Coastal versus open-ocean denitrification in the Arabian Sea

International audienceThe Arabian Sea contains one of the three major open-ocean denitrification zones in the world. In addition, pelagic denitrification also occurs over the inner and mid-shelf off the west coast of India. The major differences between the two environments are highlighted using the available data. The perennial open-ocean system occupies two orders of magnitude larger volume than the seasonal coastal system, however, the latter offers more extreme conditions (greater nitrate consumption leading to complete anoxia). Unlike the open-ocean system, the coastal system seems to have undergone a change (i.e., it has intensified) over the past few decades presumably due to enhanced nutrient loading from land. The two systems also differ from each other with regard to the modes of nitrous oxide (N2O) production: In the open-ocean suboxic zone, an accumulation of secondary nitrite (NO2?) is invariably accompanied by depletion of N2O whereas in the coastal suboxic zone high NO2? and very high N2O concentrations frequently co-occur, indicating, respectively, net consumption and net production of N2O by denitrifiers. The extents of heavier isotope enrichment in the combined nitrate and nitrite (NO3?+NO2?) pool and in N2O in reducing waters appear to be considerably smaller in the coastal region, reflecting more varied sources/sinks and/or different isotopic fractionation factors

The Arabian Sea as a high-nutrient, low-chlorophyll region during the late Southwest Monsoon

© The Authors, 2010. This article is distributed under the terms of the Creative Commons Attribution 3.0 License. The definitive version was published in Biogeosciences 7 (2010): 2091-2100, doi:10.5194/bg-7-2091-2010.Extensive observations were made during the late Southwest Monsoon of 2004 over the Indian and Omani shelves, and along a transect that extended from the southern coast of Oman to the central west coast of India, tracking the southern leg of the US JGOFS expedition (1994–1995) in the west. The data are used, in conjunction with satellite-derived data, to investigate long-term trends in chlorophyll and sea surface temperature, indicators of upwelling intensity, and to understand factors that control primary production (PP) in the Arabian Sea, focussing on the role of iron. Our results do not support an intensification of upwelling in the western Arabian Sea, reported to have been caused by the decline in the winter/spring Eurasian snow cover since 1997. We also noticed, for the first time, an unexpected development of high-nutrient, low-chlorophyll condition off the southern Omani coast. This feature, coupled with other characteristics of the system, such as a narrow shelf and relatively low iron concentrations in surface waters, suggest a close similarity between the Omani upwelling system and the Peruvian and California upwelling systems, where PP is limited by iron. Iron limitation of PP may complicate simple relationship between upwelling and PP assumed by previous workers, and contribute to the anomalous offshore occurrence of the most severe oxygen (O2) depletion in the region. Over the much wider Indian shelf, which experiences large-scale bottom water O2-depletion in summer, adequate iron supply from reducing bottom-waters and sediments seems to support moderately high PP; however, such production is restricted to the thin, oxygenated surface layer, probably because of the unsuitability of the O2-depleted environment for the growth of oxygenic photosynthesizers.Financial support was provided by CSIR through the Network Project CMM0009 to SWAN and by NSF through OCE-0327227S to JWM

Nitrogen isotopic studies in the suboxic Arabian Sea

Measurements of 15N/14N in dissolved molecular nitrogen (N2), nitrate (NO3- ) and nitrous oxide (N2O) and 18O/16O in N2O [expressed as δ15N and δ18O, relative to atmospheric N2 and oxygen (O2), respectively] have been made in water column at several locations in the Arabian Sea, a region with one of the thickest and most intense O2 minima observed in the open ocean. Microbially-mediated reduction of NO3- to N2 (denitrification) in the oxygen minimum zone (OMZ) appears to greatly affect the natural isotopic abundances. The δ15N of NO3- increases from 6‰ in deep waters (2500 m) to 15‰ within the core of the denitrifying layer (250-350 m); the δ15N of N2 concurrently decreases from 0.6‰ to 0.20‰ Values of the isotopic fractionation factor (e) during denitrification estimated using simple advection-reaction and diffusion-reaction models are 22‰ and 25‰, respectively. A strong decrease in δ15N of NO3- is observed from ~ 200m (> 11‰) to 80m (~ 6‰); this is attributed to the input of isotopically light nitrogen through nitrogen fixation. Isotopic analysis of N2O reveals extremely large enrichments of both 15N and 18O within the OMZ, presumably due to the preferential reduction of lighter N2O to N2. However, isotopically light N2O is observed to accumulate in high concentrations above the OMZ indicating that the N2O emitted to the atmosphere from this region cannot be very heavy. The isotope data from the intense upwelling zone off the southwest coast of India, where some of the highest concentrations of N2O ever found at the sea surface are observed, show moderate depletion of 15N, but slight enrichment of 18O relative to air. These results suggest that the ocean-atmosphere exchange cannot counter inputs of heavier isotopes (particularly 18O) associated with the stratospheric back flux, as proposed by previous workers. This calls for additional sources and/or sinks of N2O in the atmosphere. Also, the N2O isotope data cannot be explained by production through either nitrification or denitrification, suggesting a possible coupling between the two processes as an important mechanism of N2O production

Behaviour of boron, calcium and magnesium in Purna and Auranga estuaries (Gujarat), west coast of India

46-50Ratios of B:Cl, Ca:Cl and Mg:Cl were generally within limit of river water at one end and seawater at the other. Percentage addition or removal (PA & PR) of B, Ca and Mg was calculated using the concept of the theoretical dilution line. High percentage of removal of B at low chlorinities indicated its rapid removal in the initial encounter of river water with seawater. On the contrary, Ca was preferentially added at low chlorinities. Addition decreased as the chlorinity increased and removal was observed near to seawater chlorinity. Mg was more conservative when compared to B and Ca

Boron-chlorinity relationship in western Bay of Bengal

183-185Significant scatter in B/Cl beyond the limit of experimental error indicates that the ratio is not truly constant. B/Cl maximum, centred around 100 m, appears to be influenced by colloidal and particulate matter while the maximum at 400 m appears to be due to the influence of Persian Gulf and Red Sea water and also due to the release of boron by oxidative degradation of organic matter. At 3000 m depth the ratio is typical of Antarctic bottom water

Behaviour of Boron, Calcium & Magnesium in Mindola river Estuary(Gujarat)

90-92The well mixed Mindola River estuary experiences considerable tidal influence with a tidal range of about 3.5 m in the mouth region which reduces to 0.9m at a point 21 km upstream during neap tide. Boron is preferentially removed over the entire tidal cycle; the maximum removal being 9%. On the contrary calcium and magnesium are preferentially added over the same period though to a small extent. An inverse relationship between calcium and magnesium is probably indicative of ion-exchange reaction between calcium in clays of riverine origin and magnesium in water when the river water encounters sea water

Δ<SUP>14</SUP>C, ΣCO<SUB>2</SUB> and salinity of the Western Indian Ocean Deep waters: spatial and temporal variations

Indian Ocean Deep waters (IDW) show an increase in salinity and &#931;CO<SUB>2</SUB> between 64&#176;S and 16&#177;4&#176;N by 0.106-0.110&#8240; and 82-104 &#956;M kg<SUP>&#8722;1</SUP> respectively with a corresponding decrease in &#916;<SUP>14</SUP>C by 42&#8240; as revealed by the GEOSECS data (1977-78). The &#916;<SUP>14</SUP>C-&#931;CO<SUB>2</SUB>-Salinity relationships show better correlation in the western sector. High biological productivity induced changes and corrosive deepwaters could account for &#931;CO<SUB>2</SUB> increase in the northern regions. Reoccupation of GEOSECS stations during 1985-95 do not show distinctly discernible short-term temporal changes in &#916;<SUP>14</SUP>C, &#931;CO<SUB>2</SUB> and salinity

Water Quality of the River Damanganga (Gujarat)

94-99Water quality (pH, suspended solids, chlorides, DO, BOD, reactive and total phosphorus, nitrates and boron) of River Damanganga which receives 0.2 mld of industrial waste into its fresh water zone through Pimparia River and 3.7 mld in its tidal zone was studied at 10 stations, along a 35 km stretch. Riverine flow decreased considerably in the premonsoon season and became almost negligible after April. Strong tidal effect with a water rise >5.5 m was observed near the mouth of the river during spring tides which decreased to 3.5 m during neap tides. Tidal influence was weak at upstream stations and the industrial effluent was discharged in a zone where appreciable tidal influence occurs only during spring tides. The flood and ebb currents were fairly strong. Quality of water in the discharge zone deteriorated considerably after March (DO decreasing to about 1 mg/litre). High acid content of the effluent lowered pH of water. The discharge in the fresh water zone, presently did not affect the water quality adversely. The flushing time for the estuary was calculated to be 3 days, during February. The suspended load within the estuary was high. High concentrations of phosphate at some estuarine stations were associated with the waste water discharge

Concentration and toxicity of some metals in zooplankton from nearshore waters of Bombay

181-183Zooplankton samples collected from 4 stations located in the nearshore waters of Bombay were analysed for Cu, Co, Mn, Ni and Cd. Concentrations of Co, Mn and Ni were more in copepods and gelatinous organisms than in mysids and decapods. High concentrations of Ni and Co were noticed in zooplankton from Thana Creek which receives considerable amount of industrial effluents. Concentration of Cu and Mn was more in zooplankton obtained from offshore area. Cd concentration was invariably constant in zooplankton collected from all stations. Bioassay tests were carried out for evaluating the acute toxicity of Cu and Ni on selected groups of zooplankton. Cu was more toxic than Ni. Among the different organisms tested Sagitta, Lucifer, Ctenophores and Medusae were relatively more sensitive than Acetes and mysids