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1 Climate Sciences Sciences Dynamics Chemistry of the Past Solid Earth Techniques Geoscientific Methods and and Physics and Atmospheric Atmospheric Atmospheric Data Systems Geoscientific Earth System Earth System ¨ O. This study provides Measurement urich, Switzerland Instrumentation Hydrology and 2 O to the overlying water Ocean Science Annales Biogeosciences The Cryosphere 2 Natural Hazards Natural Hazards and Earth System and Earth O in lakes are essentially un- Model Development 2 Geophysicae O, without complete denitrification 2 , and M. F. Lehmann 1 , and the highest observed rates coinciding N label flow-through sediment incubations in 2 4970 Open Access Open Access Open Access Open Access Open Access Open Access Open Access Open Access Open Access Open Access Open Access Open 4969 Access Open Access 15 −

m over a 100-yr time horizon (Forster et al., 2007). 1 2 − ¨ urich, Sonneggstrasse 5, 8092 Z , C. H. Frame O throughout the year, with production rates ranging O h Climate 1 2 O fluxes and the contributions of the microbial pathways 2 ) reduction via denitrification was found to be the major Sciences Sciences 2 − 3 Dynamics Chemistry of the Past Solid Earth O flux. A significant portion (up to 15 %) of the total NO Techniques Geoscientific Methods and 2 and Physics and Advances in Atmospheric Atmospheric Atmospheric O fluxes to the atmosphere during seasonal mixing events. Data Systems Geoscientific Geosciences Earth System Earth System 2 Measurement Instrumentation Hydrology and Ocean Science Biogeosciences The Cryosphere Natural Hazards Natural Hazards O production in a eutrophic, EGU Journal Logos (RGB) EGU Journal Logos and Earth System and Earth 2 Model Development , C. B. Wenk O) is a potent greenhouse gas, generated through microbial nitrogen O) is a potent greenhouse gas with a global warming potential that is 1,* 2 2 O were assessed using 2 reduction is the main pathway for x . These fluxes were highest when the bottom water had completely stabilized to 2 O production pathway in the sediments under both oxygen-depleted and oxygen- 300 times higher than that of CO 2 This discussion paper is/has been under review for the journal Biogeosciences (BG). Please refer to the corresponding final paper in BG if available. Department of Environmental Sciences, University of Basel, Bernoullistrasse 30, 4056 Basel, now at: Institute of Geology, ETH Z Nitrous oxide (N ∼ to N a low-oxygen state, inducive contrast to with complete the denitrificationevidence notion that without that lake accumulation stable sediments anoxia ofand are may is N produce a particularly large significant N con- source of N 1 Introduction ing bottom water. Nitrate (NO N replete conditions in the overlying water,contribute while ammonium to oxidation did the not benthic significantly consumed N by denitrification was reduced only to N been widely neglected. Furthermore,rates and the the biogeochemical microbial pathways controlsknown. that In produce on this benthic the study, N benthic N production that produce N the eutrophic, monomictic south basinwere of a Lake significant Lugano source inbetween of Switzerland. 140 The N and sediments 2605 nmolwith N periods of water column stratification and stably anoxic conditions in the overly- Nitrous oxide (N (N) turnover processes, such asPrevious nitrification, studies nitrifier quantifying denitrification,ocean, and natural denitrification. but sources the have potential mainly role of focused terrestrial on water soils bodies and in the the global N Abstract Received: 18 February 2013 – Accepted: 4Correspondence March to: 2013 C. – V. Freymond Published: ([email protected])M. 12 and F. March Lehmann 2013 ([email protected]) Published by Copernicus Publications on behalf of the European Geosciences Union. Biogeosciences Discuss., 10, 4969–4993, 2013 www.biogeosciences-discuss.net/10/4969/2013/ doi:10.5194/bgd-10-4969-2013 © Author(s) 2013. CC Attribution 3.0 License. 1 Switzerland * benthic N NO monomictic south-alpine lake C. V. Freymond 5 25 15 20 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 2 O 2 OH), 2 O fluxes O under 2 2 O through 2 O production and 2 O sources (Forster 2 O is a free intermediate 2 O production (Otte et al., O, releasing N O is produced during the O concentration is largely 2 2 2 2 rapidly in the topmost mil- ) availability (Ward, 2008). .N 2 2 O (Wrage et al., 2001). The 2 2 concentration in the overlying O production pathways in the 2 2 O (Knowles, 1982) under anoxic ) when hydroxylamine (NH 2 − 2 , Devol, 2008; Codispoti et al., 2001). 1 O reductase enzyme, e.g. under low O 2 − O source appears to be higher under low- ects, such as eutrophication, acidification, 2 ff ) and oxygen (O 4972 4971 + 4 O flux is often challenging in aquatic systems reduction in the redox transition zone (coupled 2 ) to nitrite (NO − 3 + 4 2–5 µmol L < ] 2 concentration is reflected in a narrower oxygenated zone in 2 to nitric oxide (NO), and then N − 2 does not seem to inhibit nitrifier denitrification to the same extent or O is the most important stratospheric ozone-depleting substance cur- 2 2 erent types of microorganisms perform these pathways under overlapping via the gaseous intermediates NO and N ff O accumulation in aquatic environments (Naqvi et al., 2000; Codispoti et al., 2 2 penetration depth is closely related to the O 2 O. They also highlight that factors influencing N to N 2 conditions (Ritchie and Nicholas, 1972; Poth and Focht, 1985). However, unlike − 3 In lacustrine sediments, microbial activity consumes O Distinguishing the relative contributions of each of these major N 2 from freshwater sediments, indicateof that N lake sediments can be a significant source crease in bottom water O the sediment (Rasmussen and Jorgensen,an 1992). equally Narrow narrow redox succession zonation of leads microbialin thus processes the to (Stockdale redox et zonation al., may 2009). have1996). Changes profound Seasonal consequences cycles on of N water columntion mixing state and of stagnation can surface influence sedimentsthe the and sediments, oxida- modulate potentially the changing penetration the(Rasmussen of redox environments and redox of boundaries Jorgensen, nitrifiers into 1992). and The denitrifiers few studies that have quantified N limeters, leading to suboxicdenitrification or becomes anoxic an important conditions redox in processThe (Hunting deeper and O sediment van der horizons, Geest, 2011). where water and the sediment reactivity (Lehmann et al., 2009; Thibodeau et al., 2010). A de- NO or suboxic conditions (i.e.Denitrification [O can both produceconditions that and suppress consume the dissolved activityconcentrations (Firestone of N et the al., N 1979;between Otte et oxic al., and 1996). Furthermore, suboxiccauses rapid conditions N transitions may cause2001). “stop and go” denitrification, which quentially reduces NO importance of nitrifier denitrificationO as a N denitrification, O through the same mechanisms (Kool et al., 2011). Denitrification is the reduction of a second mechanism known as nitrifier denitrification, an enzymatic pathway that se- nitrification-denitrification, Ward, 2008). Ammonia oxidizers also produce N consumption pathways to thebecause di total N environmental conditions. Furthermore, certain microbes carryway out in more response than to one path- changesaerobic in biogeochemical oxidation conditions. of N ammonium (NH an intermediate in thetion reaction, depend decomposes primarily (Stein, 2011). onHowever, in Rates substrate sediments, of (NH aerobic ammonia ammonia oxida- be oxidation closely and coupled nitrite to oxidation anaerobic to NO nitrate can N along the land-ocean continuum byin reducing this it process back that to N mayis be also released produced to during the environmentas other under nitrification N certain (specifically, transformation ammonia conditions. reactions oxidation) N (Galloway and et nitrifier al., denitrification. 2003) such et al., 2007). However, the recentdue increase to in atmospheric human N interventionuse in of synthetic the nitrogen-based fertilizers nitrogen (CodispotiMosier cycle, et al., et in 2001; al., Bouwman particular et 1998). through al., Inhave 2002; the aquatic multiple agricultural systems, detrimental anthropogenic environmental fixedand e nitrogen the reduction (N) of loading biodiversity can bial (Galloway processes et taking al., place 2003). in In redoxfixed these transition N. zones environments, Denitrification, play micro- for an example, important can be role an in important removing mechanism for removing fixed rently being emitted toconcentration the has atmosphere increased (Ravishankara from et2011 270 al., ppb (AGAGE, 2009). in 2012), The 1750 but atmospheric (Forster therecontributions et of are the al., still major 2007) large sources tocesses uncertainties and 323 sinks with in ppb (Forster regard in soils et to al., and the 2007). the relative Microbiological ocean pro- are the most important natural N Furthermore, N 5 5 25 15 20 10 25 15 20 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | , 2 O O 2 2 ects of variable bottom wa- ff O accumulation in the deep 2 E) west of the village of Figino, 0 N labeling to assess benthic N 15 54 O fluxes from the sediments to the ◦ 2 N, 8 0 57 ◦ 4974 4973 O production in a lake. Furthermore, previous mea- 2 O producing processes in the lake, and (3) to study the 2 and a maximum depth of 95 m (Barbieri and Polli, 1992). 3 O production pathways in lacustrine sediments were studied using O accumulation in near bottom waters. Upon water column overturn 2 2 O-laden bottom waters may be advected to the surface, enhancing N 2 O production rates and pathways. 2 50cm sediment cores with 20 cm overlying bottom water were taken with a 5.7 cm ∼ The SB mixes completely from January/February to April. It hasLazzaretti an area and of Hanselmann 20.3 km (1992) and Lehmann et al. (2004a,b) described in de- The study’s objectives were (1) to estimate N In this study, N close to the pointgust, October) of and maximum 2011 (January, water May). The depth cores were of stored this upright and basin in (Fig. the dark 1) in 2010 (April, Au- hypolimnion decrease, and bywithin June/July, the the sediments redox by transition several meters zonein into the has the water bottom migrated column. waters Complete from until anoxia prevails the water column turns2.2 over again in Sampling winter. Six diameter mini gravity corer at a site (45 tail the changes in seasonalperiod, redox the conditions whole in water the columnthe southern becomes sediment/water basin. oxygenated and During interface oxic the until conditionsgenerally mixing late are in found April, spring. at together With withters the the and increased onset organic phytoplankton of matter production thermo-stratification, export in to surface wa- the hypolimnion, oxygen concentrations in the deep basin. In addition, aonly smaller outlet, sub-basin the of Tresa thethe River southern (Barbieri limited basin and water is Polli, exchange,that located 1992; the the in Lehmann northern front basins et and of al., arethat southern the 2004a). characterized basins are Due by can connected to be a bylimnology regarded distinct a can as be limnology, narrow two found so separate opening in lake at Barbieri systems and Melide. Polli A (1992) detailed anda Barbieri volume overview and of of Simona 1.14 km the (2001). lake’s 2.1 Site description Lake Lugano is locatedabove in sea southern level Switzerland/northern (Fig. 1). Italy It at is an divided altitude into of two 271 m main basins, the northern and southern 2 Sampling and methods overlying water column and totify assess the seasonal dominant variations benthic inpossible N these impact fluxes, of (2) variable to redoxcle conditions iden- on of N bottom waters during the seasonal cy- hypolimnion during thermal stratificationto in what summer causes N and fall,in begging winter, the N questionfluxes as into the atmosphere. et al., 2000; McCarthy2002a,b,c, 2003a,b) et in al., combination 2007; withproduction substrate Liikanen rates and and pathways. The Martikainen,a experiments 2003; were eutrophic, conducted Liikanen monomictic with et sediments lakeLugano. from al., in Monomixis southern and Switzerland, thewaters the makes resulting the south intermittent SB basin anoxiater an (SB) oxygenation and ideal on of study suboxia the site Lake of benthicsurements for the N testing (Wenk, bottom the 2013) e indicate high bottom water N tive importance of nitrification, nitrifier1996; denitrification, Liikanen and and denitrification Martikainen,2010). (Mengis et 2003; al., Liikanen et al., 2003b; McCrackinex and situ Elser, steady state flow-through incubations with intact sediment cores (Lavrentyev benthic environment are still not clearly identified, particularly with regard to the rela- 5 5 25 15 20 10 15 20 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | O], 20 L label 17 × N + 4 O], and 14 NaOH to 17 N 1 O concen- N 2 − 14 N-NH 14 + 15 N O concentration 14 O 2 of [ (Ammonium Chlo- (Potassium Nitrate, 16 1 + 4 − N − 3  14 incubations, where the N O] 2 . Detector-sensitivity cor- N-NH 1 15 O exchange with air. The 16 N-NO − 2 15 N 15 + 14 O] isotopologues, respectively. O N 17 O mL 16 2 15 N N and 10 µmol L 14 + 15 1 N − N O 15 14 16 + N O 15 O], [ 4976 4975 1 µmol L 17 N 16 N < 14 N ) based on the M/z 44, 45 and 46 detector sig- was established with a peristaltic pump. For each 15 1 14 1 − N N − 24 h (Gardner and McCarthy, 2009), the in- and out- 14 14 > 90 %). + O standards were compared to the respective concentra- > 2 O O concentrations and benthic fluxes 18 2 1 mL min solutions were produced to yield the following N N ∼ 3 14 N 50 %), except in the aerobic ambient O N content 14 ∼ concentration was 15 + + 4 N atom, Spectra Stable Isotopes), one with O 15 16 O. Six KNO contamination, only the lower three quarters of the Niskin bottle contents N amended incubations, [ N 2 N atom, Spectra Stable Isotopes) and one was left unamended as a control. O analyses, glass vials (21 mL) were filled from bottom to top, and allowed 2 15 15 2 N content 15 99 % N O concentrations (in nmol L 15 2 to N C). 15 > ◦ − 3 O analysis were only taken on the last day of the experiment. O concentrations and stable isotope ratios were determined using an isotope ra- For N After a conditioning period of 99 % 2 2 and [ For the tomated purge and trapstandards system were produced (Thermo using Finnigan theNO GasBench denitrifier II). method (Sigman N ettration al., standards: 2001) 0.03, to 0.1, reduce 0.2,rected IRMS 0.5, peak 1.0, areas of 1.5 nmol N tions N in a regression analysis,the and the N resulting transfer functionnals. was used These to correspond calculate to the [ 2.4 Determination of N N tio mass spectrometer (IRMS, Thermo Finnigan Deltaplus XP), coupled to an au- N to overflow for atvials were least capped two with aluminium bottleservice crimp caps volumes GmbH, with Art. to silicone Nr. septa minimizeexchange 300227). (CS N with chromatographie Subsequently, a water. The 10 mL samplesprevent He were further headspace sterilized was microbial with added activitywithin 0.2 in mL one (Sigman of hour et 10 after mol al., sampling. L 3 After 2001). days. adding Sample NaOH, treatment the samples was were done analyzed within background NH was added (final flows were sampled daily. The presented3 results incubation are days (October the 2010, average of January 2 2011, (August May 2011). 2010) In or April 2010, samples for vial. A constant flow of sampling campaign, three duplicate flow-throughcore experiments incubations were were set supplied up, withOne where water of two from the three one inlet inletride, water water reservoirs reservoir, was respectively. amended with > The labeled substrates were(final added so that the in situ concentrations were doubled Steady state flow-through experiments wereand set Lavretyev up et according al. to (2000)placed Gardner (Fig. with et 2). gastight, al. The O-ring (1991) topwere sealed caps PVC lowered on plungers into the containing the linersSubsequently, two the liners were holes. inlet removed until The water and plungers all reservoir re- (FEP, headspace 0.8 was mm connected air inner to was diameter). the released A core through with second the gastight tube holes. tubes connected the core with the sampling risk of O were used, and in Octoberunderwater. Within 2010 approximately and 6 January h from 2011,the sampling the flow-through and bags experiments after were were transportation additionally set to(6.5 stored up the lab, in a cold room at a2.3 near-in situ temperature Steady state flow-through experiments of bottom water werein sampled open using 20 10 L Lditions plastic Niskin (April bottles. containers 2010, Bottom with Mayhypolimnion, waters headspace bottom 2011). were waters for During stored were samples thermalBags, filled CEL taken stratification into Scientific during and Corporation, gastight ITP-25) oxic anoxia 25 without L in con- headspace. bags In the (Gas order deep to Sampling minimize Tedlar the during transport to the home laboratory on the day of sampling. In addition, 3 5 5 25 15 20 10 25 15 20 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | ) O O O 1 (1) 2 2 2 − ) as 1 − O flux mea- 2 ). In October 1 is the average O fluxes under concentrations additions were − 2 O isotopologues − 3 Q (2.4 µmol L − 3 17 − 2 ). By August 2010, ), 1 1 − − O fluxes were positive, N-NO in May 2011 for core 1 2 2 ). Positive fluxes indicate 15 O and O as well as the unla- 2 − 2 18 in October 2010. In January m 1 2 and − − ] from the sediment to the water + 4 2 m O), and the doubly-labeled N labeled sediment cores (Fig. 3). − O], and we therefore assume that 1 O h 16 − 3 − m 2 17 N 1 N concentration was low (0.3 µmol L N-NH − , respectively) probably reflecting the 14 O h 2 14 15 + 4 2 N − N-NO N 15 m 15 15 1 labeled cores did not exceed natural abun- − + O, + 4 4978 4977 O concentrations (nmol L 16 O 2 O to the overlying water column. N O h N 17 2 2 N labeling experiments 15 N N-NH gradient started to collapse, and suboxic to hypoxic O for all 15 N A Q ) were measured in bottom waters. In May 2011, the 2 15 2 15 1 14 N × − O fluxes of the replicate cores was most pronounced 2 14 N-N 
in + 15 O fluxes were again relatively low (202 and 195 nmol N ) accumulated in the bottom water, but NO 2 O 1 O 850 nmol N 2 − 18 N is the sediment core surface area (m ∼ O fluxes in cores with  N 2 concentration conditions, increased from 831 and 140 nmol N (11 µmol L A O concentration between the in- and out-flowing water, and thus − 14 + 4 2 2  O fluxes calculated from unamended cores showed strong varia- N O fluxes from the unamended cores, despite the two-fold increase 2 O fluxes in the O fluxes 2 2 out ] are the measured N erent months and between duplicate cores. In April and August 2010 14 O), the singly- ( O fluxes were calculated for the total N 2 [ ff in 2 ) and O O of masses 44, 45 and 46 [nmol h / ) decreased. In January 2011, samples were collected right at the begin- 16 concentration in the bottom water had risen (7.1 µmol L 1 2 concentration was comparatively high (83.7 µmol L 700 and 2 1 − N-N  (44.5 µmol L N O recovery from the N − out + 4 and NH − 3 ∼ 2 + 4 15 14  O − 3 O) according to Eq. (1) (Fig. 3). O] 2 N in January 2011 and 178 and 189 nmol N in April 2010 to 2426 and 2605 nmol N = 16 16 N-N 14 erence between the N 2 2 N O N ff − − 2 15 15 N 15 m m erence N N ff 1 1 Overall, total N In almost all incubations, except in January 2011, average N Fluxes of N N-label was detected as − − 15 15 comparable to N (di heterogeneity of the sediments (Fig. 3). 3.3 15 Generally, dance levels. Total N h and core 2, respectively). tion between di the di beled ( ( indicating that sediments releasedsurements N in unamended cores (corein 1 and situ core NO 2), which representh N and May 2011, average N concentrations of O water column was fully oxygenated again. 3.2 Benthic N Net benthic N (50.3 µmol L ning of the winter overturn. The O water: Flux sented in Table 1. Inbottom March waters and were April fully 2010,and oxygenated, the the the water NO column NH wasthe well lake was mixed stratified, so an that and anoxic the near-bottom layer NH of about2010, 2 m with width ongoing hadtom developed, stratification layer and expanded by organic another matter 6well as to decomposition, NH 7 the m into anoxic the bot- water column. NO [ any changes in the contributionswere of negligible. the natural abundance column were then calculated from the concentration changes in the in- and out-flowing 3 Results 3.1 Water column characteristics The physical and chemical bottom water parameters at the five sampling dates are pre- where [N flow rate (L h a net increase influxes N out of the sediments. 5 5 20 15 10 25 20 15 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | O O or 2 2 into into + 4 2 + 4 may, in O fluxes 2 NH 2 O produc- 15 2 , Frame and in Norwegian 1 − (August 2010), 2 2 − − also impacted the labeled cores with m N from m 1 2 − 3 15 1 seems to hinder ben- − − O production pathway 2 2 O h O h 2 N-NO 2 ) were measured in bottom 15 O production pathways. This 2 2 O 1 − erences leading to sediment core O fluxes reported here ranged be- (as low as 5 µmol L O release by denitrifiers (Codispoti ff at the bottom of the artificially oxy- 2 O production rates were in fact very 2 2 2 2 O production and bottom water oxy- − 2 O (McCrackin and Elser, 2010; Liikanen m O fluxes in 2 O throughout the year 2 1 2 − (11 µmol L 4980 4979 2 O h in a eutrophic lake in Finland. McCrackin and O production, and that O 2 O flux of 550 nmol N 2 2 2 − . Mengis et al. (1996), for example, measured net 2 m − 1 O fluxes were observed. The fact that more reduced − m 2 concentration fluctuations on a much smaller scale, e.g. 1 (April 2010) and 1115 nmol N O fluxes from unamended cores (Fig. 3) to the NH − 2 2 O fluxes increased during the stagnation period with in- 2 O h O produced by ammonia oxidizers increases substantially 2 − 2 2 O production during stratification can lead to accumulation 2 O h m and small amounts of O 2 1 − 2 − O h -laden bottom waters would have been expected to cause this type 2 2 concentrations. Enhanced N − 3 O release. 2 O fluxes of 458–542 nmol N gradient, small amounts of O 2 2 or NO O production, implies denitrification as the dominant N 2 + 4 amended cores shows no obvious stimulation of N − 3 -deficient environments may be conducive to N O, the relative yield of N O fluxes during the fully oxic April 2010 incubation were relatively high and com- We cannot rule out the possibility that other factors besides O Comparison of the total N 2 2 2 Casciotti, 2010). While weN did not observe the incorporation of genation, additional measurements atshed a light on higher the temporal impactdaily and of fluctuations spatial O (Bartoli resolution et could heterogeneity al., (Principi 2003), et or al., spatial 1994;impact di Svensson of and the Leonardson, 1996). availabilityet In of al., addition, 2013) reducing the that substrates mayan also like influence open organic the question. carbon activity Finally, of the or denitrifiersin process sulfide in of the the (Wenk nitrifier presence sediments denitrification remains of may NO also produce N activity of denitrifying bacteria. TheN likelihood of such impactsparable is to raised those by the observed factwere in that the low anoxic in August Maycannot 2010 2011, identify incubation. when Furthermore, a fluxes the direct relationship water between was N still well-oxygenated. Still, although we O et al., 2001). However, ourlow results indicate during that the N initialwhen pulses phase of of O theof perturbation destratification of period the denitrifiers’ (Januarytion redox in environment. 2011), January Rather, the the 2011 reduced suggests period fact, N that inhibit the N introduction of low concentrations of O of the O waters, and only lowconditions benthic during N stratification foster N thic N (see below). It has been suggested that the rapid injection of small quantities of O beginning of water column overturn in January 2011, together with the breakdown and oversaturation in the bottom water, as measured in 2009 (Wenk, 2013). At the lakes. NO suggests that the in situ microbialpresented processes here were not are substrate-limited real so rather that thanvary the potential seasonally, rates with rates. potential The links expectation to thatpartially water net column N supported. stratification and Total oxygenation, N creasing was anoxia from AugustSuch to high October sedimentary 2010, N when maximal values were reached. benthic N genated Lake Baldegg in static chamberused experiments. Liikanen microcosm and incubation Martikainen (2003) experimentsfluxes with of intact up sediment to coresElser 317 to nmol (2010) measure N reported N an average N 4.1 The sediments are a netLacustrine source sediments of are a N known sourceand of Martikainen, N 2003; Mengis ettween al., 140 1996). nmol The N N and were comparable to measurementsHowever, the reported October previously for 2010 othertion sampling eutrophic rate stands lakes. of out, 2605 nmol with N an exceptionally high produc- in NH respect to control cores were only observed in April 2010. 4 Discussion 5 5 25 15 20 10 25 20 15 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 2 + 4 − 3 − 3 . How- N-NH N-NO 2 15 oxidation 15 as well as + 4 2 O during ni- O produced 2 2 O reduction to as an electron -remineralizing 2 + 4 − 2 O production during 2 O produced during the 2 O production during am- from NH 2 OH decomposition). + 4 , respectively, Wenk, 2013). 2 oxidation, and thus our abil- 2 available for oxidation during + 4 − + 4 m 1 O consumption. N 2 − O produced solely by denitrification and 2 ect occurs in lake sediments. ff 4982 4981 cient than N labeled cores (Fig. 3), suggesting that denitrifi- , Wenk, 2013), once redox gradients and, pre- ffi O (Otte et al., 1996). As a result, N O production via NH O without being completely reduced to N cannot serve in the place of NO 2 2 2 2 O exported from the sediment is mainly produced + 4 O producing process in the sediment − − 3 2 2 -limitation has slowed the overall rate of ammonia m 2 O fluxes measured during these incubations indicate oxidation was not observed at the oxygenated sedi- 1 20.3 (SD) µmol N h 2 − N-NH + 4 ± 15 (e.g. denitrification) nitrifier denitrification could be an im- O production pathway in the Lake Lugano SB sediments. to ammonia oxidizers that then convert it to N 2 − 2 − 2 O production during nitrification is enhanced under low O O fluxes observed throughout the year. 2 O that was produced during NH 2 2 O even if O regeneration from organic N in the sediments reduced the degree 2 O. Although NO 2 was low but not absent during this incubation, which agrees with + 4 O recovery was only observed in the overlying water of the 2 2 O production by this mechanism. In the same line, Norton and Stark to N 2 tracer is incorporated into the pool of NH N-N − 3 5.0 (SD) and 46.2 + 4 O. O 15 2 ± NH O production was much more e 15 2 N tracer approach adopted here and is therefore beyond the scope of this study. O production through NH N-labeling of the N 2 15 during the last step of denitrification is thought to be less tightly regulated than the O production rates increased over the experimental period (data not shown). Nev- O produced by nitrifier denitrification coupled to denitrification is not possible using 15 Although methodological limitations prevent us from ruling out N N Denitrification rates were relatively high under the truly anoxic conditions observed Even in April 2010 and May 2011, when the sediment/water interface was fully oxy- 2 2 2 microorganisms to ammonia-oxidizing organismsadded could reducesoil the incubations. degree It is to feasible which that a similar e ammonia oxidation, it is clearthrough that N nitrate (and possiblyquired nitrite) to reduction produce and, the therefore, N that denitrification is re- conditions (Stein, 2011; Bange ettracer al., by rapid 2010). NH It isof possible that dilution ofity the to detect N (2011) suggest that immediate shunting of unlabeled NH that N N preceding step that reduces NOby to denitrifiers N can accumulate when denitrification rates are high. ment/water interface in April 2010seemingly and May conducive 2011 to (Fig. benthic 3),produce even nitrification. though N Only conditions were in Januaryprevious 2011 findings did that NH N high (10.2 Furthermore, the October 2010 experiment wasditions the may only not one in have which been steady-state reachedN con- before the experiments wereertheless, started. N the overall high net N ever, variability in denitrification rates between the two cores in October 2010 was very acceptor during nitrifier denitrification, wedenitrification cannot supplies rule NO out thetrifier possibility denitrification. that Distinguishing proximate between N N the in October 2010 (28.2sumably, µmol the N h microbial community hadthat stabilized. was At reduced, that was time, released up as to N 15 % of total NO incubations. Rapid oxygen consumption supported byto high rates the of sediments organic can matter reduce input ing the conditions oxygen favorable penetration to depth denitrification into close2007). the to It the sediments, is sediment/water produc- not interface (Li clearreducing et whether NO al., canonical denitrifiers carry out all of the steps involved in Significant labeled cores, but notcation in is the the predominant N genated and nitrification was likelymonia to oxidation occur was (Ward, 2008), notdenitrification N is observed. the Therefore, main it source is of N very incorporated likely into that the canonical N anoxic as there is a sourceportant of source NO ofoxidation N (and therefore the rate of N 4.2 Denitrification is the main N under conditions that stimulate their nitrifier denitrification pathway. As a result, as long 5 5 25 15 20 10 25 15 20 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | O 2 , 2001. to the overlying reduced to N 1 − 3 − O yr ) and the time span of 2 2 O in the bottom waters of 2 http://agage.eas.gatech.edu/data O before it escapes to the atmo- 2 O degassing events for regional or , as well as benthic fluxes from sev- 2 2 − 10.1046/j.1440-1770.2001.00120.x m O fluxes measured in this study, which 2 1 − O fluxes presented in this study (879 nmol , 2003. 2 4984 4983 O h , last access: 29 December 2012. 2 ¨ oscher, C. R.: Marine pathways to nitrous oxide, in: O in the water column. Surprisingly, the highest 2 reduction by denitrification was found to be the pri- O was observed after thermal stratification occurred md.txt O production during ammonia oxidation was minimal 2 2 − 3 O are produced in the sediments of the Lake Lugano 2 , 2002. mean , 1992. O, while N 2 reduced to N We would like to thank Mark Rollog for laboratory assistance, and − 3 accumulated as N mean/global − 3 O fluxes varied seasonally, and the proportion of NO 2 ) over the area of the Lake Lugano SB (20.3 km 2 appeared to be enhanced by ongoing anoxia. Hence, 1–15 % of the N re- O production can lead to the accumulation of N − 2 O budgets also awaits further investigation. 2 m 2 1 − 10.1029/2001GB001812 10.1023/B:AECO.0000007040.43077.5f 10.1007/BF00878135 O to the water column. NO O fluxes from the surface waters to the atmosphere were not addressed in this O and NO emissions from fertilized fields, Global Biogeochem. Cy., 16, 1080, 2 2 2 O h Lean, J., Lowe, D. C.,land, Myhre, R.: G., Changes Nganga, J., in2007: Prinn, atmospheric The R., constituents Raga, Physical and G., Science inment Schulz, Basis. radiative M., Report Contribution forcing, and of of in: Van Dor- the Climate Working Change Intergovernmental Group I Panel to on the Climate Fourth Change, Assess- edited by: Solomon, S., N doi: and Yoshinari, T.: The oceanicwe fixed enter nitrogen the anthropocene?, and Sci. nitrous Mar., oxide 65, budgets: 85–105, 2001. Moving targets as sediment-water interface: Interactionsdoi: and spatial variability, Aquat. Ecol., 37, 341–349, tion: changes in phosphorus andters, nitrogen Lakes as Reserv. well Res. as Manage., oxygen 6, balance 37–47, and doi: biological parame- doi: archive/global Nitrous Oxide and2010. Climate Change, edited by: Smith, K. A., Earthscan, London, 36–62, N If we extrapolate the average of total N 2 Forster, P., Ramaswamy, V., Artaxo, P., Berntsen, T., Betts, R., Fahey, D. W., Haywood, J., Codispoti, L. A., Brandes, J. A., Christensen, J. P., Devol, A. H., Naqvi, S. W. A., Paerl, H. W., Bouwman, A. F., Boumans, L. J. M., and Batjes, N. H.: Modeling global annual Bartoli, M., Nizzoli, D., and Viaroli, P.: Microphytobenthos activity and fluxes at the Barbieri, A. and Simona, M.: Trophic evolution of Lake Lugano related to external load reduc- Barbieri, A. and Polli, B.: Description of Lake Lugano, Aquat. Sci., 54, 181–183, stratification. The relevance ofglobal such N lacustrine N Acknowledgement. AGAGE Advanced Global Atmospheric Gases Experiment: Bange, H. W., Reing, A., Kock, A., and L stratified lakes, and possibly to rapid evasion to the atmosphere during periods of de- Mauro Veronesi and Stefano Beatrizotti for their help with the sediment core sampling. References sphere has yet tobenthic be N determined. Nevertheless, we argue that significant, year-round SB. It is worthranged between noting 140 that anderal 2605 the nmol other lacustrine N studies N (e.g.Mengis McCrackin et al., and 1996) Elser, are(Bouwman 2010; of et Liikanen the al., same and 2002). order Martikainen, as 2003; global average fluxes estimatedstudy for and soils remain unknown,umn and of the capacity Lake of Lugano microbial SB processes to in remove the sedimentary water N col- one year, the sediments ofwater. the The Lake spatial Lugano SB resolutionand release of 6.9 benthic t our conditions N are experiments notrapolation is doubt is small highly accompanied compared variable by in topotentially a time the large large and lake amounts uncertainty. space, area, Nevertheless, of so it that N this demonstrates ex- that terface. Net N versus N duced from NO percentage of NO and stable anoxia developed,denitrification. conditions that are generally thought to favor complete N This study demonstrated thatof the N sediments of themary Lake source Lugano of SB this are N or a not net observed source at all, even when oxic conditions prevailed at the sediment/water in- 5 Conclusions 5 5 30 25 20 15 10 25 15 20 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | , , ected ff ects of ff , 1979. , 2009. 10.1641/0006- ect ammonium ff 10.2134/jeq2002.3380 , 1992. 10.1023/a:1016015526712 and nitrogen supply, Freshwater , 2003a. 2 ects of sea salts on the forms of ni- ff ciency may decrease with increased ffi 10.1007/s10533-009-9329-5 , 2009. ects of the zebra mussel on nitrogen dynamics 10.1007/BF00878142 ff , 2004a. , 2003. , 2007. 4985 4986 N of particulate and dissolved carbon and nitrogen in 10.2136/sssaj1979.03615995004300060016x 15 ect of ammonium and oxygen on methane and nitrous δ ff , 2003. , 2011. , 2004b. , 2000. C and , 2010. 13 δ 10.1046/j.1365-2427.2003.01023.x 10.4319/lo.2009.54.6.2157 10.4319/lo.2004.49.2.0415 10.1016/s0045-6535(03)00224-8 10.3354/ame021187 10.1016/S1002-0160(07)60089-7 10.1016/j.soilbio.2010.09.030 10.4319/lo.2004.49.3.0839 10.5194/bg-7-2695-2010 Lugano. II. Seasonal andwater interface, spatial Aquat. Sci., variability 54, of 285–299, doi: microbiological processes at the sediment- doi: and the microbial community at194, the doi: sediment-water interface, Aquat. Microb. Ecol., 21, 187– tinct and significant source of nitrous oxide from soil, Soil Biol. Biochem., 43, 174–178, Biol., 48, 500–511, doi: shallow, sub-tropical Florida Bay: whyeutrophication, denitrification Biogeochemistry, e 95, 185–198, doi: aquatic sediments with continuous measurements8, of 553–560, redox potential, 2011. Int. J. Environ. Sci. Te., terannual variation ofgen the in isotopic Lake compositiondoi: Lugano: of a sedimenting long-term organic sediment carbon trap andThibodeau, study, nitro- Limnol. B.: Aerobic Oceanogr., respirationisotope 49, and rations 839–849, hypoxia of in dissolved54, the oxygen 2157–2169, Lower doi: constrain St. oxygen Lawrence sink Estuary: partitioning, stable Limnol.water Oceanogr., induced by redox potential746, in doi: sediment of Dianchi Lake, China, Pedosphere, 17, 739– for simultaneous gas and nutrient151–160, flux 2002a. measurements in sediments, Boreal Environ. Res., 7, by oxygen, nitrate, and2002b. sulfate, J. Environ. Qual., 31, 338–349, doi: temperature and oxygen availability on greenhousea gas eutrophic and nutrient mid-boreal dynamics lake, in Biogeochemistry,2002c. sediment 59, of 269–286, doi: gas dynamics in boreal, littoral sediments under raised CO and Martikainen, P. J.: Spatial and seasonal variation in greenhouse gas and nutrient dy- 3568(2003)053[0341:tnc]2.0.co;2 trogen released from estuarine andflux?, freshwater sediments: Estuaries, does 14, ion 157–166, pairing 1991. a oxide fluxes across sediment-water1293, interface doi: in a eutrophic lake, Chemosphere, 52, 1287– and Cosby, B. J.: The nitrogen cascade, Bioscience, 53, 341–356, doi: Seasonal variation of the Lake Lugano: constraints on biogeochemical49, cycling 415–429, in doi: a eutrophic lake, Limnol. Oceanogr., trous oxide production by adoi: marine ammonia-oxidizing bacterium, Biogeosci., 7, 2695–2709, nitrite, and oxygen on theSci. composition Soc. Am. of J., the 43, gaseous 1140–1144, products doi: of denitrification in soil, Soil Edn., edited by: Capone,demic D. Press, G., San Bronk, Diego, D. 199–261, A., 2008. Mulholland, M. R., and Carpenter, E. J., Aca- Qin, D., Manning,Cambridge M., University Chen, Press, Cambridge, Z., UK Marquis, and New M., York, NY, Averyt, USA, 130–234, K. 2007. B., Tignor, M., and Miller, H. L., Lazzaretti, M. A. and Hanselmann, K. W.: The role of sediments in the phosphorus cycle in Lake Lavrentyev, P.J., Gardner, W. S., and Yang, L.: E Kool, D. M., Dolfing, J., Wrage, N., and Van Groenigen, J. W.: Nitrifier denitrification as a dis- Knowles, R.: Denitrification, Microbiol. Rev., 46, 43–70, 1982. Lehmann, M. F., Barnett, B., Gelinas, Y., Gilbert, D., Maranger, R. J., Mucci, A., Sundby, B., and Liikanen, A., Flojt, L., and Martikainen, P.: Gas dynamics in eutrophic lake sedimentsLiikanen, a A., Murtoniemi, T., Tanskanen, H., Vaisanen, T., and Martikainen, P. J.: E Liikanen, A., Ratilainen, E., Saarnio, S., Alm, J., Martikainen, P. J., and Silvola, J.: Greenhouse Liikanen, A., Huttunen, J. T., Murtoniemi, T., Tanskanen, H., Vaisanen, T., Silvola, J., Alm, J., Gardner, W. S. and McCarthy, M. J.: Nitrogen dynamicsHunting, at E. the R. and sediment-water van interface der Geest, in H. G.: Predictability of bacterial activity and denitrification in Lehmann, M. F., Bernasconi, S. M., Barbieri, A., Simona, M., and McKenzie, J. A.: In- Liikanen, A., Tanskanen, H., Murtoniemi, T., and Martikainen, P. J.: A laboratory microcosm Li, Q. M., Zhang, W., Wang, X. X., Zhou, Y. Y., Yang, H., and Ji, G. L.: Phosphorus in interstitial Gardner, W. S., Seitzinger, S. P., and Malczyk, J. M.: The e Liikanen, A. and Martikainen, P. J.: E Galloway, J. N., Aber, J. D., Erisman, J. W., Seitzinger, S. P., Howarth, R. W., Cowling, E. B., Lehmann, M. F., Bernasconi, S. M., McKenzie, J. A., Barbieri, A., Simona, M., and Veronesi, M.: Frame, C. H. and Casciotti, K. L.: Biogeochemical controls and isotopic signatures of ni- Firestone, M. K., Smith, M. S., Firestone, R. B., and Tiedje, J. M.: The influence of nitrate, Devol, A. H.: Denitrification including anammox, in: Nitrogen in the Marine Environment, 2nd 5 5 30 25 30 10 25 20 20 15 15 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | , ¨ ohlke, J. K.: , 2010. O): the dom- 2 , 2007. , 1994. , 2000. 10.1023/A:1009740530221 , Biochem. J., 126, 1181–1191, 10.1890/08-2210.1 O due to intensifying anoxia on the 2 , 2001. 10.1007/bf00879967 , 2013. 10.1038/35042551 O production by nitrosomonas-Europaea 2 usion, Mar. Ecol.-Prog. Ser., 81, 289–303, ff , 1996. , 2003b. 4988 4987 under transient and dynamic aerobic and anaerobic ects of bioturbation by tube-dwelling chironomid larvae Nitrosomonas europaea ff , 2009. 10.1021/ac010088e , 2010. , 2009. , 1992. 10.4319/lo.2013.58.1.0001 O-producing pathways in bacteria, in: Methods in Enzymology: Re- 10.3394/0380-1330(2007)33[219:EOHFRO]2.0.CO;2 2 Alcaligenes faecalis O budget: Nitrous oxide emissions through the agricultural nitrogen cycle – 10.1023/A:1026070209387 2 10.1046/j.1365-2427.1996.00500.x 10.1016/j.earscirev.2008.11.003 10.1016/j.ecss.2010.08.015 10.3354/meps081289 10.1126/science.1176985 ects of hydrological flow regime on sediment-water interface and water column nitrogen ff Ph.D. thesis, Departmentpreparation, of 2013. Environmental Sciences, University of Basel,and Switzerland, in Lehmann, M.dependent denitrifiers F.: coexist Anaerobic in theOceanogr., ammonium water 58, column 1–12, oxidation of doi: a (anammox) meromictic south-alpine bacteria lake, Limnol. and sulfide- ments: a review of availabledoi: technology and observed evidence, Earth-Sci. Rev., 92, 81–97, on oxygen uptake and denitrification300, in doi: eutrophic lake sediments, Freshwater Biol., 35, 289– and Alkhati, M.:N Benthic budget nutrient ofdoi: fluxes the along St. the Lawrence Laurentian marine Channel:edited system, by: impacts Capone, Estuar. D. onPress, G., Coast. San the Bronk, Diego, D. 199–261, Shelf A., 2008. Mulholland, S., M. 90, R., and 195–205, Carpenter, E. J., Academic – an examination of nitrifier denitrification, Appl. Environ. Microb.,sediment 49, composition, 1134–1141, Aquat. 1985. Sci., 56, 243–262, doi: in a coastaldoi: sediment – role of molecular-di A bacterial method forAnal. the Chem., nitrogen 73, 4145–4153, isotopic doi: analysis of nitrate insearch seawater on and Nitrification and freshwater, Relateddemic Processes, Press Part INC, A, San edited Diego, USA, by: 486, Klotz, 131–152, M. ISBN: G., 978-0-12-381294-0, Elsevier 2011. Aca- ide production by conditions, Appl. Environ. Microb., 62, 2421–2426, 1996. oxidative and reductive processes in 1972. tems, in: Methods in Enzymology:edited Research by: Klotz, on M. Nitrification G., and Elsevier978-0-12-381294-0, Academic Related 2011. Press Processes, INC, Part San A, Diego, USA, 486, 343–368, ISBN: inant ozone-depleting substancedoi: emitted in the 21st century, Science, 326, 123–125, 1998. and George, M. D.:Indian Increased continental marine shelf, Nature, production 408, of 346–349, N doi: ing the global N OECD/IPCC/IEA phase II developmentventory of methodology, Nutr. IPCC Cycl. guidelines Agroecosys., 52, for 225–248, national doi: greenhouse gas in- E dynamics in a greatRes., lakes 33, 219–231, coastal doi: wetland (Old Woman Creek, Lakenitrous Erie), oxide production J. in Great lakes, Lakes Ecology, 91, 528–539, doi: artificially oxygenated lake, Limnol. Oceanogr., 41, 548–553, 1996. namics and their interactions65, in 83–103, the doi: sediments of a boreal eutrophic lake, Biogeochemistry, Wenk, C. B., Blees, J., Zopfi, J., Veronesi, M., Bourbonnais, A., Schubert, C. J., Niemann, H., Wenk, C. B.: Nitrogen transformation pathways, rates and isotopic signatures in Lake Lugano, Stockdale, A., Davison, W., and Zhang, H.: Micro-scale biogeochemical heterogeneity in sedi- Svensson, J. M. and Leonardson, L.: E Thibodeau, B., Lehmann, M. F., Kowarzyk, J., Mucci, A., Gelinas, Y., Gilbert, D., Maranger, R., Ward, B. B.: Nitrification in marine systems, in: Nitrogen in the Marine Environment, 2nd Edn., Stein, L. Y.: Surveying N Principi, C., Baudo, R., and Muntau, H.:Rasmussen, Spatial and H. temporal and heterogeneity Jorgensen, of Lake B. Lugano B.: Microelectrode studies of seasonal oxygen-uptake Sigman, D. M., Casciotti, K. L., Andreani, M., Barford, C., Galanter, M., and B Poth, M. and Focht, D. D.: N-15 kinetic-analysis of N Otte, S., Grobben, N. G., Robertson, L. A., Jetten, M. S. M., and Kuenen, J. G.: Nitrous ox- Ritchie, G. A. F. and Nicholas, D. J.: Identrification of sources of nitrous-oxide produced by Norton, J. M. and Stark, J. M.: Regulation and measurement of nitrification in terrestrial sys- Ravishankara, A. R., Daniel, J. S., and Portmann, R. W.: Nitrous oxide (N Naqvi, S. W. A., Jayakumar, D. A., Narvekar, P.V., Naik, H., Sarma, V., D’Souza, W., Joseph, S., Mosier, A., Kroeze, C., Nevison, C., Oenema, O., Seitzinger, S., and van Cleemput, O.: Clos- Mengis, M., Gachter, R., and Wehrli, B.: Nitrous oxide emissions to the atmosphere from an McCarthy, M. J., Gardner, W. S., Lavrentyev, P.J., Moats, K. M., Joehem, F. J., and Klarer, D. M.: McCrackin, M. L. and Elser, J. J.: Atmospheric nitrogen deposition influences denitrification and 5 5 30 15 20 10 25 30 25 20 15 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | in bottom − 2 ] 1 − − 2 10.1016/S0038- NO and NO − 3 ] [µmol L 1 , NO − + 4 − 3 NO , NH 2 ] [µmol L 1 − + 4 NH ] [µmol L 4990 4989 1 − 2 C] [µmol L ◦ [ Temp. O , 2001. April 2010August 2010October 2010January 5.5 5.7 2011 5.7May 2011 5.9 224 0 0 5.5 11 0.3 190 7.1 44.5 13.2 83.7 0.7 75.7 50.3 69.6 0.1 83.1 1.7 2.4 2.3 0 In situ temperature and concentrations of dissolved O in the production of0717(01)00096-7 nitrous oxide, Soil Biol. Biochem., 33, 1723–1732, doi: waters. DIN concentrations werements measured (Wenk, in 2013). the inlet water reservoirs of the control experi- Table 1. Wrage, N., Velthof, G. L., van Beusichem, M. L., and Oenema, O.: Role of nitrifier denitrification

Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper |

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N Tresa La Location and map of Lake Lugano. The sampling station (red triangle) is located west Steady state flow-through set-up. Fig. 2. Fig. 1. of the village ofPolli, Figino 1992). (SB), close to the point of maximum depth (modified from Barbieri and Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | erent ff NO (mass 44). 14 N 14 Core 2 Core 2 labeled labeled - - 3 3 O flux. Note the di 2 NO NO 15 15 Core 1 Core 1 Core 2 Core 2 labeled labeled + + 4 4 NH NH 15 15 Core 1 Core 1 1 Core 2 Core 2 1 unamended unamended Core 1 Core 1 0 0 NO (mass 46). Black: Total N 800 600 400 200 800 600 400 200

August 2010 August

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1000 1200 1000 1200

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-2 -1 -2 -1 N b) d) January 20 15 4993 Core 2 Core 2 Core 2 labeled labeled labeled - - - 3 3 3 NO NO NO 15 15 15 Core 1 Core 1 Core 1 O 2 Core 2 Core 2 Core 2 labeled labeled labeled + + + 4 4 4 mass 45 mass 44 mass 46 total N NH NH NH 15 15 15 Core 1 Core 1 Core 1 NO (mass 45). Dark grey: Core 2 Core 2 14 Core 2 N 1 1 15 unamended unamended unamended Core 1 Core 1 Core 1 0 0 0 600 400 200 800

800 600 400 200

April 2010

4000 3000 2000 1000 1200 1000

1200 1000

2 2 2 2

O flux [nmol N [nmol flux O N ] m h O N [nmol flux O N ] m h O 2 2 m h O N [nmol flux O N ]

-2 -1 -2 -1 -2 -1 NO and e) May 20 c) October 2010 a) 15 N O fluxes from the sediment to the overlying water. Each column represents the average value of duplicate 14 . 2 N (c) Fig. 3. experiments. Error bars show the standard error of the fluctuations over incubation time. White: Light grey: scale in