Impact of ocean warming on net primary production in the northern Indian Ocean: role of aerosols and freshening of surface ocean

Warming due to climate change stratifies the upper ocean and reduces nutrient input to the photic zone resulting in a decline in net primary production (NPP). On the other hand, climate change increases both anthropogenic aerosol input into the atmosphere and the river discharge due to the melting of glaciers on land resulting in enhanced nutrient inputs to the surface ocean and NPP. To examine the balance between these two processes, spatial and temporal variations in the rate of warming, NPP, aerosol optical depth (AOD), and sea surface salinity (SSS) were studied between 2001 and 2020 in the northern Indian Ocean. Strong heterogeneity in the warming of the sea surface was observed in the northern Indian Ocean with significant warming in the south of 12°N. Insignificant trends in warming were observed in the northern Arabian Sea (AS), north of 12°N, during winter and fall, and western Bay of Bengal (BoB) during winter, spring, and fall associated with higher levels of anthropogenic AOD (AAOD) due to a reduction in incoming solar radiation. The decline in NPP was observed in the south of 12°N in both AS and BoB and correlated inversely with SST suggesting that a weak supply of nutrients due to upper ocean stratification controlled NPP. Despite warming, the weak trends in NPP in the north of 12°N were associated with higher AAOD levels and their rate of increase suggesting that the deposition of nutrients from the aerosols seems to be compensating for declining trends due to warming. The decrease in sea surface salinity confirmed an increase in river discharge, and nutrient supply led to weak NPP trends in the northern BoB. This study suggests that the enhanced atmospheric aerosols and river discharge played a significant role in warming and changes in NPP in the northern Indian Ocean, and these parameters must be included in the ocean biogeochemical models for accurate prediction of possible changes in the upper ocean biogeochemistry in the future due to climate change.

Impact of atmospheric dry deposition of nutrients on phytoplankton pigment composition and primary production in the coastal Bay of Bengal

Atmospheric deposition of pollutants decreases pH and increases the nutrient concentration in the surface water. To examine its impact on coastal phytoplankton composition and primary production, monthly atmospheric aerosol samples were mixed with coastal waters in the microcosm experiments. These experiments suggested that the biomass of Bacillariophyceae, Dinophyceae and Chlorophyceae were increased and primary production of the coastal waters increased by 3 to 19% due to the addition of aeolian nutrients. The increase in primary production displayed significant relation with a concentration of sulphate and nitrate in the atmospheric aerosols suggesting that both decreases in pH and fertilization enhanced primary production. The impact of acidification on primary production was found to be 22%, whereas 78% was contributed by the nutrient increase. The atmospheric pollution is increasing rapidly over the northern Indian Ocean since past two decades due to rapid industrialization. Hence, it is suggested that the impact of atmospheric pollution on the coastal ecosystem must be included in the numerical models to predict possible changes in the coastal ecosystem due to climate change.

Impact of atmospheric anthropogenic nitrogen on new production in the northern Indian Ocean: constrained based on satellite aerosol optical depth and particulate nitrogen levels

Aerosols are one of the significant external sources of soluble reactive nitrogen to the surface ocean and their deposition affects the primary productivity. Owing to rapid industrialization over South and Southeast Asia, an increasing trend in atmospheric pollutants was observed over the northern Indian Ocean (NIO). To assess the contribution of the aeolian supply of inorganic nitrogen to the NIO, the available compositional data of marine aerosols collected over this basin between 2001 and 2020 were compiled. Based on the observed relationship of mass load, and particulate nitrate and ammonium concentrations with the corresponding satellite-derived anthropogenic aerosol optical depth (AAOD), the temporal, spatial, and long-term variabilities were derived for the past two decades. In particular, high aerosol mass load, nitrate and ammonium levels were observed in the coastal aerosols of peninsular India during fall and winter and they were low in summer. The atmospheric input of inorganic nitrogen to the Arabian Sea is higher (AS; 1.7 TgN per year) compared to that of the Bay of Bengal (BoB; 0.9 TgN per year) and accounts for ∼30% of the total external sources of nitrogen to the NIO. The new production, supported by external sources of nitrogen, contributes to ∼23 and 53% of export production to the oxygen minimum zone (OMZ) in the AS and BoB respectively. A significant rate of increase in the aerosol mass load (0.05-1.67 μg per m³ per year), and nitrate (0.003-0.04 μg per m³ per year) and ammonium (0.006-0.11 μg per m³ per year) concentrations was observed between 2001 and 2020, likely because of the increased emission of anthropogenic pollutants over South and Southeast Asia and their subsequent long-range atmospheric transport to the NIO. Overall, these results suggest that an enhanced contribution of atmospheric nitrogen may potentially increase (1) the N/P ratio of the surface ocean that impacts phytoplankton composition, (2) export production to the OMZ leads to intensification, and (3) sequestration of atmospheric CO₂. A decrease in primary production due to global warming is reported due to a decrease in vertical nutrient supply; however, the increase in atmospheric deposition of nutrients may compensate for this. Therefore, ocean models must be coupled with atmospheric models to better constrain the oceanic response to climate change in the NIO.

Highly Diverse Synechococcus Pigment Types in the Eastern Indian Ocean

Marine picocyanobacteria Synechococcus exhibit highly diverse pigment types (PTs) and hence possess great advantage to utilize different spectrum of light effectively and to occupy a wide range of light niches. In this study, we explored the diversity of Synechococcus PTs in the eastern Indian Ocean (EIO), surface water of Strait of Malacca (SSM), and coastal waters of Sri Lanka (SSL). All the detected PTs were phycourobilin (PUB) containing PT 3 and showed distinct distribution patterns. Low PUB PT 3a and partial chromatic acclimater PT 3eA dominated in coastal and shallow waters (SSM and SSL). In contrast, high PUB and chromatic acclimaters PT 3dA and PT 3c/3dB were mainly distributed in open ocean (EIO). PT 3dA and PT 3c/3dB occurred at similar depths of the lower euphotic layers but showed distinct distribution pattern that are partially exclusive, indicating that they compete with each other for the same light niche. Interestingly, the newly described PT 3f was detected with high relative abundances at all stations and particularly dominated in the upper euphotic layer in EIO, which was confirmed with PT-specific quantitative polymerase chain reaction (qPCR). The relative abundance of PT 3f was negatively correlated with nutrient level, implying that PT 3f is adapted to oligotrophic waters. Pronounced niche partition of different PTs was observed in the upper and lower layers of euphotic zone in EIO and SSM/SSL. Light, nutrients, and strong stratification may play important roles in the niche partition of different PTs. Further analysis about ecologically significant taxonomic units revealed high diversity within each PT at different locations, which provided insights for understanding specific PT with wide range of niches.

Monitoring bio-optical response of coastal waters surrounding the Indian subcontinent to atmospheric dust deposition using satellite data

The paper investigates the impact of atmospheric dust deposition on ocean biological productivity in association with oceanic supply of nutrients over specific regions of the Arabian Sea (20°N, 69°E) and the Bay of Bengal (20°N, 87°E) during wintertime (November-March) from the year 2012 to 2017 using satellite-based observations. During winter, selected regions are characterized by higher Chlorophyll-a (Chl-a) and major oceanic vertical supply of nutrients. Moderate Resolution Imaging Spectroradiometer onboard Aqua space-platform is used to obtain Chl-a and aerosol optical depth (AOD) data. Blended Chl-a daily product from various satellite sensors is also used. There are a total of nine cases (seven cases of the Arabian Sea region and two cases of the Bay of Bengal region) where episodic Chl-a enhancements following high AOD values are observed. Chl-a maxima lag behind AOD maxima by 1 to 4 days. Modern-Era Retrospective analysis for Research and Applications (Version-2) is used for AOD and dust deposition flux estimation. Estimated dust deposition flux ranges between 0.44 and 27.68 mg m^-2 day^-1.

Impact of dust storm on phytoplankton bloom over the Arabian Sea: a case study during March 2012

Dust storms affect the primary productivity of the ocean by providing necessary micronutrients to the surface layer. One such dust storm during March 2012 led to a substantial reduction in visibility and enhancement in aerosol optical depth (AOD) up to ~ 0.8 (AOD increased from 0.1 to 0.9) over the Arabian Sea. We explored the possible effects and mechanisms through which this particular dust storm could impact the ocean's primary productivity (phytoplankton concentration), using satellite-borne remote sensors and reanalysis model data (2003-2016). The climatological analyses revealed anomalous March 2012 in terms of dust deposition and enhancement in phytoplankton concentration in the month of March during 2003-2016 over this region. The studied dust storm accounts for increase in the daily average surface dust deposition rate from ~ 3 to ~53 mg m^-2 day^-1, which is followed by a significant enhancement in the chlorophyll-a (Chl_a) concentration (~ 2 to ~9 mg m^-3). We show strong association between a dust storm and an event of anomalously high biological production (with a 4-day forward lag) in the Arabian Sea. We suggest that the increase in biological production results from the superposition of two complementary processes (deposition of atmospheric nutrients and deepening of the mixed layer due to dust-induced sea surface temperature cooling) that enhance nutrient availability in the euphotic layer.

Seasonal changes in carbonaceous aerosols over a tropical coastal location in response to meteorological processes

Near-surface atmospheric aerosols (PM₁₀) collected from a tropical coastal location in south-west peninsular Indian region for a duration of 6 years (2012-18) (N = 461) were analysed for carbonaceous aerosol components, the less studied aerosol species. Organic carbon (OC), its water soluble-insoluble (WSOC and WIOC) components, primary-secondary (POC and SOC) fractions and elemental carbon (EC) were examined for understanding the annual, seasonal, day-night variations in abundance pattern along with associated physical and meteorological processes. Total carbonaceous aerosols accounting for 36% of the collected aerosol mass with 31.5% organic matter (OM) and 4.5% EC respectively, exhibited consistent seasonal pattern throughout the study period with high concentration during winter followed by post-monsoon, pre-monsoon and monsoon. Delineation of marine and continental components of carbonaceous species based on their relative dominance during different air-mass periods, shows that while marine aerosols were a combination of natural sources comprising of volatile, semi-volatile species and secondary organics (from marine VOC precursors); the continental aerosols were composed of anthropogenic combustion sources (fossil fuel, biomass emissions etc). Based on the measurements of OC and EC during 2005-09 and 2012-18, their long term trends (for more than a decade) were investigated. Although OC showed an increasing tendency, EC exhibited a decrease with the total carbonaceous aerosols exhibiting a gradual decreasing trend over the years, indicating that they do not strictly reverberate the reported increasing trend observed over north-central parts of India. This can be presumed to be due to the reduced anthropogenic inputs over the location owing to the control measures and policies. The strong convective activity and large scale monsoon phenomena also helps in the effective dispersion of pollutants. Making use of comprehensive measurement of carbonaceous aerosols and the previous measurements of other aerosol components, an improved chemical composition model is presented.

Atmospheric iron deposition: global distribution, variability, and human perturbations

Atmospheric inputs of iron to the open ocean are hypothesized to modulate ocean biogeochemistry. This review presents an integration of available observations of atmospheric iron and iron deposition, and also covers bioavailable iron distributions. Methods for estimating temporal variability in ocean deposition over the recent past are reviewed. Desert dust iron is estimated to represent 95% of the global atmospheric iron cycle, and combustion sources of iron are responsible for the remaining 5%. Humans may be significantly perturbing desert dust (up to 50%). The sources of bioavailable iron are less well understood than those of iron, partly because we do not know what speciation of the iron is bioavailable. Bioavailable iron can derive from atmospheric processing of relatively insoluble desert dust iron or from direct emissions of soluble iron from combustion sources. These results imply that humans could be substantially impacting iron and bioavailable iron deposition to ocean regions, but there are large uncertainties in our understanding.