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Considering Alappuzha Mud Bank in the southern Kerala coast as a typical case of biologically productive Mud Banks that form along the southwest coast of India during the Southwest Monsoon (June - September), the present study addresses several pertinent missing links between the physical environment in Mud Banks and their influence on plankton stock. This study showed that very strong coastal upwelling prevails in the entire study domain during the Southwest Monsoon, which manifests itself in the form of significantly cool, hypoxic and nitrate-rich waters surfacing near the coast. The upwelled water persisting throughout the Southwest Monsoon period was found to have fuelled the exceptionally high phytoplankton stock in the entire study area, including the Mud Bank region. Having accepted that Mud Banks are special because of the calm sea surface conditions and relatively high turbidity level in the water column around them, the present study showed that except at points close to the sea bottom, turbidity level in the Alappuzha Mud Bank was below the critical level to inhibit the plankton stock. The suspended sediments that form in the Mud Bank occasionally could be attributed to the disturbance of the bottom fluid muddy layer and their vertical spurts.

The LOHAFEX iron fertilization experiment was conducted for 39 days in the closed core of a cyclonic mesoscale eddy located along the Antarctic Polar Front in the Atlantic sector of the Southern Ocean. Mixed layer (ML) waters were characterized by high nitrate (~ 20 μM), low dissolved iron (DFe ~ 0.2 nM) and low silicate concentrations (below 1 μM) restricting diatom growth. Upon initial fertilization, chlorophyll-a doubled during the first two weeks and stabilized thereafter, despite a second fertilization on day 21, due to an increase in grazing pressure. Biomass at the different trophic levels was mostly comprised of small autotrophic flagellates, the large copepod Calanus simillimus and the amphipod Themisto gaudichaudii. The downward flux of particulate material comprised mainly copepod fecal pellets that were remineralized in the upper 150 m of the water column with no significant deeper export. DFe concentrations in the upper 200 m were not significantly affected by the two fertilizations but after day 14 showed a greater variability (ranging from 0.3 to 1.3 nM) without a clear vertical pattern. Particulate iron concentrations (measured after 2 months at pH 1.4) decreased with time and showed a vertical pattern that indicated an important non-biogenic component at the bottom of the mixed layer. In order to assess the contribution of copepod grazing to iron cycling we used two different approaches: first, we measured for the first time in a field experiment copepod fecal pellet concentrations in the water column together with the iron content per pellet, and second, we devised a novel analytical scheme based on a two-step leaching protocol to estimate the contribution of copepod fecal pellets to particulate iron in the water column. Analysis of the iron content of isolated fecal pellets from C. simillimus showed that after the second fertilization, the iron content per fecal pellet was ~ 5 fold higher if the copepod had been captured in fertilized waters. We defined a new fraction termed leachable iron (pH 2.0) in 48 h (LFe48h) that, for the conditions during LOHAFEX, was shown to be an excellent proxy for the concentration of iron contained in copepod fecal pellets. We observed that, as a result of the second fertilization, iron accumulated in copepod fecal pellets and remained high at one third of the total iron stock in the upper 80 m. We hypothesize that our observations are due to a combination of two biological processes. First, phagotrophy of iron colloids freshly formed after the second fertilization by the predominant flagellate community resulted in higher Fe:C ratios per cell that, via grazing, lead to iron enrichment in copepod fecal pellets in fertilized waters. Second, copepod coprophagy could explain the rapid recycling of particulate iron in the upper 100–150 m, the accumulation of LFe48h in the upper 80 m after the second fertilization and provided the iron required for the maintenance of the LOHAFEX bloom for many weeks. Our results provide the first quantitative evidence of the major ecological relevance of copepods and their fecal products in the cycling of iron in silicate depleted areas of the Southern Ocean., This work was supported by the Government of Spain (MINECO CTM2008-01864-E/ANT and CTM2014-59244-C3-3-R). LML was supported by the Campus de Excelencia Internacional Program (Ministerio de Educación, Cultura y Deportes, España).

Coastal upwelling and mud banks are two oceanographic processes concurrently operating along certain stretches of the southwest (Kerala) coast of India during the Southwest Monsoon period (June–September), facilitating significant enhancement in plankton biomass. Mud banks have scientific and societal attention from time immemorial, predominantly due to the large fisheries associated with them. In this paper, for the first time, the specific biophysical roles of these oceanographic processes have been discriminated, based on a focused 18 weekly/fortnightly time-series study (April to September 2014) in a mud bank-upwelling area (off Alappuzha, southwest coast of India). In conjunction with standard hydrographical and satellite remote sensing data, we utilised a FlowCAM to track the biophysical linkage in terms of plankton composition abundance and size structure at three locations (M1, M2 and M3) in the study area. During the Pre-Southwest Monsoon (April–May), the entire study area was warmer with low nitrate concentration in the surface waters, which caused lower biomass of autotrophs compared to the Southwest Monsoon (June–September). By the onset of the Southwest Monsoon (June), drastic hydrographical transformations took place in the study domain due to the Coastal upwelling, reflected as the surfacing of significantly cool, high nutrient and hypoxic waters. Concurrently, mud bank formed at location M2 due to the presence of relatively high-suspended sediments in the region, creating a localised calm environment conducive for fishing activities. In response to the hydrographical transformations in the entire study area during the Southwest Monsoon, the autotrophic plankton biomass and size structure experienced significant change. The micro-autotrophs biomass that was low during the Pre-Southwest Monsoon (av. 0.33 ± 0.2 mgC L− 1 at surface and av. 0.07 ± 0.04 mgC L− 1 at subsurface) noticeably increased during the Southwest Monsoon (av. 1.6 ± 0.4 mgC L− 1 at surface and av. 1.3 ± 0.2 mgC L− 1 at subsurface). The seasonal mean bio-volume of micro-autotrophs followed the same pattern in all three locations with the dominance of smaller individuals during the Pre-Southwest Monsoon (av. 55 ± 4.4 × 103 μm3 individual− 1 at surface and av. 67.1 ± 38.4 × 103 μm3 individual− 1 at subsurface). Relatively large phytoplankton dominated during the Southwest Monsoon (av. 77.3 ± 5.5 × 103 μm3 individual− 1 at surface and av. 90.3 ± 4.9 × 103 μm3 individual− 1 at subsurface). Similar spatial change in the plankton composition, biomass and size structure in all three locations suggested the dominant role of coastal upwelling, and not the mud bank, in shaping the dominant autotrophic plankton in the study domain. The nutrient enrichment in the region associated with the coastal upwelling facilitated the predominance of a classical short food chain capable of supporting large fish stocks. In addition to relatively calm mud bank, the hypoxic, cool upwelled waters that existed in the subsurface induce the fish stock to concentrate in the surface waters, making the relatively calm mud bank and adjoining areas a rich fishing ground for the native fishers.

The results of the first time-series measurements spanning 18-weeks (22 April to 20 September 2014) from a coastal environment (Alappuzha, southwest India), where two process of upwelling and mud banks are concurrent during summer monsoon are presented. The upwelling signals were evident from the 2nd week of May onwards and the intensity varied widely from 1st week of June to 3rd week of September. The formation of mud banks was observed during the peak upwelling period (14th June) and once formed, they were persistent till the end of monsoon season (September 2014). The time-series evolution of dissolved oxygen levels showed the incursion of hypoxic waters well in to the mud banks during peak upwelling period. The considerable reduction in the dissolved methane levels (300 to

The Arabian Sea experiences moderate to weak upwelling along the south-west coast of India, which subsequently propagates towards the north. This causes variation in plankton community composition, which is addressed in the present study. Here we report the spatial variations in distribution of phytoplankton groups along the north-south transect in the eastern Arabian Sea based on marker pigments supported with flow-cytometric and microscopic analyses. 15 phytoplankton pigments were identified using High-performance liquid chromatography (HPLC) and the chemotaxonomic software (CHEMTAX) analysis associated these to seven major group of phytoplankton. The phytoplankton biomass, chlorophyll a (Chl a) was higher in southern stations with dominance of fucoxanthin whereas, divinyl chlorophyll a (divinyl Chl a), marker pigment of Prochlorococcus was present only in the northern region. Microscopic observation revealed the dominance of larger forms; diatoms (Chaetoceros coarctatum and Nitzschia sp.) and dinoflagellates (Scrippsiella sp., Oxytoxum nanum and Oxytoxum sp.) in the southern region. Furthermore, a study of plankton size distribution showed dominance of picoplankton (fpico) followed by nanoplankton (fnano) along the northern stations with comparatively higher microplankton (fmicro) in the south. This study clearly showed the influence of different environmental conditions on the phytoplankton community as reflected in dominance of diatoms in the southern (south of 12 °N) and that of picoplankton in the northern (north of 12 °N) region.

An analytic gap exists between the lower limit of detection of oxygen (O2) by conventional sensors (LOD, ~1 – 3 µM O2) and the appearance of NO2– from NO3– reduction (~0.05 µM O2), which signifies the secondary nitrite maximum. The near-anoxic milieu where precise O2 measurements could not be made until very recently even by current optodes, favors biogeochemically important redox reactions. The ongoing improvement of optodes might make the gap vanish. Significant O2 levels near the conventional LOD for O2, when accompanied by ~ ≥0.05 µM NO2–, must be erroneous and should be deleted from historical data going back to the 1930s.

The western continental shelf of India (eastern Arabian Sea) houses the world's largest seasonally-occurring coastal low-oxygen zone. The natural abundance of nitrogen and oxygen isotopes in dissolved nitrate were measured along four transects of the shelf (off Mumbai, Goa, Mangalore and Calicut) to gain insights into nitrogen cycling in this system. Carbon and nitrogen isotope abundances in particulate organic matter were also measured to understand sources and cycling of organic matter. Three of the four transects (with the exception of Mumbai transect) experienced fully anoxic (sulphidic) conditions over the inner shelf. Denitrification signals, as indicated by a linear increase of δ15N and δ18O in nitrate in oxygen-deficient waters, were observed off Mumbai, Goa and Mangalore. The occurrence of nitrite oxidation was indicated by a decoupling of stable N and O isotopes of nitrate. The fractionation factor computed was found to be quite low (6.1‰), as compared to open ocean oxygen minimum zones, presumably due to a large contribution (>49%) of benthic denitrification to N-loss. Autochthonous production and phytoplankton composition are proposed to exert major controls on the C isotopic composition of particulate organic matter while nitrate and ammonium seem to control the N-isotope variability. Overall, this study attempts to fill the knowledge gap in terms of N and C stable isotope studies in a system that is otherwise well documented from the biogeochemical point of view.

We report here results of a time series study carried out to understand seasonal variability of phytoplankton in relation to changes in dissolved oxygen based on observations at the Candolim Time Series (CaTS) station located in coastal waters of Goa, Central West Coast of India. The water column remains well oxygenated during the non-southwest (SW) monsoon period (November–May). However, upwelling of water already depleted in dissolved oxygen during the SW monsoon, coupled with degradation of copious amounts of locally produced organic matter, leads to severe oxygen depletion including sulphate reduction beneath a thin surface layer during late summer-early autumn (August–October). The onset of oxygen-deficient conditions greatly alters the community structure of water-column phytoplankton. Pico-autotrophs and pennate diatoms were found to dominate the autotrophic biomass in such oxygen depleted waters.

The Arabian Sea hosts one of the most intense, perennial Oxygen Minimum Zones (OMZ) in the world ocean. Observations along a meridional transect at 68°E extending from 8 to 21°N showed large seasonal as well as interannual changes in the dissolved oxygen and nitrite concentrations. Unlike previous studies that used observations from the periphery of the OMZ, our observations are from its core and also allow us demarcating the southern extent of the OMZ. The southern boundary of the OMZ extended up to ~9°N during the southwest monsoon but was restricted to ~11°N during the northeast monsoon. While previous studies report ventilation from the oxygenated Indian Central Water (ICW) at intermediate depths of the OMZ, none have so far discussed how this ventilation takes place or its variability. Our observations reveal intrusion of the ICW at intermediate depths (200–500 m) into the OMZ up to 16°N, in contrast to 11°N reported in earlier studies. The oxygen variations at the southern boundary of the OMZ are coherent with those in the sea level, associated with the westward propagating Rossby waves radiated off the west coast of India. Besides modulating the oxygen distribution in the surface layer and the upper boundary of the OMZ through upwelling or downwelling, these Rossby waves can potentially influence the oxygen at intermediate depths by controlling the isopycnal mixing of the ICW within the OMZ, leading to variations in oxygen and nitrogen concentrations. Further investigations are needed to better understand the potential influence of these Rossby waves on the OMZ variability and the associated nitrogen cycling.

The oxygen minimum zone (OMZ) of the Arabian Sea is the thickest of the three oceanic OMZ. It is of global biogeochemical significance because of denitrification in the upper part leading to N2 and N2O production. The residence time of OMZ water is believed to be less than a decade. The upper few hundred meters of this zone are nearly anoxic but non-sulfidic and still support animal (metazoan) pelagic life, possibly as a result of episodic injections of O2 by physical processes. We report on discrete measurements of dissolved O2 and NO2–, temperature and salinity made between 1959 and 2004 well below the tops of the sharp pycnocline and oxycline near 150, 200, 300, 400, and 500 m depth. We assemble nearly all O2 determinations (originally there were 849 values, 695 of which came from the OMZ) by the visual endpoint detection of the iodometric Winkler procedure, which in our data base yields about 0.04 mL L−1 (~ 2 μM) O2 above the endpoint from modern automated titration methods. We acknowledge that much lower (nanomolar) O2 values have been measured recently with the STOX (Switchable Trace amount OXygen) sensor in the eastern tropical South Pacific, and that similar conditions may also prevail in the Arabian Sea OMZ. In spite of the error in O2 measurements at vanishingly low levels, we argue that the temporal trends of the historic data should still hold. We find 632 values acceptable (480 from 150 stations in the OMZ). The data are grouped in zonally paired boxes of 1° lat. and 2° long. centered at 8, 10, 12, 15, 18, 20, and 21° N along 65 and 67° E. The latitudes of 8–12° N, outside the OMZ, are treated in passing. The principal results are as follows: (1) an O2 climatology for the upper OMZ reveals a marked seasonality at 200 to 500 m depth with O2 levels during the northeast monsoon and spring intermonsoon seasons elevated over those during the southwest monsoon season (median difference, 0.08 mL L−1 [~ 3.5 μM]). The medians of the slopes of the seasonal regressions of O2 on year for each of the NE and SW monsoon seasons are −0.0043 and −0.0019 mL L−1 a−1, respectively (−0.19 and −0.08 μM a−1; n = 10 and 12, differing at p = 0.01); (2) four decades of statistically significant decreases of O2 between 15 and 20° N but an opposing trend toward an increase near 21° N are observed. The mechanisms of the balance that more or less annually maintain the O2 levels are still uncertain. At least between 300 and 500 m, the replenishment is inferred to be due to isopycnal re-supply of O2, while at 200 (or 250?) m it is diapycnal, most likely by eddies. Similarly, recent models show large vertical advection of O2 well below the pycnoclines and oxyclines. The NO2– distribution, taken as an indicator of active NO3– reduction, does not show a trend in the redox environment for a quarter of a century at a GEOSECS station near 20° N. In the entire OMZ, the regression slopes on year within seasons for the rather variable NO2– do not present a clear pattern but by other measures tended to an increase of NO2–. Vertical net hauls collect resident animal (metazoan) pelagic life in the NO2– maximum of the OMZ at O2 levels well below the lower limit of the Winkler titration; the extremely low O2 content is inferred from the presence of NO2– believed to be produced through microbial NO3– reduction. Instead of the difficult measurement by the STOX sensor, the relation between the very low O2 inferred from presence of NO2– and mesozooplankton should be studied with 100 to 150 L bottles rather than nets. The spatial (within drift stations) and temporal (daily) variability in hydrography and chemistry is large also below the principal pycnocline. The seasonal change of hydrography is considerable even at 500 m depth. Future O2 or nutrient budgets for the OMZ must not be based on single cruises or sections obtained during one season only. Steady state cannot be assumed any longer for the intermediate layers of the central Arabian Sea.

E. Boss1, J. Kesselmeier2, S. W. A. Naqvi3, and A. Neftel4 1University of Maine, 5706 Aubert Hall, Orono ME 04469-570, USA 2Max Planck Institute for Chemistry, Biogeochemistry, Joh.-J.-Becher Weg 27, 55128 Mainz, Germany 3National Institute of Oceanography, Chemical Oceanography Division, 403004 Dona Paula, Goa, India 4Agroscope Reckenholz Tanikon Research Station, Air Pollution and Climate Group, Reckenholzstrasse 191, 8046 Zurich, Switzerland

BACKGROUND Oxygen concentrations in both the open ocean and coastal waters have been declining since at least the middle of the 20th century. This oxygen loss, or deoxygenation, is one of the most important changes occurring in an ocean increasingly modified by human activities that have raised temperatures, CO 2 levels, and nutrient inputs and have altered the abundances and distributions of marine species. Oxygen is fundamental to biological and biogeochemical processes in the ocean. Its decline can cause major changes in ocean productivity, biodiversity, and biogeochemical cycles. Analyses of direct measurements at sites around the world indicate that oxygen-minimum zones in the open ocean have expanded by several million square kilometers and that hundreds of coastal sites now have oxygen concentrations low enough to limit the distribution and abundance of animal populations and alter the cycling of important nutrients. ADVANCES In the open ocean, global warming, which is primarily caused by increased greenhouse gas emissions, is considered the primary cause of ongoing deoxygenation. Numerical models project further oxygen declines during the 21st century, even with ambitious emission reductions. Rising global temperatures decrease oxygen solubility in water, increase the rate of oxygen consumption via respiration, and are predicted to reduce the introduction of oxygen from the atmosphere and surface waters into the ocean interior by increasing stratification and weakening ocean overturning circulation. In estuaries and other coastal systems strongly influenced by their watershed, oxygen declines have been caused by increased loadings of nutrients (nitrogen and phosphorus) and organic matter, primarily from agriculture; sewage; and the combustion of fossil fuels. In many regions, further increases in nitrogen discharges to coastal waters are projected as human populations and agricultural production rise. Climate change exacerbates oxygen decline in coastal systems through similar mechanisms as those in the open ocean, as well as by increasing nutrient delivery from watersheds that will experience increased precipitation. Expansion of low-oxygen zones can increase production of N 2 O, a potent greenhouse gas; reduce eukaryote biodiversity; alter the structure of food webs; and negatively affect food security and livelihoods. Both acidification and increasing temperature are mechanistically linked with the process of deoxygenation and combine with low-oxygen conditions to affect biogeochemical, physiological, and ecological processes. However, an important paradox to consider in predicting large-scale effects of future deoxygenation is that high levels of productivity in nutrient-enriched coastal systems and upwelling areas associated with oxygen-minimum zones also support some of the world’s most prolific fisheries. OUTLOOK Major advances have been made toward understanding patterns, drivers, and consequences of ocean deoxygenation, but there is a need to improve predictions at large spatial and temporal scales important to ecosystem services provided by the ocean. Improved numerical models of oceanographic processes that control oxygen depletion and the large-scale influence of altered biogeochemical cycles are needed to better predict the magnitude and spatial patterns of deoxygenation in the open ocean, as well as feedbacks to climate. Developing and verifying the next generation of these models will require increased in situ observations and improved mechanistic understanding on a variety of scales. Models useful for managing nutrient loads can simulate oxygen loss in coastal waters with some skill, but their ability to project future oxygen loss is often hampered by insufficient data and climate model projections on drivers at appropriate temporal and spatial scales. Predicting deoxygenation-induced changes in ecosystem services and human welfare requires scaling effects that are measured on individual organisms to populations, food webs, and fisheries stocks; considering combined effects of deoxygenation and other ocean stressors; and placing an increased research emphasis on developing nations. Reducing the impacts of other stressors may provide some protection to species negatively affected by low-oxygen conditions. Ultimately, though, limiting deoxygenation and its negative effects will necessitate a substantial global decrease in greenhouse gas emissions, as well as reductions in nutrient discharges to coastal waters.

The Arabian Sea is a productive basin where seasonal upwelling and convective mixing result in high surface nutrient concentrations and widespread algal blooms. The factors controlling primary productivity in the Arabian Sea are of interest because the region contains an intense oxygen minimum zone (OMZ) that is a major sink for nitrate in the ocean. A survey of iron (Fe) distribution and redox chemistry was carried out in 2007 to assess its role in Arabian Sea biogeochemistry, including an investigation of the biological response to Fe additions. Results show that Fe is strongly enriched in the eastern Arabian Sea, associated with the OMZ. Much of the Fe within the OMZ is present as Fe(II), which enhances the residence time and accumulation of Fe. In contrast, Fe concentrations are lower in the western Arabian Sea, separated from the Fe-rich OMZ by a water mass boundary. Consequently, low surface values are associated with monsoon-driven upwelling off the Omani coast. Incubations revealed that primary production during the southwest monsoon was strongly limited by Fe over much of the study area. Incubation of surface waters with Fe resulted in rapid growth of Phaeocystis and up to sixfold increase in chlorophyll. This is the first demonstration of Fe limitation in the Arabian Sea, and the first high resolution zonal survey of dissolved Fe and Fe(II) across the basin. Our findings suggest that models developed to predict the response of the Arabian Sea biogeochemistry to global warming need to consider effects on Fe inputs.

The salinity of coastal waters in the vicinity of seasonally fresh water fed estuaries changes tremendously and reportedly affects the living calcite secreting organisms like foraminifera, as well as their dead remains. The precise mechanism of adverse effect of such seasonal salinity changes on calcite secreting organisms is, however not clear. The seasonal fresh water influx from the estuaries also affects the pH and alkalinity of the coastal seawater. Therefore, to understand the effect of salinity induced pH/alkalinity variations on benthic foraminifera, living specimens of Rosalina globularis were subjected to different salinity. Additionally, water samples were collected from an estuary during both monsoon and post monsoon season to understand the relationship between salinity, pH and total alkalinity (TA). The pH decreased with decreasing salinity during both the seasons. A similar decrease in TA with decreasing salinity was also observed but only till 20 psu salinity, below which the TA increased with decreasing salinity. Even though the maximum growth was reported in specimens kept at 35 psu salinity, growth of specimens maintained at >25 psu salinity, was same. Specimens kept at 10 psu and 15 psu salinity, however were much smaller and turned opaque within two days of lowering the salinity and later on their tests dissolved within 24 and 43 days, respectively. No specimen reproduced at 10 psu and 15 psu salinity, while only a few specimens (3%) reproduced at 20 psu salinity. As compared to 10-20 psu salinity, ~60 % reproduction was observed in specimens subjected to 25-40 salinity. The specimens maintained at 20 psu salinity took twice the time to reach maturity than those subjected to 2540 psu salinity. Since a big drop in pH was observed at 10-15 psu salinity (pH 7.2 and 7.5, respectively), while the alkalinity was still higher, we suggest that fresh water influx induced drop in pH adversely affects calcification and reproduction in benthic foraminifera. The response is, however not linear as beyond a certain limit, a further increase in pH does not affect benthic foraminifera; rather they respond to salinity as per their salinity tolerance range. It is further inferred that the time required to reach reproductive maturity increases at the extreme salinity tolerance limits. Dissolution of calcareous foraminifera below 20 psu salinity, suggests that salinity induced changes control the carbonate inventory in the coastal regions subjected to seasonal fresh water influx.

The spatio-temporal variations of the δ 13 C and δ 15 N of suspended particulate organic matter (SPOM), along with ancillary chemical and hydrographic parameters, have been examined monthly for one year (October 2010 to September 2011) at a total of seven stations to refine the understanding of SPOM sources and biogeochemical pathways in the tropical Zuari Estuary, west coast of India. The dynamic nature of C and N cycling in estuaries is reflected in the isotopic variation of SPOM as a function of space and time. The results exhibit marked seasonality, with the autochthonous component and the terrestrial component being the major SPOM sources during periods of low and high river discharges, respectively. Spatially, enrichments of δ 13 C and δ 15 N from the head (δ 13 C = 29.6‰, δ 15 N = 1.78‰, C/N = 7.09) to the mouth (δ 13 C = -20.9‰, δ 15 N = 5.77‰, C/N = 6.12) of the estuary indicate downstream reduction of terrigenous influence. The mean C:N ratios varied between 4 and 9 over the study period with low values during the monsoon, in spite of large terrestrial input, which can be attributed to microbial modification of terrestrial SPOM. A significant positive correlation of δ 13 C with salinity suggests the reliability of δ 13 C as a biogeochemical tracer for organic matter provenance. Carbon: chlorophyll plots reveal suppressed autochthonous production during the southwest monsoon which may be attributed to the rapid flushing time, lightlimitation due to cloud cover and high water-column turbidity.

Measurements of total organic carbon (TOC) for two years in Kuwaiti waters showed high TOC levels (101.0–318.4, mean 161.2 μM) with maximal concentrations occurring within the polluted Kuwait Bay and decreasing offshore, indicating substantial anthropogenic component. Analysis of winter-time data revealed a large increase in density over the past four decades due to decrease in Shatt Al-Arab runoff, implying that the dissolved/suspended organic matter in surface waters of the northern Gulf could be quickly injected into the Gulf Deep Water (GDW). Our measurements together with an analysis of previously collected/published data suggest that the recent summer-time declining trend in oxygen in the GDW might be related to eutrophication. Higher preformed TOC and lower preformed dissolved oxygen contents of the high-salinity water mass that flows out of the Gulf and ventilates the mesopelagic oxygen minimum zone (OMZ) of the Northwestern Indian Ocean may cause expansion/intensification of the regional OMZ.

The Indian Ocean Dipole (IOD) and the El Niño/Southern Oscillation (ENSO) are independent climate modes, which frequently co-occur, driving significant interannual changes within the Indian Ocean. We use a four-decade hindcast from a coupled biophysical ocean general circulation model, to disentangle patterns of chlorophyll anomalies driven by these two climate modes. Comparisons with remotely sensed records show that the simulation competently reproduces the chlorophyll seasonal cycle, as well as open-ocean anomalies during the 1997/1998 ENSO and IOD event. Results suggest that anomalous surface and euphotic-layer chlorophyll blooms in the eastern equatorial Indian Ocean in fall, and southern Bay of Bengal in winter, are primarily related to IOD forcing. A negative influence of IOD on chlorophyll concentrations is shown in a region around the southern tip of India in fall. IOD also depresses depth-integrated chlorophyll in the 5–10° S thermocline ridge region, yet the signal is negligible in surface chlorophyll. The only investigated region where ENSO has a greater influence on chlorophyll than does IOD, is in the Somalia upwelling region, where it causes a decrease in fall and winter chlorophyll by reducing local upwelling winds. Yet unlike most other regions examined, the combined explanatory power of IOD and ENSO in predicting depth-integrated chlorophyll anomalies is relatively low in this region, suggestive that other drivers are important there. We show that the chlorophyll impact of climate indices is frequently asymmetric, with a general tendency for larger positive than negative chlorophyll anomalies. Our results suggest that ENSO and IOD cause significant and predictable regional re-organisation of chlorophyll via their influence on near-surface oceanography. Resolving the details of these effects should improve our understanding, and eventually gain predictability, of interannual changes in Indian Ocean productivity, fisheries, ecosystems and carbon budgets.

Variations in the Indian summer monsoon (ISM) intensity during the last 16.7 ka have been studied using organic carbon (Corg), δ 15 N of sedimentary organic matter, CaCO3, sediment texture, δ 18 OC and Mg/Ca derived sea surface temperature, δ 18 O of sea water and sea surface salinity, in a 14 C-dated sediment core from the eastern Arabian Sea. The δ 18 O in water and planktonic foraminifera shells off the central west coast of India may be controlled by the ISM intensity as this area receives high precipitation and land runoff. Also, the Corg and CaCO3 contents of sediments and δ 15 N of sedimentary organic matter may be linked to ISM-induced productivity and denitrification. The results of the present study reveal that between 16 and 15.2 ka BP the ISM was weak with minor fluctuations and started intensifying around 15.2 ka BP, at the onset of the Bolling-Allerod (B-A) event. The B-A event is characterized by higher water column denitrification rates comparable to the present day. The ISM signatures observed in the δ 18 OC record of B-A event compare well with those from Timta cave of the western Himalayas and also the Asian summer monsoon signatures from the Hulu caves in China and warming signatures in Greenland Ice Sheet Project 2 (GISP2) suggesting atmospheric tele-connections through Intertropical Convergence Zone (ITCZ). The boundary between the Younger Dryas and the Holocene is discernible with small episodes of abrupt events of increased ISM intensity. This decrease in δ 18 OC values at ~ 11.8 ka BP is contemporary with June solar insolation maximum at 30° north and the increase in methane in the GISP2 ice core supporting episodes of warmer climate and increase in ISM intensity. The ISM seems to have been most stable between 7 and 5.6 ka BP. The core exhibits periodicity of 500 that is comparable to the Atlantic water formation and the Chinese monsoon.

Reconstruction of paleo-redox conditions in a radiocarbon (14C) dated sediment core (SK-218/1), covering the past 45 ka (thousand calendar years), collected from the western Bay of Bengal (Lat: 14° 02′N; Long: 82° 00′E) at a water depth of 3307 m, has been made based on redox-sensitive element geochemistry. The high U/Th ratio, Mo enrichment, Mo/U enrichment factor ratio, Ce/Ce* We also provide the first record of the nitrogen isotopic composition (δ15N) of sedimentary organic matter in the western Bay of Bengal, a region where the mesopelagic oxygen minimum zone (OMZ) is just short of being suboxic (denitrifying) today. The sedimentary δ15N fluctuated considerably in the past, especially during the Marine Isotope Stage 3. Oscillations in δ15N were apparently in concert with those in organic carbon and nitrogen contents and could be related to climatic changes (Heinrich and Dansgaard–Oeschger events) in the North Atlantic. The Dansgaard–Oeschger 12 event appears to have exerted the most intense effect on Bay of Bengal biogeochemistry when surface productivity, as inferred from the organic carbon and nitrogen contents, was the highest recorded in the core, and the δ15N reached up to 6.3‰. Considering the probable dilution by isotopically light terrigenous organic matter, it would appear that OMZ of the Bay of Bengal had turned denitrifying. However, the absence of suboxic conditions in the sediments at this time suggests a decoupling of the benthic processes with those in the mesopelagic water column.

The mudbank formation along south-western Indian coast has been a scintillating phenomenon during monsoon season, and the source of mud is a debatable issue till now. In the midst of many hypotheses, the subterranean passage is considered a possible one but needs to have supportive scientific evidence. This present study endeavours to demonstrate the potential use of the Ground Penetrating Radar (GPR) in generating subsurface information to locate buried paleochannels in coastal zone which could act as the conduit for transport of mud from terrestrial sources to sea. The historical maps of the late 1600 s show that the present-day Vembanad Lake was a part of the nearshore area and the coastline was nearly 60 km inside, drained by the rivers from the Western Ghats. Due to shoreline migration towards the seaside since late Holocene, the present-day landforms came to existence while the earlier channels got buried under recent sand. Nearly 500-m-long GPR subsurface (6-m depth penetration) profile was generated, and various paleochannels were traced. These buried channels were categorised based on the pattern of their occurrences. Two erosional surfaces at around 1 and 2-m were mapped, and their controls on paleochannels were derived. During phase-I, many paleochannels were in active condition and due to heavy deposition during phase-II, though these channels managed to maintain themselves, but the width of their courses got narrowed and in recent times they got filled up and buried. The pre-monsoon and monsoon GPR profiles signify the high degree of wetness characteristics of these buried channels during monsoon season and their possibility of being passages for possible transport of water charged with fine clay materials through the unconsolidated in-filled material in these buried channels. This demonstrative and first-hand subsurface information through GPR studies would further support to facilitate in consolidating the subsurface evidence for strengthening the subterranean passage concept in the mudbank area of the south-western coast of India.

The relative grazing impact of Noctiluca scintillans (hereafter referred only Noctiluca) and copepods (Acrocalanus gracilis, Paracalanus parvus, Acartia danae and Oithona similis) on the phytoplankton community in an upwelling-mudbank environment along the southwest coast India is presented here. This study was carried out during the Pre-Southwest Monsoon (April-May) to the Late Southwest Monsoon (August) period in 2014. During the sampling period, large hydrographical transformation was evident in the study area (off Alappuzha, Southwest coast of India); warmer Pre-Southwest Monsoon water column condition got transformed into cooler and nitrate-rich hypoxic waters during the Southwest Monsoon (June-August) due to intense coastal upwelling. Copepods were present in the study area throughout the sampling period with a noticeable increase in their abundance during the Southwest Monsoon. On the other hand, the first appearance of Noctiluca in the sampling location was during the Early Southwest Monsoon (mid-June) and thereafter their abundance increased towards the Peak Southwest Monsoon. The grazing experiments carried out as per the food removal method showed noticeable differences in the feeding preferences of Noctiluca and copepods, especially on the different size fractions of phytoplankton. Noctiluca showed the highest positive electivity for the phytoplankton micro-fraction (av. 0.49 ± 0.04), followed by nano-fraction (av. 0.17 ± 0.04) and a negative electivity for the pico-fraction (av. -0.66 ± 0.06). In total ingestion of Noctiluca, micro-fraction contribution (83.7%) was significantly higher compared to the nano- (15.7%) and pico-fractions (0.58%). On the other hand, copepods showed the highest positive electivity for the phytoplankton nano-fraction (av. 0.38 ± 0.04) followed by micro- (av. -0.17 ± 0.05) and pico-fractions (av. -0.35 ± 0.05). Similarly, in total ingestion of copepods, nano-fraction (69.7%) was the highest followed by micro- (28.9%) and pico-fractions (1.37%). The grazing pressure of Noctiluca on the total phytoplankton was found to be 27.7% of the standing stock and 45.6% of the production, whereas in the case of copepods, it was 9.95% of the standing stock and 16.6% of the production. The study showed that the grazing pressure of Noctiluca on the total phytoplankton as well as larger phytoplankton fraction was 2.8- and 8-folds higher than that of the copepods. This suggests the leading role of Noctiluca as an effective grazer of larger phytoplankton along the southwest west coast of India, especially during the Peak/Late Southwest Monsoon.

Isotopic composition of nitrate (δ15N and δ18O) and particulate organic matter (POM) (δ15N and δ13C) were measured in Tillari Reservoir, located at the foothills of the Western Ghats, Maharashtra, western India. The reservoir that is stratified during spring-summer and autumn seasons but gets vertically mixed during the Southwest Monsoon (SWM) and winter is characterized by diverse redox nitrogen transformations in space and time. The δ15N and δ18O values of nitrate were low (δ15N = 2–10 ‰, δ18O = 5–8 ‰) during normoxic conditions but increased gradually (highest δ15N = 27 ‰, δ18O = 29 ‰) when anoxic conditions facilitated denitrification in the hypolimnion during spring-early summer. Once nitrate was fully utilized and sulphidic conditions set in, NH4+ became the dominant inorganic N species, with δ15N ranging from 1.3 to 2.6 ‰. Low δ15N (~−5 ‰) and δ13C (−37 ‰ to −32 ‰) of POM co-occuring with high NH4+ and CH4 in sulphidic bottom waters were probably the consequence of microbial chemosynthesis. Assimilation of nitrate in the epilimnion was the major controlling process on the N-isotopic composition of POM (δ15N = 2–6 ‰). Episodic low δ15N values of POM (−2 to 0 ‰) during early summer coinciding with the absence of nitrate might arise from N-fixation, although further work is required to confirm the hypothesis. δ13C-POM in the photic zone ranged between −29 ‰ and −27 ‰ for most parts of the year. The periods of mixing were characterized by uniform δ15N-NO3− and δ18O-NO3− at all depths. Higher POM (POC and PON) contents and C / N values with lower δ13C-POM during the SWM point to allochthonous inputs. Overall, this study, the first of its kind in the Indian subcontinent, demonstrates the potential of stable carbon and nitrogen isotopes to gain insight into biogeochemistry of Indian reservoirs where the monsoons play an important role in controlling vertical mixing and dynamics of carbon and nutrients.

The National Institute of Oceanography (NIO) with its headquarters at Dona Paula, Goa, and regional centres at Kochi, Mumbai and Visakhapatnam, is one of the 37 constituent laboratories of the Council of Scientific & Industrial Research (CSIR), New Delhi. CSIR-NIO was established on 1 January 1966 as the most important outcome of the International Indian Ocean Expedition (IIOE), the largest international collaborative effort ever undertaken in the oceans. The institute has since grown into a multidisciplinary oceanographic research institution of international repute. The principal focus of research has been on observing and understanding special oceanographic characteristics of the Indian Ocean.

A novel Gram-staining-negative, rod-shaped, non-motile bacterium, designated strain LW9T, was isolated from a water sample collected from Lonar Lake of Buldhana district, Maharashtra, India. Colonies and broth cultures were reddish orange due to the presence of carotenoid pigments. Strain LW9T was positive for catalase, ornithine decarboxylase and lysine decarboxylase activities and negative for gelatinase, oxidase, urease and lipase activities. The predominant fatty acids were iso-C15 : 0 (31.3 %), iso-C16 : 0 (9.3 %), anteiso-C15 : 0 (7.3 %), iso-C16 : 1 H (6.1 %), summed feature 3 (comprising C16 : 1ω7c/C16 : 1ω6c; 5.9 %), iso-C17 : 1ω9c (5.4 %) and iso-C17 : 0 3-OH (5.0 %). Strain LW9T contained MK-7 as the major respiratory quinone. The polar lipids consisted of phosphatidylethanolamine, two unidentified aminolipids and seven unidentified lipids. The DNA G+C content of strain LW9T was 40.5 mol%. 16S rRNA gene sequence analysis indicated that the type strains of Indibacter alkaliphilus and Aquiflexum balticum , two members of the family Cyclobacteriaceae (phylum ‘ Bacteroidetes ’) were the most closely related strains with sequence similarities of 93.0 and 94.0 %, respectively. Other members of the family Cyclobacteriaceae showed sequence similarities T is proposed as the representative of novel species in a new genus, Cecembia lonarensis gen. nov., sp. nov. The type strain of the type species, Cecembia lonarensis, is LW9T ( = CCUG 58316T = KCTC 22772T). Emended descriptions of the genera Indibacter , Nitritalea and Belliella are also proposed.

Phytoplankton and bacterial pigment compositions were determined by high performance liquid chromatography (HPLC) and liquid chromatography-mass spectrometry (LC-MS) in two freshwater reservoirs (Tillari Dam and Selaulim Dam), which are located at the foothills of the Western Ghats in India. These reservoirs experience anoxia in the hypolimnion during summer. Water samples were collected from both reservoirs during anoxic periods while one of them (Tillari Reservoir) was also sampled in winter, when convective mixing results in well-oxygenated conditions throughout the water column. During the period of anoxia (summer), bacteriochlorophyll (BChl) e isomers and isorenieratene, characteristic of brown sulfur bacteria, were dominant in the anoxic (sulfidic) layer of the Tillari Reservoir under low light intensities. The winter observations showed the dominance of small cells of Chlorophyll b-containing green algae and cyanobacteria, with minor presence of fucoxanthin-containing diatoms and peridinin-containing dinoflagellates. Using total BChl e concentration observed in June, the standing stock of brown sulfur bacteria carbon in the anoxic compartment of Tillari Reservoir was estimated to be 2.27 gC m−2, which is much higher than the similar estimate for carbon derived from oxygenic photosynthesis (0.82 gC m−2. The Selaulim Reservoir also displayed similar characteristics with the presence of BChl e isomers and isorenieratene in the anoxic hypolimnion during summer. Although sulfidic conditions prevailed in the water column below the thermocline, the occurrence of photo-autotrophic bacteria was restricted only to mid-depths (maximal concentration of BChl e isomers was detected at 0.2% of the surface incident light). This shows that the vertical distribution of photo-autotrophic sulfur bacteria is primarily controlled by light penetration in the water column where the presence of H2S provides a suitable biogeochemical environment for them to flourish.

Subsurface waters over the western Indian continental shelf experience seasonal anoxia towards the end of the southwest monsoon season. During a 3-day study carried out at the Candolim time series site (off the coast of Goa), dimethylsulphide (DMS) concentrations showed a 40-fold increase to a maximum of 442 nM at 25 m depth compared to the oxygenated surface waters. This extremely high DMS was found to be associated with relatively low chlorophyll a, low phytoplankton cell counts and a high concentration of hydrogen sulphide. However, total dimethylsulphoniopropionate, total dimethylsulphoxide and methanethiol concentrations were quite low and unlikely to account for the DMS build-up through presently known pathways of DMS production. While there are several possible mechanisms for the observed accumulation of DMS, we were unable to pinpoint the exact pathway of DMS production. Future work will involve investigation of the source of DMS through sediment slurry experiments, to explore this interesting link between the carbon and sulphur cycles under anoxic conditions.

Eukaryotic phytoplankton such as diatoms and prymnesiophytes produce biogenic halocarbons in the ocean that serve as important sources of chlorine and bromine to the atmosphere, but the role of cyanobacteria in halocarbon production is not well established. We studied distributions of chloroform (CHCl3), carbon tetrachloride (CCl4), methylene bromide (CH2Br2) and bromoform (CHBr3) in relation to phytoplankton composition, determined from pigment analysis complemented by microscopic examination, for one month in coastal waters of the eastern Arabian that experienced a Trichodesmium bloom that typically occurs during the Spring Intermonsoon season. High concentrations of zeaxanthin (23 μg l−1), alpha beta betacarotene (6 μg l−1) and chlorophyll a (67 μg l−1) were found within the bloom whereas the marker pigment concentrations were low outside the bloom. CHCl3 and CCl4 occurred in relatively high concentrations in surface waters whereas CH2Br2 and CHBr3 were restricted to the subsurface layer. Chlorinated halocarbons were positively inter-correlated and with CHBr3. The observed spatial and temporal trends in brominated compounds appear to be related to the abundance of Trichodesmium although correlations between concentrations of brominated compounds with various marker pigments were poor and statistically non-significant. The results support the existence of multiple sources and sinks of halogenated compounds, which might obscure the relationship between halocarbons and phytoplankton composition.