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Continuous measurements of hydrographic, hydrodynamic, and water quality showed marked diurnal, tidal, and seasonal variabilities in Kuwait Bay, a stressed coastal system in the northwestern Arabian/Persian Gulf. Advection of water masses and seasonality in vertical mixing regulated the Bay's hydrographic and water quality properties. Intensive stratification in summer had substantial implications on the Bay environment. Kuwait Bay constantly exports dense bottom water laden with dissolved inorganic nutrients and organic matter to the central basin of the Gulf. The export was largest in August under strong water column stratification. These in-situ findings agreed well with earlier studies that corroborated Kuwait Bay as an important area where the phenomenon of reverse estuarine circulation originates in the Gulf. Thus, Kuwait Bay is a significant source of nutrients and organic matter to the Gulf Deep Water that flows into the core of the oxygen minimum zone in the northwestern Indian Ocean.

The redox speciation of iron was determined during the iron fertilization LOHAFEX and for the first time, the chemiluminescence assay of filtered and unfiltered samples was systematically compared. We hypothesize that higher chemiluminescence in unfiltered samples was caused by Fe(II) adsorbed onto biological particles. Dissolved and particulate Fe(II) increased in the mixed layer steadily 6-fold during the first two weeks and decreased back to initial levels by the end of LOHAFEX. Both Fe(II) forms did not show diel cycles downplaying the role of photoreduction. The chemiluminescence of unfiltered samples across the patch boundaries showed strong gradients, correlated significantly to biomass and the photosynthetic efficiency and were higher at night, indicative of a biological control. At 150 m deep, a secondary maximum of dissolved Fe(II) was associated with maxima of nitrite and ammonium despite high oxygen concentrations. We hypothesize that during LOHAFEX, iron redox speciation was mostly regulated by trophic interactions., This work was funded by the Spanish Government via MINECO (CTM2008-880 01864-E/ANT, CTM2014-59244-C3-3-R) and MCIN/AEI/10.13039/501100011033 (PID2020-115291GB-I00) projects.

A series of flash floods that swamped urban drainage systems in Kuwait in November-December 2018 drastically altered coastal hydrography. The phytoplankton responded quickly to the nutrient supply from land and reduced salinity despite elevated turbidity, as evident from rapid increases in chlorophyll a and net community production. Microphytoplankton was dominated by diatoms and the photosynthetic ciliate Myrionecta rubra. Both field observations and microcosm experiments suggested that although increased nutrient availability stimulates diatom growth, microzooplankton grazing controls their outbursts. This study revealed that in a hypersaline system similar to the northern Arabian Gulf, extreme events like flash floods have immediate but short-lived corollaries on coastal phytoplankton due to synergistic effects of bottom-up and top-down factors. The patterns are comparable to those reported from other tropical and sub-tropical systems.

The Western Continental Shelf of India experiences water-column anoxia during late summer/early autumn, thereby providing conducive conditions for methane (CH 4) accumulation. Measurements of CH4 along with other ancillary physico-chemical variables were made during the period of anoxia (September–October) in 2003, 2005 and 2009. The shelf waters experienced variable oxygen deficiency (hypoxic to anoxic) in space and time. Maximum observed accumulation of CH4 was only ~104 nmol/L although the bottom waters were strongly reducing with hydrogen sulphide (H 2S) concentration reaching up to 26.6 μmol/L. Methane showed a reasonable positive correlation with H2S. The observed CH4 accumulation over the Indian shelf was lower than previously reported from the major Eastern Boundary Upwelling systems, especially off Namibia. It is proposed that the low organic loading arising from lower productivity and consequently weak sedimentary methanogenesis may largely account for the lower CH 4 concentrations in anoxic bottom waters over the Indian shelf. Also, while anoxia may limit CH4 oxidation, sedimentary inputs are probably a more important source of water column CH4. In spite of the moderate CH4 build up in bottom waters, CH4 saturation at the sea-surface observed in the present study (89–1041%) was relatively lower than observed in 1996–1997 (~110–2521%). The sea to air flux of CH4 ranged from −0.52 to 26.56 μmol m−2 d−1, marginally higher than previously reported from the upwelling system in the western Arabian Sea.

Monthly measurements of nitrate, nitrite, ammonium and phosphate at three stations off Kuwait during 2002-2015 revealed considerable inter-annual variability, broadly corresponding to fluctuations in the Shatt-al-Arab River discharge, but a lack of secular increasing trend. Nutrient enrichment experiments during two seasons revealed nitrate uptake, chlorophyll build-up and growth of micro-phytoplankton, even in the presence of ammonium, provided the availability of phosphate. Primary production was mostly nitrogen limited, but anthropogenic nitrogen supply may eventually make it phosphorus limited, especially in summer and in the open Gulf. Anthropogenic nutrient inputs appear to have enhanced biological productivity of the northern Gulf, but heterotrophic consumption, indicated by high respiration rates, probably prevented accumulation of phytoplankton biomass, accounting for the observed lack of chlorophyll increase over the past three decades. Consequently high total organic carbon and emerging hypoxia in the Gulf may lead to expansion/intensification of the oxygen minimum zone of the Arabian Sea.

The fate of the enormous amount of reactive nitrogen released to the environment by human activities in India is unknown. Here we show occurrence of seasonal stratification and generally low concentrations of dissolved inorganic combined nitrogen, and high molecular nitrogen (N2) to argon ratio, thus suggesting seasonal loss to N2 in anoxic hypolimnia of several dam-reservoirs. However, 15N-experiments yielded low rates of denitrification, anaerobic ammonium oxidation and dissimilatory nitrate reduction to ammonium—except in the presence of methane (CH4) that caused ~12-fold increase in denitrification. While nitrite-dependent anaerobic methanotrophs belonging to the NC10 phylum were present, previously considered aerobic methanotrophs were far more abundant (up to 13.9%) in anoxic hypolimnion. Methane accumulation in anoxic freshwater systems seems to facilitate rapid loss of reactive nitrogen, with generally low production of nitrous oxide (N2O), through widespread coupling between methanotrophy and denitrification, potentially mitigating eutrophication and emissions of CH4 and N2O to the atmosphere., The fate of anthropogenic nitrogen (N) remains understudied in South Asian water bodies despite its impact on water chemistry and quality. Here the authors show that N loss in Indian freshwater reservoirs is tightly coupled to methanotrophy, which has helped curb eutrophication and greenhouse gas emissions.

Incubation experiments under trace metal clean conditions and ambient oxygen concentrations were used to investigate the response of microbial assemblages in oxygen minimum zones (OMZs) to additions of organic carbon and copper, two factors that might be expected to limit denitrification in the ocean. In the OMZs of the Eastern Tropical North and South Pacific, denitrification appeared to be limited by organic carbon; exponential cell growth and rapid nitrate and nitrite depletion occurred upon the addition of small amounts of carbon, but copper had no effect. In the OMZ of the Arabian Sea, neither carbon nor copper appeared to be limiting. We hypothesize that denitrification is variable in time and space in the OMZs in ways that may be predictable based on links to the episodic supply of organic substrates from overlying productive surface waters.

Using multiple geochemical proxies including specific biomarkers (dinosterol, phytol, stigmasterol and b-sitosterol) measured in a high-sedimentation rate core collected from the inner shelf (depth � 45 m) off Goa (India), we reconstruct surface productivity, which is mainly controlled by the monsoon upwelling in this region, during the last ca. 700 a. Surface productivity appears to have varied in tandem with the Konkan-Goa rainfall and sunspot activity during the instrumental period (last 250 a). The productivity proxies also covary with the total solar irradiance reconstructed for the period beyond the instrumental era, within the considerable uncertainty of the age model. This suggests that solar forcing may control coastal upwelling intensity and biological productivity in the eastern Arabian Sea on decadal to centennial timescales. During the late Anthropocene (last ca. 50 a), steep increases in all four biomarkers indicate greatly enhanced productivity in response to high solar irradiance as well as anthropogenic inputs of nutrients. Copyright # 2008 John Wiley & Sons, Ltd.

Dissolved iron and Fe(II) were measured in the oxygen minimum zone (OMZ) of the Arabian Sea in September 2004. The OMZ is a well-demarcated feature characterized by high rates of denitrification, and a deep nitrite maximum coinciding with oxygen levels below 1 μmol L -1 . This zone is significantly enriched in dissolved Fe relative to overlying and underlying waters and up to 50% of the dissolved Fe is present as Fe(II). The maxima in Fe(II) are at the same depth as the deep nitrite maxima, centered around 200-250 m. They coincide with a local maximum in total dissolved Fe, suggesting that Fe accumulates at this depth because of the greater solubility of Fe(II) over Fe(III). Fe(II) is thermodynamically unstable even at submicromolar oxygen levels, so active biological reduction is the most plausible source. To our knowledge, this is the first report of a potential link between Fe reduction, elevated dissolved Fe concentrations, and nitrite accumulation within an OMZ. Denitrification has a high Fe requirement associated with the metalloenzymes for nitrate and nitrite reduction, so in situ redox cycling of Fe has important implications for the nitrogen cycle.

Despite their importance for the global oceanic nitrogen (N) cycle, estimates of N fluxes in the Arabian Sea remain in considerable uncertainty. In this report, we summarize current knowledge of important processes, including denitrification, N2 fixation and nitrous oxide emissions. Additionally, we discuss anthropogenic impacts on the N cycle in the region. Existing studies suggest that the Arabian Sea is a significant source of N2O, and a major sink for fixed-N mainly due to enhanced rates of denitrification that occur in suboxic portions of the water column in the Arabian Sea. Sedimentary denitrification is small compared to water column denitrification, and additions of fixed-N via N2 fixation also are small compared to pelagic denitrification. As a consequence, the fixed-N budget of the Arabian Sea is dominated by an advective supply from the south, and by the sink arising from pelagic denitrification. Although relatively small compared to the advective supply, inputs of fixed-N from runoff and from the atmosphere may have significant impacts on surface waters and on the coastal waters of western India, and these inputs are rising because of human activities. Overall, the Arabian Sea’s nitrogen cycle is likely to respond sensitively to climate change and, in turn, have an impact on climate via its N2O and denitrification components.

Transfers of some important biogenic atmospheric constituents, carbon dioxide (CO2), methane (CH4), molecular nitrogen (N2), nitrous oxide (N2O), nitrate View the MathML source(NO3-), ammonia (NH3), methylamines (MAs) and dimethylsulphide (DMS), across the air–sea interface are investigated using published data generated mostly during the Arabian Sea Process Study (1992–1997) of the Joint Global Ocean Flux Study (JGOFS). The most important contribution of the region to biogeochemical fluxes is through the production of N2 and N2O facilitated by an acute, mid-water deficiency of dissolved oxygen (O2); emissions of these gases to the atmosphere from the Arabian Sea are globally significant. For the other constituents, especially CO2, even though the surface concentrations and atmospheric fluxes exhibit extremely large variations both in space and time, arising from the unique physical forcing and associated biogeochemical environment, the overall significance in terms of their global fluxes is not much because of the relatively small area of the Arabian Sea. Distribution and air–sea exchanges of some of these constituents are likely to be greatly influenced by alterations of the subsurface O2 field forced by human-induced eutrophication and/or modifications to the regional hydrography.

Denitrification often occurs in the water column, underlying zones of intense productiv- ity and decomposition in upwelling regions. In the denitrifying zone off the southwest coast of India, high concentrations of nitrite (>15 µM) and nitrous oxide (>500 nM) have been reported near the sediment-water interface (

Understanding and quantifying ocean–atmosphere exchanges of the long-lived greenhouse gases carbon dioxide (CO2), nitrous oxide (N2O) and methane (CH4) are important for understanding the global biogeochemical cycles of carbon and nitrogen in the context of ongoing global climate change. In this chapter we summarise our current state of knowledge regarding the oceanic distributions, formation and consumption pathways, and oceanic uptake and emissions of CO2, N2O and CH4, with a particular emphasis on the upper ocean. We specifically consider the role of the ocean in regulating the tropospheric content of these important radiative gases in a world in which their tropospheric content is rapidly increasing and estimate the impact of global change on their present and future oceanic uptake and/or emission. Finally, we evaluate the various uncertainties associated with the most commonly used methods for estimating uptake and emission and identify future research needs.

A closed eddy core in the Atlantic Subantarctic Southern Ocean was fertilized twice with two tons of iron (as FeSO4) to test whether iron addition enhances downward particle flux into the deep ocean. The ~300 km2 fertilized patch was occupied for 39 d. Chlorophyll-a and primary productivity doubled after fertilization, and photosynthetic quantum yield (FV/FM) increased from 0.33 to ≥0.40. Silicic acid was at limiting concentrations (

Historically, the Indian Ocean (IO) has received relatively little attention from the oceanographic community and therefore remains substantially undersampled compared to the Atlantic and Pacific oceans. This situation is compounded by the IO being a dynamically complex and highly variable system under monsoonal influence. The biogeochemical and ecological impacts of this complex physical forcing are not yet fully understood. Specific questions and hypotheses have emerged from recent studies that have yet to be tested, such as the potential role of zooplankton grazing versus iron limitation in controlling phytoplankton production in the Arabian Sea (AS). Furthermore, the IO is a globally important denitrification zone, and it also appears to be a region where N 2 fixation rates are significant. However, there are still large uncertainties in the rate estimates for both. The IO is also warming rapidly, but the impacts of this warming on the biota, carbon uptake, and nitrogen cycling are unquantified. The increasing population density and rapid economic growth of the countries surrounding the Bay of Bengal and eastern AS make these regions' coastal environments particularly vulnerable to anthropogenic influences. Warming and anthropogenic effects might also impact the huge myctophid stocks in the AS and other commercially valuable species. These potential influences and their socioeconomic ramifications need to be explored. Deployment of coastal and open ocean observing systems in the IO has created new opportunities for carrying out biogeochemical and ecological research. International research efforts should be motivated to exploit these opportunities for addressing the questions identified in this chapter.

The eastern Arabian Sea contains the only eastern-boundary-type upwelling environment in the entire Indian Ocean, albeit on a seasonal basis. During the southwest monsoon, when the surface current flows equatorward, upwelling brings oxygen-poor, nutrient-rich subsurface waters to the Indian continental shelf that turn anoxic (sulfate-reducing) by late summer due to exhaustion of oxygen and nitrate by heterotrophic microorganisms. This natural oxygen-deficient system, by far the world's largest in the coastal ocean, is apparently more intense now than it was three decades ago. This is consistent with the sedimentary record, which indicates that productivity over the past few decades has been the highest ever in the last seven centuries. However, a trend of ongoing intensification is not seen in the data collected during the last 10 years at a coastal time series station off Goa. These data, nevertheless, show considerable interannual variability with the most severe anoxia having occurred in 2001, adversely impacting local fisheries (especially demersal fish catch). While increased nutrient loading to the coastal zone largely through atmospheric deposition of nitrogen most likely occurred in the last few decades, contributing to a shift to fully anoxic conditions, the observed interannual variability suggests that subtle changes in local hydrography/circulation could also modulate coastal anoxia in the region.

Despite a history of exploration dating back to the classical era and its leading role as a pathway for trade and cultural exchange for the great civilizations of those times, the Indian Ocean has consistently been subject to less attention in the modem era in terms of oceanographic enquiry. The cornerstone of the Sustained Indian Ocean Biogeochemical and Ecosystem Research (SIBER) initiative has been to promote more frequent and persistent research activities that encompass the entire Indian Ocean basin and to facilitate international cooperation to realize these objectives. This volume's chapters are derived from the plenary talks given by the attendees of the first SIBER conference and are a blend of current knowledge reviews and new results. Thus this collection of papers represents an interdisciplinary contribution to the Indian Ocean literature by the leading members of the Indian Ocean research community.

We compared recently published sedimentary records of δ 15 N from several coastal areas affected by both natural and anthropogenically produced shallow hypoxia with the objective of testing this as a proxy for denitrification in coastal settings. We examined the eastern boundary systems of continental shelves off western India and Peru, which appear to be experiencing intensification of bottom-water oxygen depletion, most likely as a consequence of intensification of eastern boundary coastal upwelling over the last few decades. In systems that are significantly affected by an enhanced inventory of nutrients from organic matter in soils due to continental erosion following colonial land clearing (e.g., Chesapeake Bay), fertilizer and wastewater runoff (e.g., western Indian shelf and Long Island Sound), the productivity increase is largely local and induced by anthropogenic activity. The western Indian shelf thus experiences a double effect, being both an upwelling zone and prone to nutrient enrichment from land. While in other regions of both natural and anthropogenic bottom-water hypoxia, sedimentary δ 15 N has undergone significant increases over the Anthropocene; in the eastern Arabian Sea, an opposite trend is noticed despite historical water-column measurements revealing a contemporaneous intensification of denitrification. Plausible causes are discussed here in detail, which led us to conclude that the sedimentary δ 15 N may not always work as a reliable proxy of denitrification in coastal regions.

The Arabian Sea (AS) oxygen minimum zone (OMZ) is one of the largest pelagic low-oxygen environments in the open ocean. It is responsible for the removal of up to 60 Tg of nitrogen annually, roughly an eighth of the global fixed nitrogen sink. Although denitrification has long been believed to be the major process responsible for fixed nitrogen loss from the oceans, recent studies show that anaerobic ammonium oxidation (anammox) is potentially a more important process involved. We have investigated the phylogeny of both anammox and denitrifying microbes in the AS, and here we report on their diversity in terms of their characteristic genes. Denitrifiers were targeted using nirS and nirK genes and anammox bacteria with the 16S rRNA gene. The nirK gene was amplified from all the samples, but nirS gene could only be detected when nitrite was present. The distribution of phylotypes was related to the concentration of nitrite and the apparent stage of denitrification. Most nirK or nirS genes from the AS had low identities with other published sequences. The closest identities were to sequences from other water column denitrifying environments rather than sedimentary or terrestrial environments. Phylogenetic analysis of the nirS and nirK genes revealed overall lower diversity than the very high diversities reported from estuarine and sedimentary environments. 16S rRNA partial gene sequences revealed very limited diversity among anammox sequences. All the anammox sequences were >98% identical to each other and were similar to sequences from other marine and water column environments, which are distantly related to Scalindua sp. Quantification using quantitative polymerase chain reaction assays showed that both nirS genes and anammox 16S rRNA genes were more abundant at depths with higher nitrite concentrations. The presence and abundance of genes indicative of both processes suggest that both canonical denitrification and anammox likely occur in the OMZ of the AS. However, the abundance of the nirS gene, one of the genes responsible for canonical denitrification, was an order of magnitude higher than the abundance of the anammox 16S gene, indicating that denitrifiers are numerically dominant in this environment. The greater diversity of nirS and nirK genes relative to anammox genes both among and within stations also suggests that the denitrifier assemblages are more dynamic in response to environmental conditions.

The anaerobic oxidation of ammonium (anammox) contributes significantly to the global loss of fixed nitrogen and is carried out by a deep branching monophyletic group of bacteria within the phylum Planctomycetes. Various studies have implicated anammox to be the most important process responsible for the nitrogen loss in the marine oxygen minimum zones (OMZs) with a low diversity of marine anammox bacteria. This comprehensive study investigated the anammox bacteria in the suboxic zone of the Black Sea and in three major OMZs (off Namibia, Peru and in the Arabian Sea). The diversity and population composition of anammox bacteria were investigated by both, the 16S rRNA gene sequences and the 16S-23S rRNA internal transcribed spacer (ITS). Our results showed that the anammox bacterial sequences of the investigated samples were all closely related to the Candidatus Scalindua genus. However, a greater microdiversity of marine anammox bacteria than previously assumed was observed. Both phylogenetic markers supported the classification of all sequences in two distinct anammox bacterial phylotypes: Candidatus Scalindua clades 1 and 2. Scalindua 1 could be further divided into four distinct clusters, all comprised of sequences from either the Namibian or the Peruvian OMZ. Scalindua 2 consisted of sequences from the Arabian Sea and the Peruvian OMZ and included one previously published 16S rRNA gene sequence from Lake Tanganyika and one from South China Sea sediment (97.9-99.4% sequence identity). This cluster showed only

Estimates of the amount of denitrification in the Arabian Sea are inconsistent, and so two methods of calculation of ‘original’ nitrate concentrations used in earlier studies have been reviewed. Nitrite at intermediate depths was not restricted to the Persian Gulf water; concentrations were maximal much shallower than the depths of the Persian Gulf water. The nitrate-salinity relationship used to calculate ‘original’ nitrate concentrations in a recent study would lead to an underestimate of nitrate deficits because the approach requires denitrification to be restricted to Persian Gulf water. A method involving oxidative ratios has been modified and used to calculate nitrate deficits. The deficits so calculated lie between the values reported earlier. The degree of denitrification decreases steadily southward. The results indicate that denitrification extends up to 11 or 12°N latitude. Taking 30 years as the average residence time of water for the layer in which denitrification occurs, the denitrification rate in the Arabian Sea has been calculated as about 3.2 × 10 12 gy −1 . This represents about 5% of the annual global denitrification.