Wilde, Hendrik Pieter Jacobus de

English

Proceedings - University of Groningen

   University of GroningenNitrous oxide and methane in marine systems.Wilde, Hendrik Pieter Jacobus deIMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:1999Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Wilde, H. P. J. D. (1999). Nitrous oxide and methane in marine systems. s.n.CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 12-10-2022CHAPTER 6Summary and concluding remarksThe present increase in the atmospheric concentrations of nitrous oxide (N2O) andmethane (CH+) is of concern since together these gases account for about 20o/a of theenhanced global warming due to human activit ies. In addition N2O is also involved inthe destruction of the stratospheric ozone layer. In order to understand the causesresponsible Íbr the atmospheric increases, all sources and sinks of N2O and CH4 need tobe quantified. The aim of this thesis is to contribute to a better assessment of the marineenvironment as a source of atmospheric N2O and CH+, thereby focusing on the role o1'estuaries, coastal waters, and upwelling regions. Research concentrated on: (l) thequantif ication of the distribution of N2O and CH4 in contrasting marine environments;(2) the assessment of the resulting gas ernissions to the atmosphere; and (3) theidentif ication of the microbial processes underlying the emissions. The main findings ofthis research are summarised below, and are followed bv concluding remarks.N2O in the northwest Indian OceanA study on N2O was carried out in the Somali Basin, northwest Indian Ocean, duringthe height of the southwest monsoon season. The strong monsoon winds, inducingupwelling of cold subsurface water off the Somalian coast, had a rnajor impact on thebiogeochemistry of the region. The vertical N2O distribution over the water column ofthe Somali Basin exhibrted two maxima: ( I ) a sharp subsurface maximum at about 150m depth, characterised by N2O supersaturations of up to - 10007o with respect toatmospheric solubil ity, and (2) a broad deep water maximum around 800 m depth,characterised by saturations up to -800Vo. Below the wind mixed layer, N2O wasalways negatively correlated with oxygen and positively with nitrate, indicating thatN2O was produced by nitrif ication. The average yield of N2O production, relative tonitrate production, was approximately 0. I7Vo. At oxygen concentrations below l5-20pmol dm-3, the N2O yield during nitrification increased to about 0.35Vc. Denitrificationwas of minor importance, since the oxygen concentration in the Somali Basin neverdropped below approximately l0 prmol dm-J.1 t 4 Chapter 6The NzO saturation in the suríace waters was inversely correlated with temperature,indicating that monsoon-driven upwelling of cold N2O-rich water controlled thesaturation degree of N2O in surface waters. Outside the upwelling region. surfacewaters were only slightly supersaturated with respect to the atmosphere, whereas theN2O saturation in freshly upwelled waters amounted to 3307a. The strong monsoonwrnd driving the upwelling, also induced strong vertical mixing and effective air-seagas exchange. As a consequence, N2O emissions into the atmosphere reached values aslrigh as 260 to 500 pmol m-2 d-l, which is nearly 3 orders o[ nragnitude above theglobally mean oceanic N2O Í1ux. By using temperature as an indicator of upwelling,satellite images of sea surface temperature were applied to extrapolate the observedN2O emissions in space and time. It was estimated that during the two months of mostintense upwelling, the Somalian upwelling region may account for 2.5 - 57o of thegf obal marine N2O emission, from an area of less than 0.0llVo of the world ocean.N2O in the Scheldr esturryDissolved N2O was studied in the Scheldt estuary in relation to the distributions ofnitrif ication activity, oxygen, NH+*, and NO3-, along the estuarine salinity gradient.The Scheldt estuary is characterised by elevated loadings of organic matter andnutrients, which induce intense mineralisation, and associated epletion of oxygen inthe upper estuary. Dissolved N2O was always supersaturated with respect to theatmosphere. Saturation ranged from about 30007o in the oxygen-poor waters at lowsalinit ies to slight supersaturation at the mouth of the estuary. N2O production largelyoriginated from nitrif ication in the water column. The location and intensity ofnitrification was controlled by: (1) the oxygen gradíent along the estuary, ranging fiomstrong undersaturation at low salinities to complete saturation at sea, and (2) the NHa+concentration of the river watet-entering the estuary. In the oxygen-poor waters at lowsalinities, the highest nitrification activity was about 6 pmol N dm-3 h-1. The N2O yield,associated with nitrif ication, varied spatially and seasonally. Between 0.1 and 0.47o ofthe oxidised NH4+ was converted to NrO, rather than to NO3-. These yields are almostsimilar to the values observed in the Indian Ocean, as discussed before. A comparisonwith a data set from 1978, reported in literature, indicated that the main locus of N2Oproduction has shifted upstream during the last decades, in response to the graduallydecreasing eutrophication and increasing oxygen concentrations in the estuary. Due tothe residence time of about two months, nearly all N2O was lost to the atmospherewithin the estuary rather than being transported to the open sea. The average N2O fluxper  uni t  area amounted to about  75 pmol  m-2 d-1.  which is  comparable to the f luxes inthe Somalian upwelling region. but larger than values reported for most other estuaries.Theb e 2emiestirCH.Ci n isup(tranl2 r, ) wr.10.30%remiproco r i o1one(46nortp lutparet imrsuprtheEStUbutsed.ExtthalthisliftlCotISI ITSummary and concluding remarksThe mean annual emission from the Scheldt estuary to the atmosphere was estimated tobe 2.8 x 108 g. Extrapolation to the global scale indicated that the world-wide estuarineemission rnay be as large as 1.5 Tg yr-1. cc'rresponding to about 25o/o of the presentestimates of the global marine N2O source.CH,t in the Rhine esturrvConcentrations of CH4 were measured in the Rhine estuary, its lower tributaries, andin its plume off the Dutch coast during 4 seasons. Surface waters were alwayssupersaturated with CH4 rclative to the atmospheric concentration, along the entiretransect from river to the coastal sea. Highest CH4 concentrations (0.45 - 0.85 pmol drn-3; were measured in the low salinity region of the estuary (S = l-4), corresponding to140-330 times supersaturation, which is high compared to other estuaries. Only about30Vo oÍ the CH4 in the estuary was supplied by its major tributaries, whereas thcremaining ' l}Vo was produced in situ, l ikely by the estuarine sedirnents. The in situproduction in the estuary was positively related to temperature. suggcsting that CH.1originated from a microbial source.The errission of CHa fiorn the estuary to the atmosphere (6 - l2 kmol d-1.) was aboutone order ol'magnitude lower than the CH4 export to the coastal zone of the North Sea(46 - 1l I krnol d-l). In the coastal zone, thc estuarine water mass was t.ransportednorthward by the residual current in a coastal plurne extending 25 km offshore. In thisplume, the distribution pattern of CHa followed the freshwater fl 'action, with contoursparallel to the shoreline. Coastal CH4 concentrations ranged frorn 250 nmol dm--l 1-100times supersaturated) near the mouth of the Rhine estuary, down to 3 nmol 4p-3 lslightsupersaturation) outside the plume of the Rhine. The atmospheric loss in the plume ofthe Rhine (183 - 288 kmol d') was 3 to 4 times higher than the CH4 exported by theestuary. Consequently, most of the coastal CH4 was not supplied by the Rhine estuary,but rather by the coastal sediments. Per unit area, the calculated CHa supply by coastalsediments is 2 orders of magnitude lower than the CHa supply by estuarine sediments.Extrapolation of the CH4 emission from the Rhine estuary to the global scale suggeststhat the world-wide estuarine CH4 emission amounts to 0.4 - 1.4 Tg CHa yr-1. Althoughthis emission corresponds to about 10Vo of the total marine CH4 source, it contributeslitt le to the global CH4 budget.Comparison of air-sea CH4 fluxes derived by tw'o methodsOn a research platform off the Dutch coast, air-sea exchange of CH4 wassimultaneously quantif ied by two different rnethods. According to the first method,l l 5.h temperature,controlled theegion, surfacee, whereas therong monsoon'fective air-searched values asude above the' of upwelling,; the observednonths of most.5 - 57o of therld ocean.Jistributions ofl inity gradient.ric matter andn of oxygen inrespect to thewaters at lowduction largelyd intensity of,, ranging fromC (2) the NH4+r waters at 10wIhe N2O yield,.I and 0.4Va ofelds are almostA comparisonr locus of N2Or the gradually)stuary. Due tohe atmosphere:rage N2O fluxto the fluxes inother estuaries.Chapter 6l l 6lluxes were calculated from the measured concentration difference across the air-seainterÍ-ace, multiplied with a transÍ'er velocity derived from windspeed on the basis of anempirical relationship. This method underlies alI present estimates of marine trace gasemissions. According to the second method, the gradient technique, fluxes were derivedfrom the product of the turbulent exchange coeÍficient in the atmosphere and thevertical CH4 gradient in the air above the sea surface. Fluxes derived by the gradientmethod were on average 6 times larger then fluxes based on measured air-seaconcentration differences and estimated transfer velocities. This discrepancy, well-known from similar studies on CO2, cannot be explained by experimental errors, nor bylocal variabil ity in CHa concentrations. More sirnultaneous measurements by bothmethods are required to resolve this issue.CONCLUDING REMARKSF ltrres and extrapo I aÍionsAll studies described in this thesis confirm the hypothesis that a relatively largeproportion of the global marine emission originates from estuaries. coastal waters, andupwelling regions, although the relative importance of the sources differs for N2O andCHa.Estuaries were estimated to contribute about 257o t"o the total marine N2O source,and about l07c Ío the total marine CH4 source. However, estuaries are a much moreimportant factor in the global N2O budget than in the global CH4 budget, since theyacrcount fbr about 60/o of the total N2O source, whereas their contribution to the totalCH4 source is only about 0.2c/a, due to the overwhelming importance of terrestrialsources.The study on N2O in the Somalian upwelling region suggests that the role ofupwelling regions as sources of atmospheric N2O may sti l l  be underestimated. Bange etal. (1994) reported that upwcll ing regions contribute about 3%, to the global marineN2O emission. Given our estimate that the Somalian upwelling region alone mayaccount Íbr 2.5 - 57o of the global marine N2O emission during the two months of mostintense upwelling, it is l ikely that the estimated emissions from upwelling regions needupward revision. This conclusion is supported by the l imited data from other upwellingregior . rs  (e.g.  Law and Owens,  1991;Codispot iet  a l . ,  1993).  Exis t ing est imares of  N2Oernissions from wind driven upwelling regions have been derived by combinin g in situN2O saturat ions wi th annual  mean wind speeds.  This approach may resul t  inunderestirnation of the N20 emission, since upwelling of N2O-rich water is coupled tohigh wind speed in these areas. Therefore future research in upwelling regions shouldfbcustransemlsal louestllrFacttTIlocatprodrpositiNithe ScharanitriÍ:lowerthe athat IT1concrequmattmagcH.emiÍutLIaptN2prccalreÍthttwSummary and concluding remarks ttlthe air-seabasis of anre lrace gasere derived:re and thehe gradientred air-searncy,  wel l -rors, nor byrts by bothively largewaters, andlr N2O and[2O source,much moresince theyto the totalI terrestrialrhe role ofd. Bange et,bal marinealone mayths of most:gions needr upwellingrtes of N2Oning ln sira/  resul t  incoupled toLons houldÍbcus on the height of the upwelling season, when the combined occurrence of high gastransÍèr velocities and rapid resupply of N2O from subsurface layers sustains huge N2Oemissions to the atmosphere. Finally, satell i te images of sea surface temperature.allowing extrapolation of fluxes in space and time, can be an important tool to improveestimates of the emission Íiom upwelling regions.F acto rs c o nt rol lin g gas produc tiottThis  thesis  ind icates that  d issolved oxygen is  a key parameter  contro l l ing thelocation, the rate, and the efficiency of the microbial processes responsible for theproduction of N2O and CH4. In addition, production oÍ'NzO and CH4 was shown to bepositively related to the availability oforganic carbon and nutrients.Nitrification in the water column was the dominant source of N2O production in boththe Somali Basin and the Scheldt estuary, since these contrasting environments werecharacterised by the same range of oxygen concentrations. The N2O yield duringnitrif ication in both systems was almost equal, with highest N2O yields coinciding withlowest oxygen concentrations. In addition, N2O production in the Scheldt was related tothe availabil ity of NHa+, supporting the hypothesis of Firestone and Davidson (1989)that N2O production increases with increasing availability of N-compounds.The study on CH4 in the Rhine estuary indiqated that CH4 production was largelyconcentrated in the sediments, as was expected from the strict anoxic conditionsrequired for CH4 production. It was argued that the abundant availabil ity of organicmatter in the estuarine sediments induced CH4 production rates that were two orders ofmagnitude higher than CH4 production in sandy coastal sediments. Nevertheless, theCH4 emission from the coastal plume of the Rhine was much higher than the CH4emission Íiom the estuary itself, because of its larger aerial coverage. Consequently,future research in estuaries hould explicit ly include studies in the coastal plumes.Finally. this thesis showed that different marine systems require different analyticalapproaches. In the open ocean, mostly characterised by slight supersaturation of bothN2O and CHa, it is vital to optimise the accuracy of the analytical-computationalprocedures, since a small bias in absolute concentrations can induce huge errors in thecalculated fluxes. In contrast, highly dynamic systems, l ike estuaries and upwellingregions, require sampling with high resolution and fast analytical equipment. to coverthe large spatial and temporal variation in concentrations, which may span more thantwo orders of maenitude.

Nitrous oxide and methane in marine systems.

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Nitrous oxide and methane in marine systems.

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Proceedings - University of Groningen