Interactive comment on “ A simple method for air / sea gas exchange measurement in mesocosms and its application in carbon budgeting ” by J .

Czerny and colleagues present a straightforward method to quantify the air/sea gas exchange in mesocosm experiments. The method is based on the introduction of known amounts of N2O into mesocosms and a continuous monitoring of the outgassing of the compound. Assessed gas exchange velocities are then (based on literature data) used to calculate fluxes of CO2 with maximally reachable precision. This will benefit the quality of field and lab experiments that make use of mesocosms. The paper is well written and it comprehensively conveys the rationale behind the approach. The authors have made great effort to be very exact and to include a maximal number of side-parameters into their considerations. The sensitivities of the methods towards changes in these parameters, however, are seldom stated and an estimation of the


Introduction
Pelagic mesocosms represent large volume (mostly between one and 10 m 3 ) experimental units used to gather experimental data on natural plankton communities (Petersen et al., 2003).Most mesocosm studies currently focus on investigating ecological interactions applying standard oceanographic methods on subsamples of the enclosed water.In principal, such experiments also provide the opportunity to understand biogeochemical element fluxes such as air/sea gas exchange and export.For this purpose, in situ measurements using the whole enclosure as experimental vessel have to be elaborated, in order to avoid problems occurring when extrapolating from bottle incubations to the mesocosm.But mesocosms are generally open towards the atmosphere allowing for air/sea gas exchange, making it difficult to calculate production or consumption of volatile compounds inside an experimental unit.
Climate relevant trace gases and other volatile carbon compounds produced in marine environments are increasingly investigated for their potential climate feedbacks and have been measured in previous mesocosm experiments (Sinha et al., 2007;Archer et al., 2012;Hopkins et al., 2012).Precise knowledge of air/sea gas exchange rates is needed, in order to compare measured aquatic concentrations of gases between various experiments or even with open ocean conditions, and to calculate turnover rates.
Not only in the context of global change, CO 2 entering the surface ocean, being fixed via photosynthesis, feeding the food chain and sinking into the ocean interior is of special interest to biogeochemical experimentalists.Calculating carbon fluxes from water column pools of inorganic and organic carbon quantitatively related to air/sea fluxes and export rates could largely improve the understanding of the system (Czerny et al., 2012a).For a direct integrated estimates of cumulative net community production, changes in CT have to be corrected for CO 2 air/sea gas exchange and eventually for calcification and evaporation.In previous mesocosm experiments in a Norwegian Fjord (Delille et al., 2005) and indoors (Wohlers et al., 2009;Taucher et al., 2012), net Figures community carbon production (NCP), calculated on the basis of measured changes in total dissolved inorganic carbon (CT) were presented.Delille et al. (2005) used a parameterisation for wind dependent boundary layer thickness achieved from experimental data compiled by Smith (1985) to calculate air/sea gas exchange.Wind speed, the crucial input parameter, was set zero, because the mesocosms were closed to the atmosphere and moored in a sheltered surrounding.Whereas most parameterisations result in zero gas exchange at zero wind speed (Wanninkhof, 1992), laboratory derived data by Smith et al. (1985) can be used to calculate air/sea gas exchange in calm environments.Under calm conditions, gas exchange is governed by other energy inputs than wind, e.g.thermal convection due to evaporation and temperature changes (Liss, 1973).Here, the general assumption was made that calm conditions in the mesocosm are comparable to the conditions in the experimental tanks used by Smith et al. (1985) to determine the boundary layer parameterizations.However, surface turbulence in many mesocosm experiments is unlikely to be very low.Active mixing systems, wave movement of the surrounding water, thermal mixing or the deployment of sampling gear might create turbulence within the enclosures, comparable to quite windy conditions.Taucher et al. (2012) for example, found wind speeds of more than 6 m s −1 to be necessary for balancing the carbon budget in a Kiel indoor mesocosm experiment, applying the Smith et al. (1985) calculation.While parameterisations for wind speed dependent gas exchange over the ocean are obviously not suitable for calculating mesocosm air/sea gas exchange, direct measurement of exchange velocities in an enclosed water volume can be easily done.
Here, we present a simple method for direct measurements of air/sea gas exchange rates in mesocosm experiments using N 2 O as a tracer.The conversion of measured exchange rates to those of CO 2 and other gases is explained.We are providing a detailed description of the method and calculations including a discussion of prerequisites to achieve high quality data.
The measurement protocol and results are explained using a KOSMOS (Kiel Off Shore Mesocosm for future Ocean Simulation) (Fig. 1a) experiment on ocean Introduction

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Full acidification in the Arctic as a model.Applicability of the method in the Kiel indoor mesocosm facility is further explained and discussed.

N 2 O addition
One litre of saturated N 2 O solution was prepared via bubbling of seawater for two days in a narrow measurement cylinder covert with Parafilm ® .Additions of the solution to the mesocosm were calculated using solubility constants by Weiss and Price (1980) with respect to salinities (S) and temperatures (T ).
The targeted concentrations of N 2 O should be adapted to the setup in order to achieve mesocosm to air fluxes, which can be measured at good precision over reasonable time scales.Here, seawater tracer concentrations were chosen in accordance to the highest certificated reference material for N 2 O analyses available in our lab (∼ 55 nmol kg −1 ).
Assuming a background concentration of 13 nmol kg −1 , 40 nmol kg −1 of medical grade N 2 O was added.Based on experience, a surplus of approximately 20 % was added to the mesocosms to account for losses unavoidable during handling of the solution.
Addition of the solution to the mesocosms (about 1-2 ml m −3 ) can be calculated according to the formulation: where V ad is the volume of N 2 O stock solution added (ml), V w the volume of the mesocosm (l), (ad) the desired addition (mol l −1 ), and K TS is the solubility constant by Weiss and Price (1980) for S and T of the N 2 O stock solution (mol l −1 atm −1 ) prepared at one atmosphere.Introduction

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Full A syringe with a large inlet diameter was used to transfer the stock solution carefully.Filling of the syringe was done slowly as vacuum increases undesired outgassing of N 2 O.The stock solution was first diluted with filtered seawater in 25 l carboys, which were filled almost to the brim.The content of the carboys was homogeneously distributed to the mesocosms by using the pumped injection device "Spider" (Riebesell et al., 2012).

Sampling
Three of the nine mesocosms were sampled every second day using integrating water samplers (IWS, Hydrobios).Triplicate samples, representative for the 15 m deep water column, were drawn directly from the sampler.The water was filled bubble free into 50 ml headspace vials via a hose reaching to the bottom of the vial.The vial volume was allowed to overflow about four times before closing.Vials were closed with butyl rubber plugs (N20 Machery and Nagel), crimp sealed and stored at room temperature after addition of 50 µl of saturated mercury chloride solution.

Measurement procedures
Measurement of aquatic N 2 O concentrations was performed via gas chromatography with electron capture detection (Hewlett Packard 5890 II), using a headspace static equilibration procedure as described by Walter et al. (2006).The GC was equipped with a 6'/1/8" stainless steel column packed with a 5 Å molecular sieve (W.R. Grace and CO) and operated at a constant oven temperature of 190 • C. A 95/5 argon-methane mixture (5.0,Air Liquide) was used as carrier gas. 10 ml of helium (5.0,Air Liquide) headspace was added and manually injected after equilibration was achieved by manual shaking and storage for at least 10 h at equilibration temperature of 21 Full a minimum of three data points close to sample concentrations.Headspace to water phase ratios in the vials was determined gravimetrically.Dissolved inorganic carbon was determined via colorimetric titration using a SOMMA system and total alkalinity (TAlk) via potentiometric titration (Dickson, 1981).CO 2 concentrations, partial pressures and pH (total scale) were calculated from DIC and TAlk measurements with the program CO2SYS by Lewis and Wallace (1995).For more details on carbonate chemistry see Bellerby et al. (2012).
Determination of salinity and temperature in the mesocosms was performed with a data logger-equipped hand held multisensory CTD 60M (Sea and Sun Technology).Volume of the mesocosms was determined with the same instrument using sodium chloride additions of ∼ 0.2 g kg −1 as a tracer (Czerny et al., 2012b).
Wind velocity and direction measured at 10 m height onshore about one mile from the mooring site were provided by the staff of the AWI-PEV Station in Ny Ålesund.

Setup of the Svalbard 2010 ocean acidification experiment
Nine 15 m deep KOSMOS mesocosms each with a diameter of 2 m were moored end of May 2010 in the Kongsfjorden, Svalbard.Seven different CO 2 treatment concentrations were achieved through addition of CO 2 saturated seawater.While the ambient (∼ 180 µatm pCO 2 ) control treatment was replicated twice, the seven enriched mesocosms followed a gradient up to ∼ 1420 µatm pCO 2 .Development of the enclosed natural plankton community was followed for 30 days after CO 2 manipulation, including addition of inorganic nutrients on day 13.For more details see Riebesell et al. (2012) and Schulz et al. (2012).

Results and discussion
Concentrations of N 2 O added on day four decreased in the enriched mesocosms from initially measured ∼ 50 nmol kg −1 on day 6 to ∼ 30 nmol kg −1 on day 28 (Fig. 1b).Introduction

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Full Concentrations measured in the fjord close to the mesocosms were slightly oversaturated compared to atmospheric equilibrium values, calculated for in situ seawater T , S and atmospheric mixing ratios measured close by on Zeppelin Mountain.Despite variable wind conditions, the concentration decrease inside the mesocosms could be fitted (R 2 = 0.96) using the relationship:

Calculation of CO 2 fluxes from changes in N 2 O concentrations
Daily N 2 O fluxes were calculated from the fitted N 2 O concentration decrease over time and converted to volumetric units.Changes in the N 2 O inventory, derived using the determined volume of the mesocosms (method described in Czerny et al., 2012b) were used to calculate fluxes in µmol cm −2 h −1 (F N 2 O ) across the water surface according to: where I w 1 is the fitted bulk water N 2 O inventory in µmol per mesocosm on t 1 and I w 2 on t 2 with ∆t as the time interval between t 1 and t 2 in h, while A is the nominal surface area of the mesocosm in cm 2 .A N 2 O transfer velocity (k N 2 O ) in cm h −1 is than calculated by dividing F N 2 O by the concentration gradient according to Eq. ( 4): where C N 2 Ow is the fitted bulk water N 2 O concentration (µmol cm transfer velocity for any other gas using its Schmidt numbers to correct for gas specific properties as shown for the transfer coefficient of CO 2 (k CO 2 ) in Eq. ( 5): (5) The Schmidt number for N 2 O (Sc N 2 O ) published by Rhee (2000), and the Schmidt number for CO 2 (Sc CO 2 ) derived from diffusion coefficients published by J ähne et al. (1987) were used.Fluxes for CO 2 (F CO 2 ) can then be calculated by multiplication of k CO 2 with the diffusion gradient between bulk water CO 2 concentrations and calculated equilibrium concentrations with the atmosphere as:

Chemical enhancement of CO 2 air/sea gas exchange
Another correction has to be applied to derive accurate CO 2 fluxes in calm environments like the KOSMOS mesocosms.As CO 2 , reacts with water, unlike N 2 O, CO 2 gas exchange might be chemically enhanced due to buffering of diffusive concentration change by equilibration reactions within the boundary layer.Other than inert gases, exchanged CO 2 diffuses not necessarily through the boundary layer, but can be also formed from bicarbonate close to the interface.This applies only at low wind speeds due to slow CO 2 hydration kinetics, and not when mixing is considerably faster.Thus, chemical enhancement is thought to be insignificant under turbulent conditions relevant for open ocean CO 2 exchange (e.g. when k > 5 cm h −1 ), but applies to the conditions found inside the mesocosms (k ∼ 1.8-2.5 cm h −1 ) (Wanninkhof et al., 1996).Moreover, the state of the carbonate system determines the extent of chemical enhancement, being negligible at pH < 6 and substantial at pH > 8.In the Svalbard ocean acidification experiment, the treatment pH tot (total scale) ranged from 7.5 to 8.3 (Bellerby et Introduction

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Full al., 2012), therefore chemical enhancement created a pH effect on carbon flows that must be considered.To correct for this, theoretical parameterisations by Hoover and Berkshire (1969) were chosen, as currently no empirical parameterisations exist sufficiently describing the process in natural seawater (Wanninkhof et al., 1996).The enhancement factor α, the ratio between chemical enhanced flux and not enhanced flux can be calculated using Eq. ( 7): 2 being the first and second stoichiometric equilibrium constants for carbonic acid and H + the proton concentration.Q = rτD −1 0.5 in cm −1 , where r = r 1 + r 2 K * w H + −1 (s −1 ), with r 1 being the CO 2 hydration rate constant for reaction two (s −1 ) and r 2 is the rate constant for reaction three (l mol −1 s −1 ) from Johnson (1982) and K * w is the equilibrium constant for water.D is the diffusion coefficient for CO 2 by J ähne et al. (1987).The boundary layer thickness z (cm) can be calculated from determined transfer velocity (z ).All constants used here can be found in Zeebe and Wolf-Gladrow (2001).Using the Hoover and Berkshire (1969) model, input conditions similar to our experimental conditions in Svalbard (T = 5 • C, S = 35, z = 0.002 cm, pH tot = 8.2) result in enhancement of about 8 % (α = 1.082).For the same conditions, but at a temperature of 25 • C, CO 2 gas exchange would be enhanced by about 48 % (α = 1.479).Chemical enhancement factors using more complex models published by Smith (1985), Emerson (1975), Quinn andOtto (1971), and Keller (1994) give very similar results to the Hoover and Berkshire (1969) model (Wanninkhof et al., 1996).Experimental data from tank experiments reproduce calculated chemical enhancement relatively well, i.e.Hoover and Berkshire, (1969), Wanninkhof et al., (1996), Degreif (2006), Liss (1973).The simple fit derived from enhancement experiments in natural Baltic Introduction

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Full seawater published by Kuss and Schneider (2004) is not recommended for use, as influences of T , S and z are not considered.The relevance of chemical enhancement for ocean CO 2 exchange is controversial as the calculation of k from wind speed over the ocean itself still bears considerable uncertainty.As k in our experiments is measured directly, comparability to experimental results is quite straight forward.
Due to low temperatures during the Svalbard experiment, chemical enhancement of ∼ 3 to 7 % is very low (Fig. 6).The influence of about 3 degree warming during the experiment in June 2010 is overall larger than the calculated difference arising from pH treatments (Fig. 3).Strong pH-dependent chemical enhancement could produce artificial treatment effects in the carbon budget estimates especially in ocean acidification studies.Biological carbon uptake estimates by Silyakova et al. (2012) and Czerny et al. (2012a) at arctic temperatures are relatively unaffected by enhancement of this magnitude and possible uncertainties therein.
Evidence for a strong increase in chemical enhancement due to enzymatic catalysis by free carbonic anhydrases as suggested by Berger and Libby (1969) was not found in later experiments (Goldman and Dennett, 1983;Williams, 1983), but it might be interesting to reconsider this question in future mesocosm experiments.The lack of empirical data coverage on chemical enhancement parameterisations in seawater poses the major quantitative uncertainty for NCP estimates based on CO 2 air/sea gas exchange using the presented method.Especially in setups were temperatures are high, the proportion of CO 2 exchange relying on theoretical considerations is high compared to the directly measured flux.Underlying variability in CO 2 fluxes shown in Fig. 2 is caused by variability in measured aquatic CO 2 concentrations, responsible for the CO 2 diffusion gradient.The uncertainty in fluxes caused by errors in carbonate system determination seem to be of similar magnitude or larger than the error introduced from N 2 O analytics.The natural source of oceanic background N 2 O concentrations is biological production.N 2 O is produced predominantly as a side product of nitrification, when ammonia is incompletely oxidised in the course of deep remineralisation at low oxygen concentrations.Yet, most parts of the ocean are near equilibrium with the atmosphere (mean global saturation 103 %) (see Bange et al. (1996) and references herein), whereas N 2 O oversaturation is predominantly found in tropical regions rather than in cold and temperate waters (Walter et al., 2006).Detectable nitrification in the euphotic zone was hypothesised to be also a source of N 2 O (Dore and Karl, 1996;Santoro et al., 2010), but this was not yet directly observed.Physiological results (Goreau et al., 1980;Loescher et al., 2012) suggest possible N 2 O production by nitrification in fully oxygenated waters to be very low.However, even relatively high surface layer nitrification rates, as found in upwelling regions (Rees et al., 2011), are orders of magnitudes to low to significantly bias the large fluxes caused by deliberate N 2 O addition.Remineralisation of detritus at the bottom of the mesocosm could possibly be a source of N 2 O. Conditions allowing for extensive remineralisation of accumulated organics inside pelagic mesocosms should thus be avoided.It is further strongly recommended to measure background natural N 2 O concentrations preferably inside non enriched experimental units, because N 2 O is not considered as an inert gas.

Potential errors and uncertainties
Due to convection caused by slight temperature changes in the surrounding water (Fig. 4) and an evaporation induced salinity increase (Schulz, 2012), the mesocosms in the Svalbard KOSMOS study could be considered to be homogeneous on timescales relevant for air/sea gas exchange.For referring from N 2 O fluxes to those of inert gases, it might be irrelevant whether exchange is limited by mixing processes on the air sea interface or within the water column.Yet, if a permanent stratification is formed inside the mesocosm, the decrease of N 2 O bulk water concentration can not be used to calculate Introduction

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Full mesocosm-atmosphere CO 2 exchange.Processes modulating the concentrations of biologically active compounds such as CO 2 are usually variable along the light gradient.Therefore, due to shallow primary production, considerable differences in the surface gradient of CO 2 might emerge compared to N 2 O surface gradients that are governed by diapycnal mixing.For stratified mesocosms, gas exchange calculations require the integration of information about vertical distribution of tracer and gases of interest.Therefore, N 2 O inventories have to be determined by integrated water samples independently from surface gradients determined from discrete surface water samples.
For the experiments in the KOSMOS mesocosms, gas permeability can be neglected as the bag material used is 0.5 to 1 mm thick.Estimates based on (Desmopan ® 385, Bayer) the raw material of our bag foil (Walopur ® , Epurex Films) revealed that the fraction of lateral gas exchange trough the bag were in the order of 1-2 % of the total flux.Differences between N 2 O and CO 2 in the material specific permeability of the thermoplastic polyurethane used to manufacture the bags could have the potential to cause systematic errors.Such differences seemed at first unlikely because of the general similarity of N 2 O and CO 2 in diffusivity and solubility, but permeability specifications for Desmopan 385 suggest a considerably higher permeability for N 2 O (Bayer Materi-alScience, TPU TechCentre).If thin foil is used for mesocosms, a material with good gas barrier properties should be chosen and exact permeabilities should be known for the gases of interest.
When k N 2 O is translated into transfer velocities of poorly soluble gases, dissolution and adhesion of some organic compounds in and on the plastic material could cause a lateral sink of these substances additionally to the permeability issue.

Processes driving gas exchange in mesocosms
The concentration of N 2 O (C N 2 O ) decreased quite steadily over the whole experimental period (Fig. 1b).This indicates that N 2 O fluxes were controlled by the diffusion gradient to the atmosphere.Variable external forcing by wind or waves as commonly observed Figures

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Full in natural environments was of minor importance.Wind measurements at Bellevaja station at 10 m above sea level reported velocities of up to 5 m s −1 during the experiment (Fig. 5).The water surface of the mesocosms, however, is sheltered from direct wind sheer by the two meter high plastic walls of the bag (Fig. 1a; Riebesell et al., 2012).
Waves that were able to propagate through the mesocosms were only observed on the mooring site on three days when stronger winds were blowing along the fjord from southeast, the most exposed wind direction.Enhanced gas exchange during three days with waves of ∼ 60 cm could not be resolved by our measurements.However, CO 2 gas exchange inside the mesocosms was here measured to be consistently about three times higher than calculated flux at zero wind (Fig. 6, stagnant film thickness calculated according to Smith, 1985, chemical enhancement according to Hoover and Berkshire, 1969) as performed by Delille et al. (2005).Applying a quadratic wind depended function (Wanninkhof, 1992) at constant wind speed of 3.15 m s −1 , resulting fluxes are very close to our empirical estimate over the whole period.Measured wind speeds during the experiment were generally lower than this (mean 2.1 m s −1 ), and accordingly calculated mean air/sea gas exchange was also lower outside than inside the mesocosms.This suggests different processes to be driving gas exchange in mesocosms compared to open waters.Rinsing of the plastic walls when waves are propagating through the setup presumably leads to enhanced surface renewal compared to open water.Slight temperature changes in the surrounding water mass were immediately heating or cooling the bags (Fig. 4), this probably caused a considerable thermal convection that kept the experimental units relatively homogenous throughout the experiment.Last but not least the extensive daily sampling with water samplers and probes contributed to gas exchange by active perturbation of the mesocosm surface.

Mesocosm proportions
Exchange velocities (k) in other mesocosm setups deployed in more sheltered surroundings, standing on land or inside climate controlled rooms might be lower or higher,

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Full depending on methodology used for sampling, temperature control, active mixing and gas specific permeability of the mesocosm material.Even more important than these influences on k, is the ratio between the mesocosm volume and its surface area (A / V ), when exchange rates are normalised to units of water (kg −1 or l −1 ).In an exemplary 15m deep KOSMOS mesocosm (Fig. 1a), holding ∼ 45 m 3 of water, CO 2 gas exchange over 3.14 m 2 (A / V = 0.07) surface area is causing relatively moderate changes in aquatic concentrations despite large diffusion gradients (Fig. 2).Taking the example of the Kiel indoor mesocosm (Fig. 7a) of about 1.4 m 3 at 2 m 2 surface (A / V = 1.4), concentration change in response to the same gas exchange flux is orders of magnitude faster.Additionally, air/sea gas exchange velocities are accelerated by continuous active mixing, necessary to keep plankton organisms in suspension (Fig. 7a).While after 20 days ∼ 50 % of the N 2 O added was still present during the Svalbard study (Fig. 1b), the same tracer concentration was virtually gone after 5 days in the shallow indoor mesocosm (Fig. 7b).Here, inorganic carbon uptake by phytoplankton can be rapidly compensated by ingassing of CO 2 from the atmosphere.Ocean acidification experiments in setups with A / V similar to the Kiel indoor mesocosm would lose their treatment CO 2 within a few days.Therefore, treatment levels in such experiments might be maintained using continuous measurement and control technology.Resulting controlled treatment levels are beneficial when physiological questions are investigated.However, CO 2 drawdown does not occur, and therefore DIC concentration change cannot be used to calculate NCP.Another option is to artificially decrease the surface area by covering the mesocosm with a low permeability transparent film.A thin polyurethane foil mounted on a light frame and floating on the surface, efficiently minimised air/sea gas exchange (Fig. 7a).Samples were drawn through a tube build into this cover.If covers are used, reducing the surface area to a minimum, it has to be considered that the remaining open surface should be equally large.Detailed data on this issue are missing, but as it can be imagined that this approach is very sensible to the size of the remaining interface, air/sea gas exchange should be measured in all experimental units.Introduction

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Conclusion and outlook
Direct measurement of N 2 O air/sea gas exchange can be used to estimate accurate CO 2 fluxes in various mesocosm setups, whereas common parameterisations for air/sea gas exchange are difficult to adapt to mesocosm conditions and are therefore prone to systematic errors.The commonly used parameterisations are also prone to large errors in open waters when applied on a local scale especially in productive coastal waters (Wanninkhof, 1992;Liss, 1973), although, they reproduce global CO 2 fluxes fairly well.Even problems usually present when estimating open ocean air/sea gas exchange using parameterisations are empirically solved by measuring transfer velocities (k) directly.The influence of sea surface microlayers of surface active organic molecules is discussed to be responsible for large discrepancies in gas exchange between productive coastal waters and open-ocean conditions (Frew, 1997;Kock et al., 2012).The effect of these surfactants, possibly produced in high amounts during phytoplankton blooms in mesocosms, is difficult to include in theoretical calculations, but inherently included in direct measurements.Future mesocosm experiments combining the close observation of biological, chemical and physical processes might offer the chance to bring more light into origin and composition of organic surface microlayers.
Of the four community production estimates published for the Svalbard 2010 experiment, NCP calculated from changes in dissolved inorganic carbon corrected for air/sea gas exchange (Czerny et al., 2012a;Silyakova et al., 2012) seems to be quantitatively most plausible.Although, overall quantity compares relatively well with results from oxygen and in situ 13 C-primary production estimates (Tanaka et al., 2012;de Kluijver, 2012), comparability to 14 C incubation data presented by Engel et al. (2012) is week.Much higher 14 C fixation rates can be plausibly explained by the shallow (∼ 1 m) incubation depth, while oxygen incubations obviously experienced more intermediate light and temperature conditions at ∼ 4 m depth.Therefore, bioassays are more useful to compare community production between the treatments rather than giving Introduction

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Full quantitative cumulative estimates for in situ carbon uptake (Tanaka et al., 2012).As incubations were performed only at one depth, it is impossible to integrate these data over time and depth in respect of variable light and temperature gradients.Summing up incubation results to achieve cumulative estimates would lead to further error propagation, whereas NCP calculated from in situ inorganic carbon measurements is per se cumulative and propagation of single measurement errors cannot occur.Further development of the N 2 O tracer concept is focussed on using it not only to determine air/sea gas exchange in stratified mesocosms, but also to estimate diapycnal mixing between surface layer and deep water inside mesocosms.For this purpose, N 2 O gradients developing over time, will be correlated to high resolution profiles of oxygen pH and salinity, measured with CTD sensors.Especially in temperate turbid waters, surface layer production is mostly not restricted by mesocosm length but by light penetration.The photoautotrophic surface layer communicates to some extent with a more nutrient rich deep layer where heterotrophic processes dominate.Budgeting these more naturally structured mesocosms is not only an interesting challenge, but will also introduce new ecological aspects connected to upward and downward elemental fluxes into the biogeochemical interpretation of the mesocosm system.Introduction

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Full Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | −3 ) at the point in time and C N 2 Oaw the calculated (Weiss and Price, 1980) equilibrium concentration of N 2 O with the atmosphere at prevailing bulk water T and S. k N 2 O can be translated into a Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Analytical errors in N 2 O fluxes were small, calculated on the basis of standard deviation of the measured values around the fit (±1.54 nmol kg −1 ).Resulting uncertainty in the determination of daily CO 2 fluxes are shown as error bars in Fig. 2. Here, k for maximum and minimum N 2 O results were used to calculate CO 2 fluxes.Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper |

Fig. 2 .Fig. 3 .
Fig. 1.(a) Drawing of a KOSMOS mesocosm in the configuration used for the Svalbard experiment.(b) N 2 O concentrations during the experiment.Diamonds represent the measured concentration inside the 3 examined mesocosms; blue squares are fitted daily concentrations according to Eq. (2), circles are background N 2 O concentrations measured in the surrounding fjord, triangles are calculated equilibrium concentrations from atmospheric measurements at in situ T and S. Shaded areas indicate periods when waves occurred at the mooring site.