Ocean color (OC) is an important proxy for studying primary productivity in oceans, as it is measured by satellite sensors through the interaction of incident light with optically active substances present in the upper water column up to the euphotic zone depth (EZD). These optically active substances include suspended sediments, colored dissolved organic matter (CDOM), and chlorophyll a (Chl a) (Frolov et al., 2012). Water leaving radiance measured at the visible portion of the electromagnetic spectrum measures the presence of these optically active substances on the ocean surface. The net primary productivity (NPP), on the other hand, measures how much inorganic carbon is fixed by phytoplankton per unit volume of water per unit of time (Sakalli, 2017). In addition to NPP, other biophysical parameters such as sea surface temperature (SST), EZD, mixed layer depth (MLD), CDOM, nitrate, phosphate, and sea surface salinity (SSS) play a crucial role in regulating phytoplankton biomass, ocean productivity, and community structure. Therefore, a better understanding of these parameters is essential for studying marine ecosystems and their response to various environmental factors (Mutshinda et al., 2013). The ocean primary productivity, which is the base of ocean food web and a key process for fixing energy within the ecosystem, is directly influenced by these parameters. It removes atmospheric carbon dioxide and supports life on land and in the oceans (Arthur et al., 1987; Balch et al., 1992; Frey et al., 2017). Primary productivity also influences planktivorous and predatory organisms, mammals, and seabirds via zooplankton dynamics (Capuzzo et al., 2018; Hossain et al., 2020; Raymont, 2014), and plays a major role in ocean biogeochemical, ecological, and biological processes (Mohanty et al., 2014; Sarker and Wiltshire, 2017). Moreover, marine phytoplankton fixes 30−50 billion tons of carbon annually, equivalent to 40%−50% of the global total, via this process (Raymont, 2014). Observing interannual or long-term productivity variations provides insight into how changes in the ocean environment propagate from primary producers to higher trophic levels (Salgado-Hernanz et al., 2019). The productivity variations are determined by phytoplankton concentration together with photosynthetically available radiation (PAR) and SST in the EZD since phytoplankton blooms depend on the availability of sunlight, nutrients, and mechanisms that regulate them seasonally (Behrenfeld and Falkowski, 1997). Coastal and estuarine ecosystems exhibit maximum temporal and spatial variability in productivity due to variability in biological, biochemical, and physical properties of coastal waters, such as monsoonal wind circulation, river fluxes, coastal ocean circulation, upwelling, rainfall, and open ocean vertical stratification (Abdul-Hadi et al., 2013; Hendiarti et al., 2004; Satpathy et al., 2011). The Bay of Bengal (BoB) is an important region located to the east of the Indian subcontinent, receiving freshwater input from several major rivers such as the Ganges, Brahmaputra, Meghna, Krishna, Godavari, Mahanadi, Sittang, Irrawaddy, and Salween. Despite its significance, the BoB is known to be less productive compared to the Arabian Sea, located to the west of India. This could be attributed to the inability of the monsoon winds to break the upper-ocean stratification caused by high freshwater input from rivers and the torrential rainfall in the BoB. These conditions inhibit the mixing of water layers and consequently reduce the availability of nutrients, which are crucial for primary productivity (Saikranthi et al., 2019). In addition, the limited penetration of sunlight due to intense cloud cover and the persistent presence of turbid coastal waters, extending offshore as mud plumes, also play a significant role in inhibiting sustained high productivity in the northern BoB (Bharathi et al., 2018).

The BoB is both environmentally and economically an important region, as millions of people rely on this regional sea for their livelihood. Marine and estuarine fisheries play a crucial role in terms of livelihood, food sources, and supporting the national economy of the bordering countries (Hussain and Hoq, 2010; Shamsuzzaman et al., 2017). The abundance and distribution of fisheries depend heavily on the seasonal primary productivity of the Bay. A few studies reported the BoB’s seasonal productivity and SST distribution with different approaches and for different periods (Mahesh et al., 2020; Pramanik et al., 2019; Zhou et al., 2020). Hence, this study aims to investigate the relationship between biophysical parameters such as NPP, SST, EZD, SST, and SSS in regulating phytoplankton biomass, ocean productivity, and community structure in the BoB. The study also aims to examine the interannual or long-term productivity variations to gain insight into how changes in the ocean environment propagate from primary producers to higher trophic levels. Specifically, the study will focus on determining the relationship between physical parameters and NPP and how this relationship is affected by varying SSS and SST in the BoB. Additionally, the study will explore the impact of wind reversal to flux on these relationships, building on previous research. Overall, this research aims to contribute to a better understanding of the complex interactions between biophysical parameters and their impact on ocean productivity and ecosystem dynamics in the BoB.

Monthly, seasonal, and interannual spatio-temporal distribution of NPP, MLD, SST, SSS, and EZD have been analyzed for the northern BoB using five sub-grids. NPP spatio-temporal changes are caused by variations in phytoplankton biomass and environmental factors such as light penetration, nutrients, salinity, temperature, and surface turbulence, as indicated by SST, SSS, and EZD (George et al., 2013; Kong et al., 2019). The BoB is located within the tropics, and nutrient discharge from the rivers and estuaries dominates its seasonal cycle of primary productivity, in addition to a seasonally changing wind system. The availability of light and nutrients generally control the primary productivity in BoB; in addition, seasonally reversing monsoonal wind, freshwater input, and remote atmospheric forcing, the IOD profoundly affects the productivity in the BoB. The seasonal reversal in distribution of NPP, especially at Z2 and Z1, is happening because of these biophysical processes. Upwelling processes in the BoB only occur close to the coast within 40 km (Shetye et al., 1991). Seasonally reversing monsoonal wind forcing facilitates mixing of highly stratified northern BoB, which brings nutrients to the photic zone. For the northernmost Z1 and Z2, a continuous supply of nutrients from the major Ganges-Brahmaputra-Meghna river system and subsequent mixing promotes primary productivity, which was observed consistently in our study.

The circulation dynamics of Z3 is dominated by the East Indian Coastal Current (EICC), which flows equatorward from August to December and poleward (northeast) from February to May (Chaitanya et al., 2015; Shetye et al., 1996). This zone experiences strong seasonal upwelling and efficient mixing due to the reversal of monsoonal winds and shelf bathymetric features. During the SW monsoon, the east coast of India experiences favorable winds for upwelling (Shetye et al., 1991). At the same time, EICC affects Chl a distribution in Z3 by affecting nutrient transport, light penetration, and temperature in the euphotic zone (Shanthi et al., 2015). During the SW monsoon, our analysis showed the highest level (900.90 mg/(m²·d)) of NPP in Z3, which falls under the east coast of India. Productivity is probably driven by the monsoon season with shallower thermoclines, more vigorous vertical mixing, less rain (Currie et al., 2013; Wiggert et al., 2009). Some studies (Behrenfeld, 2014; Sarker et al., 2020; Sarker and Wiltshire, 2017) reported a declining trend of primary productivity across the BoB. However, our long-term data showed a perceptible decline in Z4 only. Our finding showed that decadal change in NPP varied from 952.5 mg/(m²·d) to −59.7 mg/(m²·d). Overall, four coastal zones (Z1−Z3 and Z5) experienced positive change, which means an increase in productivity, whereas negative change, meaning reduced productivity, was found in Z4. A slightly lower increase was found in Z5 because it does not receive a considerable amount of freshwater input like Z2, and the effect of seasonally reversing wind is less pronounced. Z4 is an open ocean environment and is far away from the influence of coastal turbid water and experiences poor vertical mixing. It is also the lowest productive zone, and Fig. 16 shows the increasing trend of SST in this zone. Chaitanya et al. (2015) found that Chl a concentration in the open ocean was low, between 0.06 mg/m³ and 5.5 mg/m³. Productivity in the open ocean is generally low due to strong stratification associated with barrier layers, which prevents the exchange of nutrients from deep water to surface water. The occasional passage of tropical cyclones over Z4 helps to break the strong stratification and thereby inject nutrients into the photic zone that enhances productivity. However, several studies have reported an expanding dead zone in the middle of the BoB, and decadal reduced, lower productivity and warmer water indicate stratification and reduction of mixing depth, hence an intense oxygen minimum zone in Z4 (Diaz and Rosenberg, 2008; Sridevi and Sarma, 2020). The results of our study indicate that the MLD in Z3 and Z4 exhibited a decreasing trend over the two-decade period with a negative correlation with the NPP. This finding suggests that the water column in these zones may be becoming less mixed over time. Such a decrease in mixing could potentially have significant implications for the biological productivity of the area. A less mixed water column can result in less nutrient cycling and lower levels of primary productivity, which could ultimately impact higher trophic levels in the ecosystem. However, our findings reveal that opposite scenarios occurred in Z1, Z2 and Z5. Across Z1−Z5, the NPP ranges from 5315mg/(m²·d) to 346.7 mg/(m²·d). Seasonally, NPP varied significantly. According to our findings, NPP distribution in Z2 is highest during SE monsoon, while it is higher in Z1 during NE monsoon. Despite being close to each other, these are completely opposite scenarios. The BoB is also exposed to powerful tropical cyclones from October to December. From 2005 to 2020, about fifteen major cyclones traversed across the BoB, and these months (October and November) exhibit the most productivity on the eastern Indian coast and adjoining open sea due to cyclone-induced upwelling and vertical mixing, which favors higher productivity (Latha et al., 2015). Several studies found that seasonal and inter-annual variability in SSS is highest close to the northern part and minimum in the central zones (Akhil et al., 2014; Chaitanya et al., 2015). In the northern BoB, especially Z1−Z3 and Z5, the SSS varies with the amount of river fluxes. The SSS exhibits the greatest variability at Z1 (32.4−22.6) and followed by Z2 (28.9−16.8), over the course of the study. This is due to the fact that they are mostly located at the mouth of major rivers. Z2 and Z1 showed the highest variability during SW and NE monsoons, whereas Z3 experienced frequent fluctuations in variation trends during the same seasons. In Z1, SSS varied from 3.89 to 6.67 during the SW monsoon, with 6.67 found during the SW monsoon. The lowest variations (1.0 to 0.2) and highest SSS confidence (~32 to 30) were found in Z4 because this is far from the coast and river influence. SSS’s decadal change (2010−2020) varied from 1.05 to −4.42 across the zones. The negative values observed from Z2 indicate salinity decreasing over time due to higher freshwater inputs and rainfall from an intense monsoon season. This can also be caused by more Himalayan ice melting due to global warming. Although changes in SST are associated with changes in Chl a, NPP, species physiological response, and distribution (Behrenfeld et al., 2006; Dunstan et al., 2018), r values between calculated monthly SST and NPP are low (0 to 0.2) from Z1 to Z4, whereas a negative r value has been found in Z5. However, the decadal change (2010−2020) of SST ranged from 0.56℃ to 0.19℃, which indicates SST is increasing over time. These changes might be due to rapidly increasing atmospheric CO₂ concentration and climate changes (D’Mello & Prasanna Kumar, 2016; Levitus et al., 2001). Oceanic water mass dynamics are governed by temperature, which is a key indicator of climate change (Roemmich et al., 2012). Approximately 90% of the excess heat added to the earth’s climate system since the 1960s is stored in the oceans (Levitus et al., 2001). Based on studies (Alory et al., 2007; Chambers et al., 1999; D’Mello & Prasanna Kumar, 2016; Dong et al., 2014), there has been an overall warming of the ocean over the past half-century. A significant increase in water temperature in the BoB is also supported by D’Mello and Prasanna Kumar (2016), who reported an increase in SST at a rate of 0.014℃ per year in the BoB from 1960 to 2011.

Daily and monthly imagery and gridded data sets were analyzed to study the spatio-temporal variations in NPP, EZD, MLD, SST, and SSS in the northern BoB from January 2005 to August 2020, and the findings would contribute to a better understanding of tropical ocean ecology of a regional sea under long term climate change scenarios. The BoB, as a regional northward extension of the Indian Ocean, does not experience large-scale seasonal and spatial variability in primary productivity and SST. However, seasonal variability in freshwater discharge from the rivers along the northern bay affects the SSS distribution significantly, particularly for the two northern zones, Z1 and Z2. Z2 is the most productive of the five zones that we have analyzed, and it is driven by a heavy influx of nutrients from the rivers and the mixing due to seasonally reversing monsoon winds. We also noticed that sea surface temperature and sea surface salinity do not correlate with the region’s primary productivity, implying that other regional phenomena, viz., coastal upwelling, mixing from wind reversal, and passage of power cyclones, etc., played a significant role in the primary productivity. EZD is deeper during the summer monsoon while shallower during the wintertime, with a corresponding increase in productivity. The bay is highly stratified from the large influx of freshwater discharge from the Ganges, Brahmaputra, and Meghna rivers. Turbidity also plays a critical role in inhibiting primary productivity in the northern head bay region. The BoB is relatively less productive due to stratification and less efficient mixing or upwelling compared with the nearest Arabian Sea, with ephemerally elevated productivity. More research is needed to further understand the variations and impacts of localized processes on primary productivity.

The authors thank the US Embassy in Dhaka, Bangladesh, and the US Department of State for sponsoring undergraduate exchange program at Florida Gulf Coast University to carry out this research in Lab of Jose.