 5 28.211 32.144 30.786 1.096 0.529 1.280
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Table 3. Seasonal SST average and variability
Zone SST average SST variability Northeast monsoon Pre-monsoon Southwest monsoon Northeast monsoon Pre-monsoon Southwest monsoon 1 27.65 28.47 29.76197 0.339 0.581 0.513 2 27.81 28.95 29.72703 0.660 1.025 0.593 3 28.10 28.52 29.59749 0.216 0.643 0.374 4 28.28 28.72 29.24756 0.130 0.147 0.102 5 28.87 28.84 29.3195 0.868 1.218 0.678
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