Low primary production in the Bay of Bengal (BoB) could not be explained as this region receives nutrients from atmospheric deposition, rivers, eddies, and mixing due to depression/cyclones. In addition to nutrients, BoB also receives significant amount of suspended particulate matter (SPM) from the major rivers and their concentrations are in an order of magnitude higher than elsewhere in the open ocean region. Here we show that the removal of phosphorus (P) through adsorption on SPM may be a potential mechanism to decrease in primary production in the northern BoB. Significant fraction of P removal (5–50% of total dissolved inorganic phosphate) in association with SPM was observed in the BoB. The magnitude of removal of P through SPM is linearly related with dissolved inorganic phosphate (DIP), particulate organic carbon (POC) in the water column suggesting that P is removed in association with organic matter. The fraction of P removed from that of DIP showed inverse relation with salinity, and linear relation with SPM concentration suggesting that SPM brought by river discharge removed P from the water column. The P removed by SPM in the mixed layer showed inverse relation with mixed layer integrated primary production in the open sea region but its impact is negligible in the coastal waters. The laboratory experiment by measuring primary production in the presence of different quantities of SPM concentrations confirmed decrease in primary production due to removal of phosphate in the BoB.

$\bf{Highlights}$

$\bullet$ Significant fraction of phosphate adsorbs on to the suspended particles and it is bio-non-available.

$\bullet$ Phosphate severely controls primary production in the northern Bay of Bengal.

$\bullet$ The removal of phosphate is more in the coastal waters, it does not control primary production due to higher input than removal.

$\bullet$ High N/P ratios is caused by phosphate removed on association with suspended matter.

The tropical Indian Ocean is a net sink for the carbon dioxide (CO$_{2}$) in the atmosphere and phytoplankton production plays a crucial role in CO$_{2}$ fixation and determines the direction of CO$_{2}$ flux at the sea-to-air interface. In order to assess the influence of phytoplankton composition and primary production on pCO$_{2}$ levels in the Indian Ocean, sampling was conducted at 25 stations during the spring intermonsoon period under the auspices of the Indian GEOTRACES program. The pCO$_{2}$ was significantly correlated with salinity due to the discharge of low pCO$_{2}$ water by major rivers to the Bay of Bengal (BoB). The stronger negative correlation observed between pCO$_{2}$ and major phytoplankton marker pigments, net primary production and oxygen saturation levels suggesting significant influence of biological processes on pCO$_{2}$ levels in the Indian Ocean. This study indicates that pCO$_{2}$ levels are strongly modulated by biological processes than hitherto hypothesized as solubility pump in the Indian Ocean.

$\bf{Highlights}$

$\bullet$ Surface pCO$_{2}$ levels are undersaturated in the Indian Ocean compared to atmosphere, except Arabian Sea.

$\bullet$ Oligotrophic conditions prevailed in the entire tropical Indian Ocean.

$\bullet$ Picophytoplankton (cyanobacteria) is the dominant phytoplankton in the Indian Ocean.

$\bullet$ Significant relation between phytoplankton groups and pCO$_{2}$ indicates strong biological control on surface pCO$_{2}$.

Knowledge of relations among ocean biogeochemical and cloud properties will help to plan experiments necessary to understand the mechanisms and processes underlying the links between ocean and atmosphere interactions. Here, we explored the associations between ocean biogeochemical and cloud properties in a region that seasonally experiences polluted and pristine atmospheric conditions in winter and summer, respectively. The implications of ocean surface chlorophyll-a and biogeochemical fluxes (sea salt, dimethyl sulphide and organic fraction in sea spray) to cloud properties (cloud effective radii (R$_{e}$), cloud optical thickness, and cloud droplet number concentration(CDNC)) were studied using MODIS (Terra, Collection 6, L3) monthly data from 2001 to 2015 along with reanalysis information. We have adopted a climatological averaging approach in time (monthly, seasonal and annual) and space (coastal, open and total (basin) Arabian Sea). This approach was used to minimize incompatibility, if any, between ocean and cloud properties arising from spatio-temporal lags due to different dynamics in the respective boundary layers. The trends in monthly means suggest decreases in chlorophyll-a and CDNC, while R$_{e}$ increased over the Arabian Sea basin during 2001–2015. Variability at the basin scale (expressed as standard deviation in each month, SD) exceeded mean values of respective months for chlorophyll-a, whereas it was nearly half of the mean values for CDNC. An increase in R$_{e}$ seems facilitated more during warmer 2011–2015 than in the 2001–2010 period, which coincided with the decrease in CDNC. Fifteen-year monthly mean climatologies suggest considerable associations among ocean biogeochemical indices and cloud properties, which is more conspicuous during summer monsoon. Increase in sea salt flux appears to account for the higher values of R$_{e}$ in June–July over the basin due to strong monsoon wind. Inverse relations between chlorophyll-a and R$_{e}$ are indicative of smaller droplets that resulted from new particles formed from and or facilitated by marine biogeochemical emissions. Decline in new particle production due to decrease in surface chlorophyll-a and the growth of particles facilitated by increase in warming, seem responsible for increase in R$_{e}$ and decrease in CDNC from 2001 to 2015. Using chlorophyll-a as the main proxy for ocean biogeochemical indices, we demonstrated that connections between ocean biogeochemistry and clouds are sustained in both small and large scales in space and time over the Arabian Sea.

Polluted aerosol transport from South Asia containing oxides of nitrogen and sulphur and their deposition on surface may acidify coastal waters. To test this hypothesis, we have conducted experiments involving (a) variability in aerosol composition at a coastal station (Visakhapatnam, central east coast of India)during 2013–2014 (monthly) and 2015–2016 (weekly observations), and (b) simultaneous observations of aerosols over land and adjacent coastal water in winter of 2013 and 2016. The annual average composition of aerosols during this study was dominated by SO$_4$$^{-2}$(48%) followed by NO$-3$$^-$ (15%). Sulphate (${\sim}$12 ${\mu}$g m$^{-3}$) exhibited high concentrations in Fall Intermonsoon (FIM) and winter monsoon (WM), whereas higher nitrate (3–4.5 ${\mu}$g m$^{-3}$) concentrations were observed during summer monsoon (SM) and FIM. The mean [NO$_3$$^-$/SO$_4$$^{2-}$] ratio of 0.32 suggests that atmospheric aerosol over the study region is contributed by transportation of fossil fuel burning emissions from the subcontinent by high-altitude large-scale wind circulation. The concentrations of NO$_2$ and SO$_2$ varied from 17.2 to 34.3 and 11.5 to 16.4 ${\mu}$g m$^{-3}$, respectively with mean [SO$_2$/NO$_2$] ratio of 0.57 and [SO$_4$$^{2-}$/SO$_4$$^{2-}$+SO$_2$] ratio of 0.33 indicates coalburning (power plants/industries) and fossil fuel burning may be the major source of atmospheric dust in the study region. Comparison of total cations and anion concentrations indicate aerosols are acidic in FIM and SM and mixed nature (acidic/basic) in WM but near neutral in spring Intermonsoon (SIM).Simultaneous experiments revealed that about 5–45% of the atmospheric aerosols were deposited within 10 km from the coast. The in vitro experiments indicated that the deposition of atmospheric aerosols resulted in a measurable decrease in pH of surface seawater and displayed significant relationship betweendecrease in pH and concentration of NO$_3$$^-$/SO$_4$$^{2-}$, but it was weaker with NO$_2$/SO$_2$ suggesting former ions contribute significantly in lowering pH of coastal waters than latter. The impact of decrease in pH on acid–base equilibrium, carbonate chemistry and gas exchanges need to be assessed.

Primary production is reported to be a fraction of heterotrophic carbon demand in the Bay of Bengal (BoB), and it is attributed to the unavailability of inorganic nutrients and faster sinking of organic matter in association with mineral particles. The contribution of nutrients through external sources to total primary production is low (${\le}$5%), suggesting internal cycling of nutrients is important in the BoB. Organic nutrients support primary production in the absence of inorganic nutrients in the BoB. It was noticed that about 45% of particulate organic carbon (POC) production is exudated as dissolved organic carbon (DOC). Therefore, the total organic carbon production is revised to twice that of the earlier estimate and it is sufficient to support heterotrophic carbon demand. Balance among the ventilation of oxygen by anticyclonic eddies, strengthening due to cyclonic eddies and salinity stratification controls the oxygen levels in the OMZ than hitherto hypothesized as ballasting of organic matter. The stable isotopic composition of nitrogen in nitrate and particulate organic nitrogen (PON) does not evidence a significant contribution of anthropogenic nitrogen in the BoB. This negates the hypothesis that anthropogenic inputs modify the biogeochemistry of BoB. The deposition of anthropogenic aerosols decreases the pH of surface waters in the western BoB, whereas a decrease in salinity due to an increase in freshwater flux due to warming of the Himalayan glacier may increase pH and decrease pCO$_2$ levels. As a result, BoB is turning into more sink for atmospheric CO$_2$, which is contrasting to that of elsewhere in the global ocean.

Deposition of atmospheric dust is reported to acidify surface waters in the northern Bay of Bengal (BoB). To examine the spatial variability in content and composition of total suspended matter (TSP), aerosol samples were collected at four locations (Damra, Chilika, Vizag and Chennai) along the east coast of India in the marine atmospheric boundary layer (MABL) to evaluate its impact on pH of surface waters due to deposition on surface waters using microcosm experiments. The concentration of total suspended matter (TSP) and [SO$_4$$^{2–}$ + NO$_3$$^–$] increased from southern (146 and 6.16 ${\mu}$g m$^{–3}$, respectively) to northern coastal BoB (197 and 34.57 ${\mu}$g m$^{–3}$, respectively) due to the influence of pollutants from Indo-Gangetic Plain (IGP) in the north and dominant marine sources in the southern coastal BoB. The ionic balance in aerosols suggested that acidification potential (neutralization potential) increased (decreased) from southern to northern BoB. The dissolution of aerosols in surface seawater lowered pH by 0.018 ± 0.002 to 0.135 ± 0.005 in the coastal BoB with a higher decrease in the north than south. Our study suggests that aerosol dissolution in seawater results in ocean acidification in proportion to acidic anions (e.g., SO$_4$$^{2–}$, NO$_3$$^–$). In addition, organic acids, such as carboxylic acids, aromatic (Benzoic acid) and hydroxy acids(Lactic and glycolic acids) also contribute significantly to ocean acidification and their contribution needs further evaluation.