Hydrogen peroxide as a natural tracer of mixing in surface layers

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<ul><li><p>Hydrogen peroxide as a natural tracer of mixingin surface layers</p><p>Norman M. Scully1, *, Warwick F. Vincent1, David R.S. Lean2, Sally MacIntyre3</p><p>1 Dpartement de biologie &amp; Centre dtudes nordiques, Universit Laval, Sainte-Foy, Qubec,G1K 7P4 Canada</p><p>2 Department of Biology, University of Ottawa, P.O. Box 450, Station A, Ottawa, Ontario,K1N 6N5 Canada </p><p>3 Marine Science Institute, UCSB, Santa Barbara, California, 93106 USA</p><p>Key words: H2O2, tracer, mixing, lakes, eddy diffusion coefficient, UV.</p><p>ABSTRACT</p><p>Vertical eddy diffusion coefficients (Kz) were estimated for the epilimnion of lakes using hydrogenperoxide (H2O2) as a natural, photochemically produced tracer. Modelled profiles of H2O2 pro-duction were combined with observed profiles from the photochemically active region and thewaters below in which H2O2 had penetrated to calculate Kz values for the epilimnion of Lakes Erieand Ontario, and for two bays in a small lake in Ontario, Canada (Jacks Lake). Kz values in bothLake Erie and Lake Ontario were 105 to 102 m2 s1 during our study period. Values of Kz fromJacks Lake reflected the prevailing wind conditions but also varied with site. In a wide (3 km) baywith relatively clear water, Kz values ranged from 105 m2 s1 during calm periods to 104 m2 s1during windy periods. The Kz values were lower in the smaller bay with higher concentrations ofdissolved humic material, increasing from 106 m2 s1 during calm periods to 104 m2 s1 duringwind-induced mixing. The differences in Kz, as determined by this photochemical tracer, a func-tion of lake surface area and wind speed support the applicability of H2O2 in quantifying verticalmixing in the surface layer of small and large lakes.</p><p>Introduction</p><p>Microstructure temperature profiles have revealed that mixing patterns in lakes aremore complex than depicted in the traditional limnological literature (Spigel andImberger, 1987). The epilimnion of lakes can no longer be considered a homo-genous layer of uniform mixing but rather a composite of many sublayers that varyin thickness and mixing velocities (Imberger and Spigel, 1987; MacIntyre, 1993).This heterogeneity can play an important role in controlling light exposure andnutrient cycling. The residence time of plankton within each sublayer will influencetheir growth and physiological responses (Vincent, 1990; Frenette et al., 1996) in-</p><p>Aquat.sci.60 (1998) 1691861015-1621/98/020169-18 $ 1.50+0.20/0 Birkhuser Verlag, Basel, 1998 Aquatic Sciences</p><p>* Corresponding author.</p></li><li><p>cluding for instance damage-repair responses to solar UV radiation (Przelin et al.,1991; Milot-Roy and Vincent, 1994). The estimation of the mixing of particles at the surface of lakes is therefore critical and can be determined through calculationof a vertical eddy diffusion coefficient (Kz) in units of m2 s1 with the equation; F = Kz (dS/dZ) where F is the flux of S and (dS/dZ) the concentration gradient.Thus once the Kz is known, the flux and residence time of dissolved substances (e.g.,nutrients, oxygen) and small particulates (e.g., bacterioplankton, small celledphytoplankton) can be estimated for the various sublayers of lakes. </p><p>A variety of methods are available to measure turbulent diffusion in naturalwaters but each has logistic and interpretational difficulties. Temperature gradientsin the water column obtained through fine structure temperature profiles in combi-nation with the measurement of heat balance terms is a traditional approachtowards determining lake-wide Kz values (Jassby and Powell, 1975). However, hori-zontal advection can cause significant changes in the vertical heat distribution andcan result in misleading estimates of vertical exchange rates unless sampling is lake-wide (Jellison and Melack, 1993).</p><p>High resolution microstructure temperature profiles (vertical resolution of mm)can provide valuable information on mixing processes in lakes (Imberger and Pat-terson, 1990; MacIntyre, 1993) and an instantaneous depiction of the turbulencefield. However limnologists are generally more interested in determining verticalmovement over longer time scales. Lakewide average values of eddy diffusivitiescould be calculated from a time-series of microstructure profiles but the necessarysampling intensity over time and space may be logistically difficult and expensive. </p><p>Acoustic techniques have been used to measure turbulence (Menemenlis, 1994;Farmer et al., 1987). Since turbulent eddies cause fluctuations in the refractive indexfor sound, they can be detected using acoustical instrument arrays. In general such techniques are more applicable to oceans where the density fluctuations aregreater. </p><p>Chemical tracers have been used in many studies of vertical mixing (Spigel andImberger, 1987; Maiss et al., 1994; West et al., 1996). West et al. (1996) for exam-ple compared vertical diffusivities derived from microstructure measurements andtracer diffusivities determined from the vertical movement of sulfur hexafluoride(SF6) in the hypolimnion of a Swiss lake. The values obtained from the two methodswere within a factor of two. Tracers such as SF6 provide reliable estimates of Kz butrequire that the tracer to be followed over substantial depth and time scales. </p><p>Hydrogen peroxide has a number of properties which make it a potentiallyattractive choice as a tracer for lake and ocean mixing. It is produced near the sur-face at a rate predicted from measured variables (UV irradiance, quantum yield,dark decay rates; Scully et al., 1995) and its subsequent redistribution through thewater column could provide a measure of vertical mixing. When UV radiation isabsorbed by dissolved organic carbon (DOC), photochemical reactions may resultin part of the energy reducing dissolved ground state O2 into superoxide (O2).Superoxide may then disproportionate to the metastable chemical species hydrogenperoxide (H2O2) (Cooper et al., 1989a). H2O2 has a relatively long half-life (420 h;Cooper et al., 1994) unlike other photochemically produced reactive oxygen species(e.g., t1/2 = 3 ms for singlet oxygen; Hoign et al., 1989; Hoign, 1990). Since the timescale for mixing processes in the diurnal mixed layer is approximately 0.5 to 30 h</p><p>170 Scully et al.</p></li><li><p>(Imberger, 1985), photochemically produced H2O2 has the appropriate kinetics ofproduction and loss to be useful as a natural hydrodynamic tracer. </p><p>Although several authors have identified the potential value of using measuredconcentrations of hydrogen peroxide as a tracer (Johnson et al., 1986; Cooper et al.,1994; Miller 1994; Scully and Vincent, 1997), no studies to date have used [H2O2] ina quantitative manner to determine mixing rates in natural waters. Sikorski andZika, (1993a, b) modelled the distribution of [H2O2] in the Caribbean Sea. Howeverthey used chemical and physical parameters to reconstruct [H2O2] profiles ratherthan use [H2O2] as a quantitative guide to mixing rates. In this study we developeda method to estimate epilimnetic eddy diffusivities using H2O2. We obtained pro-files of [H2O2] from four North American lake sites that differed greatly in windexposure and with contrasting mixed layer characteristics. The two sites were on theLaurentian Great Lakes and two were from bays in a small lake. The H2O2 deriveddiffusivities were calculated using a budget approach from the flux of H2O2 betweenthe photochemically active region and the underlying waters.</p><p>Materials and methods</p><p>Study sites</p><p>Experiments were conducted in four contrasting environments (Figs. 1 and 2). TheLake Ontario Station 403 (4336 N / 7813 W) was in the exposed (fetch &gt;200 km)</p><p>H2O2 as a tracer of mixing 171</p><p>Figure 1. Lake study areas and sampling sites</p></li><li><p>mid-lake region far from inshore influences. Likewise the Lake Erie Station 84(4156 N/8139 W) was in the middle of the Central Basin. These stations were characterized by low DOC and mesotrophic concentrations of chlorophyll a (CHLa)(Table 1). </p><p>Jacks Lake (4441 N/7802 W) is a small lake located on the edge of the Canadian Shield 100 km north of Lake Ontario that is distinguished by several dif-ferent bays having different optical properties. Brookes Bay is a sheltered colouredembayment (7.6 mg DOC L1) whereas Sharpes Bay is a larger less coloured (5.6 mgDOC L1) bay with a twofold greater fetch. Lake Ontario and Lake Erie containmuch lower DOC (</p></li><li><p>Cooper et al. (1989b), Cooper et al. (1994) and Scully et al. (1995) in their previousstudies of H2O2 dynamics. A comprehensive bathymetry of Jacks Lake is providedby Rowan et al. (1995).</p><p>Temperature profiling</p><p>Temperature profiles at 0.5 m intervals were obtained in Lakes Ontario and Erieusing a Seabird CTD profiler. Temperature profiles were determined at 1 m inter-vals in Sharpes and Brookes Bay using a YSI temperature-oxygen meter.</p><p>H2O2 profiling</p><p>At the Lake Ontario station, during a 48-h period on August 4 and 5 1993, concen-trations of hydrogen peroxide were measured every 3 hours at 10 depths throughthe mixing zone. Similarly, the Lake Erie station was sampled from July 12 to 13,1994 (Scully et al., 1995). The Lake Ontario and Lake Erie stations were sampled atmost times at depths of 0.5, 1, 15, 20, 25 m and every 2.5 m from 2.5 to 12.5 m fromthe CSS Limnos with 6-L Niskin bottles mounted on a rosette. </p><p>During the period of June 23 to 25 1993, a 72-h diel experiment was conductedat the Brookes Bay and Sharpes Bay sites in Jacks Lake (Scully et al., 1995). Lakewaters were sampled every 3 h at a central station in each bay using a 6-L Van-Dornbottle. There was a 30 min difference in sampling times between the two sites.</p><p>Hydrogen peroxide concentration was determined using the horseradish per-oxidase (HRP) scopoletin method by measuring the enzyme mediated loss of fluorescence for scopoletin (Cooper et al., 1988; Scully et al., 1995). For this analy-sis, 20 ml aliquots of lakewater were transferred to a cuvette and placed in a TurnerDesigns Fluorometer equipped with a near-UV lamp with a 365 nm excitation filter(Corning C/S 7-60) and a 490 nm emission filter (Turner 2A and 65A). The samplewas then injected with 100 mL of 0.5 M phosphate buffer (pH 7.0) and 40 mL of0.0096 g L1 scopoletin and the fluorescence then measured. Then 20 mL of a 0.01 Mphosphate buffered 4 g L1 HRP solution containing 2 mM phenol was added andthe sample fluorescence read again after 0.5 min. Calibration curves were obtainedthrough standard additions for each of the waters. Sample concentrations were cal-culated from the slope of the change in fluorescence as a function of the [H2O2](Scully et al., 1995). The slope of the calibration curves varied between waters butwere linear across the measured concentration range. The samples were analyzed intriplicate, with a coefficient of variation (CV) that was typically less than 5%.</p><p>Surface [H2O2] were integrated to give areal estimates (mg m2) for two strata inthe surface mixed layer of each lake: the photochemically active region and theunderlying stratum that extended down to the seasonal thermocline. The photo-chemically active region was defined as the depth stratum where the bulk (99%) ofthe total water column H2O2 is produced. The production rate for this stratum wasdetermined by integrating from the surface to the sampling depth nearest the 1% ofsurface H2O2 production; this can also be approximated using the depth of 10% ofsurface 380 nm radiation at solar noon (N. Scully, unpublished). The depth limit ofthe seasonal mixed layer (i.e. depth of the seasonal or parent thermocline) was</p><p>H2O2 as a tracer of mixing 173</p></li><li><p>estimated from temperature profiles at the time of sampling as 15 m for Lake Erie,10 m for Lake Ontario, 5 m for Sharpes Bay and 3 m for Brookes Bay. The lowerlimit of the photochemically active region (and upper depth limit for the secondstratum) was estimated from H2O2 production profiles as 7.5 m for Lake Erie, 5 mfor Lake Ontario, 1 m for Sharpes Bay and 0.6 m for Brookes Bay. </p><p>Spectral irradiance</p><p>Spectral irradiance (280400 nm every 2 nm) was measured from 6 to 21 h every 15 min using an OL 752 Optronics scanning spectroradiometer (Scully et al., 1995).Subsurface measurements were also made so that attenuation coefficients could becalculated every 2 nm from 280 to 400 nm (Scully and Lean, 1994; Scully et al.,1995).</p><p>Modelled H2O2 profiles</p><p>Modelled depth profiles of H2O2 were determined by multiplying wavelength spe-cific quantum yields (fAl) from Scully et al. (1996) by the total number of photonsabsorbed at that wavelength over a specific depth range Dz. These values were thenintegrated over the UV region (280400 nm) and corrected for decay using thein situ H2O2 decay constants obtained from Scully et al. (1995). The CV for [H2O2]analysis for both fAl and decay constant determination were typically less than 5%. </p><p>In situ dark decay constants for H2O2 were calculated from the slope of the natu-ral logarithm of the nighttime areal concentration over time (Cooper et al., 1994;Scully et al., 1995). These H2O2 decay constants were then applied to calculatedecay corrected net areal production by subtracting H2O2 lost through decay pro-cesses. These values were 0.032, 0.040, 0.041, 0.126 h1 for Lake Erie, Sharpes Bay,Lake Ontario and Brookes Bay respectively and were assumed not to vary withdepth. fAl values were determined for each lake water, with the exception of LakeErie for which Lake Ontario data were used. At wavelengths 300, 340, 360 and 400 nm fAl values were 3.8, 1.5, 0.39, and 0.56 104 for Lake Ontario, 1.9, 0.42, 0.39and 0.14 104 for Sharpes Bay and 2.8, 1.6, 0.97 and 0.12 104 for Brookes Bay(Scully et al. 1996). Apparent quantum yield values every 2 nm were obtainedthrough interpolation from the slope of the relationship of the natural logarithm of fAl over wavelength. </p><p>At any depth z, the spectral irradiance (El) is given by: </p><p>El(z) = Eol exp(klz) (eqn. 1)</p><p>It is assumed that the production of H2O2 is proportional to the absorbed pho-tons El (z)/z per unit depth-scale at wavelength l. Multiplying this value byquantum yield (fAl) and summing the wavelengths which dominate production(280400 nm) gives an estimate of H2O2 production at depth z:</p><p>(d [H2O2]/dt)z = fAl / z El (z) = fAl kl El (z) (eqn. 2)l l</p><p>174 Scully et al.</p></li><li><p>where:</p><p>(d[H2O2]/dt)z = modelled production at depth z (mmol m3 s1) fAl = apparent quantum yield at a specific wavelength l (Scully </p><p>et al., 1996)Eol = surface photon flux at a specific wavelength l (mmol m2 s1) </p><p>(Scully et al., 1995).Eol exp(kl z) = photon flux at depth z at a specific wavelength l (mmol m-2 s-1).</p><p>Modelled production values were then vertically integrated and expressed as arealvalues (Pm, mmol m2 s1) for the photochemically active region.</p><p>Eddy diffusivity calculations</p><p>Epilimnetic eddy diffusion coefficients (Kz) for H2O2 were estimated using a onedimensional model at the four sites throughout the day for the photochemicallyactive region and the underlying stratum within the mixed zone. Modelled arealproduction of hydrogen peroxide was combined with observed areal p...</p></li></ul>

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