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 SO42− (48%) followed by NO3− (15%). Sulphate (~12 μg m−3) exhibited high concentrations in Fall Intermonsoon (FIM) and winter monsoon (WM), whereas higher nitrate (3–4.5 μg m−3) concentrations were observed during summer monsoon (SM) and FIM. The mean [NO3−/SO42−] 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 NO2 and SO2 varied from 17.2 to 34.3 and 11.5 to 16.4 µg m−3, respectively with mean [SO2/NO2] ratio of 0.57 and [SO42−/SO42−+SO2] ratio of 0.33 indicates coal burning (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 between decrease in pH and concentration of NO3−/SO42−, but it was weaker with NO2/SO2 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.

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 (Re), 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 Re 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 Re 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 Re in June–July over the basin due to strong monsoon wind. Inverse relations between chlorophyll-a and Re 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 Re 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.

Knowledge on the variability in quantity and compositions of various size groups of aerosols is important to understand their sources and their role in biogeochemical and climate processes. Here, we studied total suspended particles (TSP), PM10 and PM2.5 for their quantitative and water soluble compositional (F−, Cl−, SO42−, NO3−, NH4+, Na+, K+, Ca2+, and Mg2+) distributions, and to understand their nature and potential sources at Goa and Visakhapatnam on the west and east coasts, respectively, of India. While the mean concentrations of TSP were found to be 117 ± 44 and 85 ± 51 μg/m3 its maximal levels occurred in spring intermonsoon (SIM; 141 ± 52) and winter monsoon (WM; 155 ± 145 μg/m3) seasons at Goa and Visakhapatnam, respectively. PM10 and PM2.5 exhibited higher ranges at Visakhapatnam than Goa. The increase in PM2.5 abundance from WM to SIM at Visakhapatnam seems to occur in coincidence with decrease in TSP favored by topography and ambient meteorological conditions. Locally released and seasonally transported (from land and sea) constituents contributed to the observed variability in aerosol compositions. Sulphate dominated the aerosol composition at both Goa (57–64%) and Visakhapatnam (43–55%) followed by NO3− (5–16% and 6–18%, respectively) where the former component was higher in PM10 and PM2.5. The NO3− was more in TSP. Relations between SO42− and NH4+ suggested possible presence of NH4HSO4. Examination of ionic ratios and balance suggested near neutrality in PM10 and PM2.5 while TSP was acidic at the both locations. Notable relations between Ca2+ and NO3−, particularly in PM10 at Goa, indicated their release from mining related activities.

While the disciplines of oceanography and limnology often operate in isolate, freshwater, estuarine, and marine ecosystems are intricately linked. The emphasis of this special issue for Limnology and Oceanography is on the aquatic continuum and the connectivity between aquatic ecosystems from headwater streams and inland waters, to coastal and marine systems. Changes in the transport and transformation of elements as well as ecological functions occur along this aquatic continuum. Assemblages of organisms change in a way that reflects the ecological and biogeochemical conditions of the aquatic gradient. Here, we highlight research progress in limnology and oceanography across the aquatic continuum and at the interfaces of headwaters to oceans. Contributions explored nutrient and carbon dynamics which included release, transportation, transformation, and stoichiometry from freshwaters to marine. The special issue also explored food web continua, including functional changes, biodiversity gradients, and photosynthesis and respiration comparisons among ecosystems at different points in the continuum. Rapid improvements in biomolecular techniques, use of long‐term datasets, applications of novel statistical methods, and improved upscaling methods can transform the way aquatic scientists are describing biological organisms and communities from freshwaters to oceans. One important conclusion is the recognition that anthropogenic activities such as invasive species and nutrient pollution trigger challenge the current concepts of aquatic continua including the river continuum concept, the land to ocean continuum, river to estuary systems, and the submarine groundwater discharge. Both limnologists and oceanographers have much to gain from exchanging information with one another, especially in light of global change.

The aerosols from continents contain relatively higher amounts of inorganic nutrients than those of marine origin and can make a notable contribution to the coastal biological productivity. To test this hypothesis, the composition of aerosols over the city of Visakhapatnam (central east coast of India) were studied when continental flow was dominant and its impact on phytoplankton biomass was estimated through microcosm experiments between September 2013 and November 2014. Higher nitrate (NO3−) and ammonium (NH4+) concentrations were observed in the aerosols collected in January while higher phosphate (PO43−) was observed in September. Simultaneous observations of aerosols over the city and neighboring coastal waters revealed that the concentrations of nitrate in ambient aerosols ranged from 0.56 to 1.89μgm−3 and 0.09 to 0.86μgm−3, respectively. Our results suggest that 52–89% of city's aerosols borne nitrogen deposited over waters within 10km from the coastline. Microcosm experiments were conducted by spiking the surface water samples, collected from the coastal Bay of Bengal (BoB), with the dust borne nutrients. Upon spiking, dissolved inorganic nitrogen (NO3−+NH4+) increased from 0.3 to 11.7μmolL−1 and the N:P ratio increased from 2 to 97. This led to enhanced phytoplankton biomass (1.5 to 4 times) upon spiking. The increase in phytoplankton biomass was linearly related to dissolved N:P ratios in water as aerosol deposition increased the N:P ratios in the microcosms, leading to phytoplankton growth. Though aerosols did not contribute to bioavailable silicate, our microcosm experiments showed linear relationships between ambient silicate phytoplankton biomass, and concentrations of Fucoxanthin (a marker pigment for diatoms). This indicates that the availability of silicate in coastal waters facilitated dominant diatom growth in the presence of higher N:P ratios due to aerosol deposition. The deposition of soluble aerosol nitrogen appears to support ~3 to 33% of the biological production in the coastal waters off Visakhapatnam with higher contribution in winter (~33%) than in summer (10%). This study suggests that atmospheric deposition of nutrients enhances phytoplankton biomass in waters along the central east coast of India during the winter monsoon period, in particular, supporting the hypothesis stated above.

A study was conducted to understand the intra- and inter-seasonal variability of dissolved oxygen and nutrients in a tropical monsoon estuary (Zuari in Goa, India). We adopted a dual sampling approach with (a) daily or alternate day sampling at a fixed location in the mid-estuarine zone and (b) longitudinal transect sampling from freshwater end to mouth during spring and neap tides of each month for about a year. Multivariate statistical analyses of oxygen and nutrients were carried out to evaluate the hypotheses: (i) biogeochemical processes chiefly regulate their variability and (ii) anthropogenic inputs lead to material accumulation in the estuary. Multivariate statistical analyses helped identify the controlling factors of the oxygen and nutrient variability. Our results significantly revealed (i) physical forcings (freshwater discharge and tidal circulation, these also facilitate sedimentary releases) are more important than biogeochemical processes in determining oxygen and nutrient variability in the water column and (ii) the monsoon driven regular annual flushing makes the system resilient to human interference as the Zuari estuary returns to normalcy by postmonsoon every year. Our study identified the significance of subsurface discharges in transporting mining effluents from the river basin. Results also suggest that extrapolation of controlling factors of biogeochemical variables at a fixed location to the entire estuary is untenable since the relative dominance of forcings vary in time and space in the estuary.

Processes in natural waters are highly variable in time and space. Although changes are expected in short-time scales, how short one could get to measure reliably is subjective to sampling strategies and methodologies. Here, we show that sub-hourly changes in surface waters dissolved oxygen, nutrients, and pigments are measurable and significant in an estuarine system. Tidal circulation has been found to strongly influence the observed changes and has implications to material fluxes in and out of estuaries.

The role of microzooplankton (MZP) in the pelagic trophodynamics is highly significant, but the responses of marine MZP to increasing CO2 levels are rather poorly understood. Hence the present study was undertaken to understand the responses of marine plankton to increasing CO2 concentrations. Natural water samples from the coastal Bay of Bengal were incubated under the ambient condition and high CO2 levels (703–711 μatm) for 5 days in May and June 2010. A significant negative correlation was obtained between phytoplankton and MZP abundance which indicated that phytoplankton community structure can considerably be controlled by MZP in this region. The average relative abundances of tintinnids under elevated CO2 levels were found to be significantly higher (68.65 ± 5.63% in May; 85.46 ± 9.56% in June) than observed in the ambient condition (35.68 ± 6.83% in May; 79 ± 5.36% in June). The observed dominance of small chain forming diatom species probably played a crucial role as they can be potentially grazed by tintinnids. This fact was strengthened by the observed high negative correlations between the relative abundance of major phytoplankton and tintinnids. Moreover, particulate organic carbon and total bacterial counts were also enhanced under elevated CO2 level and can serve as additional food source for ciliates. The observed responses of tintinnids to increasing CO2 might have multiple impacts on the energy transfer, nutrient and carbon cycling in the coastal water. The duration of the present study was relatively short and therefore further investigation on longer time scale needs to be done and might give us a better insight about phytoplankton and MZP species succession under elevated CO2 level.

The Bay of Bengal is considered to be a low productive region compared to the Arabian Sea based on conventional seasonal observations. Such seasonal observations are not representative of a calendar year since the conventional approach might miss episodic high productive events associated with extreme atmospheric processes. We examined here the influence of extreme atmospheric events, such as heavy rainfall and cyclone Sidr , on phytoplankton biomass in the western Bay of Bengal using both in situ time-series observations and satellite derived Chlorophyll a (Chl a ) and sea surface temperature (SST). Supply of nutrients through the runoff driven by episodic heavy rainfall (234 mm) on 4–5 October 2007 caused an increase in Chl a concentration by four times than the previous in the coastal Bay was observed within two weeks. Similar increase in Chl a , by 3 to 10 times, was observed on the right side of the cyclone Sidr track in the central Bay of Bengal after the cyclone Sidr . These two episodic events caused phytoplankton blooms in the western Bay of Bengal which enhanced ~40% of fishery production during October–December 2007 compared to that in the same period in 2006.

Experiments conducted aboard the R/V Hakuho Maru (Cruise KH-02-3) in the autumn of 2002 were designed to study the pathways of atmospheric aerosol transport, selected trace elements and major ions composition of particles and quantify their dry deposition over the East China Sea. We determined the concentrations of trace elements Al, Mn, Pb, Se, and Zn and major ions Na+, SO42− and NO3− in aerosol particles. Aerosols were continuously collected in fine and coarse size fractions with a cut off at 2.5 μm. Particle number densities (volumes) were determined in four size ranges between 0.1 and <0.5 μm. High Al concentrations in the coarse mode fractions suggest continental origin of these aerosols. Weather charts revealed conducive meteorological conditions for northeastward air mass transports from the Asian continent. Back trajectory analysis results corroborated with the meteorological observations. Nss-SO42−, Mn, Pb, Se and Zn were more abundant in the fine than coarse mode fraction, but NO3− was associated with the latter. About three-fold higher Mn/Al ratio in fine mode than in coarse aerosols (0.042 vs. 0.015, respectively) and a strong correlation between non-crustal Mn and nss-SO42− suggest that fine Mn originates from industrial sources. Similarly, non-crustal Pb and Zn in the fine fraction were again significantly correlated with nss-SO42− indicating major contributions from anthropogenic activities. Our results highlight the fact that atmospheric pollutants from Asia, including heavy metals and major ions, are carried to the East China Sea even during non-dusty periods. It is necessary to obtain the atmospheric deposition fluxes, wet and dry, throughout the year both by periodic and continuous atmospheric measurements in order to find the biogeochemical significance of Asian aerosols to the East China Sea and North Pacific Ocean.

Dimethyl sulphide (DMS) is a biogenic gas of climatic significance on which limited information is available from the Indian Ocean. To fill this gap, we collected data on DMS and total dimethylsulphoniopropionate (DMSP t ) by participating in a dozen cruises. Here, we discuss the variability in DMS and DMSP t in the north and central Indian Ocean in terms of their spatial and temporal variation. DMS and DMSP t exhibited significant spatial and temporal variability. Apart from the concentration gradients in DMS within the Arabian Sea, Bay of Bengal and Central Indian Ocean basins, differences in average abundances were conspicuous between these basins. The Arabian Sea contained more DMS (mixed layer average was 7.8 nM) followed by the Bay of Bengal (2.8 nM) and the Central Indian Ocean (2.7 nM). The highest concentrations of DMS and DMSP t (525 nM and 916 nM, respectively) were found in upwelling regimes along the west coast of India during the Southwest monsoon and fall intermonsoon seasons. Average surface DMS was the highest in the Arabian Sea. On the other hand observed sea-to-air fluxes of DMS were higher in the Bay of Bengal due to the prevalence of turbulent conditions. In the Arabian Sea wind speeds were low and hence the sea-to-air fluxes. The total diffusive flux of DMS from the study area to atmosphere is estimated to be about 1.02 × 10 12 g S y −1 , which contributes to 4.1–6.3% of the global DMS emission

Intense upwelling during summer and convection in winter are believed to drive higher biological productivity in the Arabian Sea than in the Bay of Bengal. Although the Arabian Sea receives substantial atmospheric deposition of dust aerosols, its role in biological activity is unknown. We have analyzed chlorophyll‐a (SeaWiFS), absorbing aerosol index (TOMS), surface winds (NCEP), and modeled dust deposition and SST (OI) data during two distinct seasons June–August (JJA, summer months) and October–December (OND, winter months) for the period 1997–2004. Climatologies of physicochemical properties have been developed from World Ocean Atlas 2001 (WOA01). Our results suggest that despite the strong vertical supply of nutrients in the western and central Arabian Sea regions, maximal chlorophyll‐a was limited to the former region in both JJA and OND periods, suggesting the importance of atmospherically transported substances in determining chlorophyll abundance in the North Indian Ocean. Time‐averages (1997–2004) revealed chlorophyll abundances in northwestern regions are larger than in other regions of the respective basins. The NW regions of the Arabian Sea and the Bay of Bengal have exhibited contrasting chlorophyll distribution patterns during El Niño years (1997–1998 and 2002–2003; positive SST anomalies); decreased and increased chlorophyll contents in respective regions. Following the passage of tropical cyclones, SeaWiFS records depicted large areas in the Arabian Sea to experience intensified chlorophyll production with strong wind speeds of 55–65 knots whereas its enhanced production occurred only in small patches even under the influence of Orissa Super Cyclone of October 1999 (wind speed up to 140 knots) due to strong stratification.

Characteristics of trace gases (O3, CO, CO2, CH4 and N2O) and aerosols (particle size of 2.5 micron) were studied over the Arabian Sea, equatorial Indian Ocean and southwest part of the Bay of Bengal during the monsoon transition period (October–November, 2004). Flow of pollutants is expected from south and southeast Asia during the monsoonal transition period due to the patterns of wind flow which are different from the monsoon period. This is the first detailed report on aerosols and trace gases during the sampled period as the earlier Bay of Bengal Experiment (BOBMEX), Arabian Sea Monsoon Experiment (ARMEX) and Indian Ocean Experiments (INDOEX) were during monsoon seasons. The significant observations during the transition period include: (i) low ozone concentration of the order of 5 ppbv around the equator, (ii) high concentrations of CO2, CH4 and N2O and (iii) variations in PM2.5 of 5–20μg/m3.

The export of carbon through the biological pump from the surface to the deep ocean has a direct influence on the removal of CO 2 from the atmosphere. This is because the carbon is sequestered for only a few days to months in the surface while the carbon removed from the surface to deep waters takes hundreds of years to re-enter the atmosphere. The highest dissolved inorganic carbon (DIC) is expected in the deep waters of the North Pacific due to longer age of waters. On contrary, the higher deep water DIC is found in the northern Indian Ocean than elsewhere in the World Oceans. The sinking fluxes of particulate organic (POC) and inorganic carbon (CaCO 3 ) are found to be the highest in the northern Indian Ocean. The rates of bacterial respiration, organic carbon regeneration and inorganic carbon dissolution are also found to be the highest in the northern Indian Ocean than elsewhere. A most efficient biological pump appears to be operating in the northern Indian Ocean that transports surface-derived organic/inorganic carbon to deeper layers where it is converted and stored for longer times in dissolved inorganic form.

Data on the distribution of dimethylsulphide (DMS) and dimethylsulphoniopropionate (DMSP) in relation to phytoplankton abundance in different oceanic environments is important to understand the biogeochemistry of DMS, which plays an important role in the radiation balance of the earth. During the summer monsoon of 2001 measurements were made for DMS and DMSPt (total DMSP) together with related biological parameters in the Bay of Bengal. Both DMS and DMSPt were restricted to the upper 40 m of the water column. Diatoms accounted for more than 95% of the phytoplankton and were the major contributors to the DMS and DMSPt pool. The mean concentration of DMS in the upper 40 m was observed to be around 1.8+/-1.9 nM in the study area, while DMSPt concentrations varied between 0.7 nM and 40.2 nM with a mean of 10.4+/-8.2 nM. The observed lower DMSPt in the northern Bay in spite of higher mean primary productivity, chlorophyll a and phytoplankton cell counts seemed to result from grazing. Though salinity divides the Bay into different biogeochemical provinces there is no relation between salinity and DMS or DMSPt. On the other hand DMS was linearly related to chlorophyll a:phaeopigments ratio. The results suggest the need for deeper insight into the role of diatoms in the biogeochemical cycling of DMS.

South Asia includes the Indian subcontinent (India, Pakistan, Bangladesh and Nepal), as well as Sri Lanka,Maldives and Mauritius Fig. 3.1The region is one of the most densely populated in the world, with present population densities of 100–500 persons km-2. Although total land area comprises only about 3% of the world’s land masses, the 1990 population was 21.3% of the global total and by 2025 is expected to rise to -24%. The urban population in the region is increasing rapidly. In 1980 the percentage of the population living in urban areas was 23% for India, 28% for Pakistan and 11% for Bangladesh. By 2000 estimates had increased to 38%, 37% and 21% respectively. Several of the world’s most polluted cities are found in South Asia. Calcutta, Delhi, Mumbai, Karachi and Dhaka are examples of megacities that produce unacceptably high emissions of health-endangering gaseous and particulate matter into the atmosphere. Once considered only a local problem, such urban pollution is now recognized to have regional and even global implications.

Marine bioinvasion - introduction of marine organisms alien to local ecosystem through ship hulls and ballast water - has serious consequences to native biota, fishery and general coastal ecosystem. Over 80% of the world cargo is mobilized transoceanically and over 12 billion tones of ballast water is filled at one part of the ocean and discharged at the other. These ballast waters offer conducive situation for bacteria, viruses, algae, dinoflagellates and a variety of macro-faunal larval/cyst stages to translocate to alien regions, usually along the coasts of the continents. As an example, there are over 18 species of animals and plants documented along the Indian coasts as those that might have got invaded and established. They can cause deleterious effects to local flora and fauna through their toxigenic, proliferative and over-competitive characteristics. This article points out the threats arising out of marine bioinvasion and various technological developments needed to deal with this unavoidable scourge in global shipping transport.

Using in situ data collected during 1992–1997, under the Indian programme of Joint Global Ocean Flux Study (JGOFS), we show that the biological productivity of the Arabian Sea is tightly coupled to the physical forcing mediated through nutrient availability. The Arabian Sea becomes productive in summer not only along the coastal regions of Somalia, Arabia and southern parts of the west coast of India due to coastal upwelling but also in the open waters of the central region. The open waters in the north are fertilized by a combination of divergence driven by cyclonic wind stress curl to the north of the Findlater Jet and lateral advection of nutrient-rich upwelled waters from Arabia. Productivity in the southern part of the central Arabian Sea, on the other hand, is driven by advection from the Somalia upwelling. Surface cooling and convection resulting from reduced solar radiation and increased evaporation make the northern region productive in winter. During both spring and fall inter-monsoons, this sea remains warm and stratified with low production as surface waters are oligotrophic. Inter-annual variability in physical forcing during winter resulted in one-and-a-half times higher production in 1997 than in 1995.