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What can be done with echosounder bubble fluxes in a lake or reservoir : Spatiotemporal variability and atmospheric emissions. Tonya DelSontro Eawag & ETH Zurich , Switzerland. The Bible. Helge Balk and Alex Rynskiy. Simrad EY60/EK60 split - beam echosounder

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Tonya DelSontro Eawag & ETH Zurich , Switzerland

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Whatcanbedonewithechosounderbubblefluxes in a lakeorreservoir: Spatiotemporalvariabilityandatmosphericemissions

Tonya DelSontro

Eawag & ETH Zurich, Switzerland


The Bible

Helge Balk

and

Alex Rynskiy


Simrad EY60/EK60 split-beam echosounder

with 120 kHz, 7° beam angle transducer

Woh

Wohlensee,

Switzerland

Lake Kariba,

Zambia/

Zimbabwe


Bubble Size Calibration


  • How an echosoundercanhelpillustratethespatiotemporalvaribilityofebullition

  • Howatmosphericechosounderfluxes, whicharedependent on bubblesizeand a dissolutionmodel, comparewithothermethods


Lake Kariba (Zambia/Zimbabwe)

  • Subtropical

  • Oligotrophic

  • Monomictic

    • Anoxichypolimnion

  • 3 yearres. time

  • Level ± 4 m

  • Oneofworld‘slargestreservoirs

    • 5250 km2, 280 km long

    • 100 m maxdepth

  • Dendritic

    • 2100 km ofshoreline

1) Howimportantisebullition

in a large reservoir?

2) Large-scalespatialvariability

relatedtoriverinflows?

DelSontro et al. (2011) ES&T 45: 9866-9873


Methodology

  • Comparingebullitiondynamics in bayswithriverinflowswithbayswithout

  • Echosounderandsurfacechambersurveys

Hydroacousticprocessingnote:

- Erased all non-bubbletargets

- Estimatedflux in 5 m segments


Ebullitiondynamics in riverbays

Frequency

Magnitude

  • Data richness & newaspects traditional methodscannotreveal


Small-scalespatialvariability in a hotspot

  • Can makegeneralizationsthatitisrelatedtobathymetry, sedimentationdynamics, or ??


Large-scalevariability in surfaceemissionsandmethodcomparison

  • River deltas do emitmore CH4, mostly via ebullition

  • Ebullitionhotspots (riverdeltas) moreobvious via acoustics


Large-scalevariability in surfaceemissionsandmethodcomparison

Surface Chambers

EchosounderSurfaceFlux

  • Echosoundercoverage an orderofmagnitudemorethanchambercoverage

  • Chambers integrateover all fluxes, eventhelowones –

  • But methodsarewithinerror


Wohlensee (Lake Wohlen), Switzerland

  • Run-of-river reservoir

  • 2.5 km2

  • Meandepth, 10 m

  • 2 dayres. time

  • Waterlevel ± 10 cm


Methodology

  • Reverseditforbubbledensity in thepresenceoffish (all non-bubbletargets)

  • Segment divisionbased on bubble-to-non-bubbleratios (nostandard bin length)


Spatiotemporalemissionvariabilityin a localizedhotspot

  • Obviousdifferenceseven on consecutivedays


Bubble sizevariability per segment

  • Meanbubblediameterillustratesonepossiblecauseforfluxvariability


Bubble sizedistribution

  • Larger bubblescontributemoreto total gas volume

  • Variability in meanbubblesizedistribution per segment, but preferredsizerangeis 4-6 mm diameter


Surfaceemissionmethodcomparison

  • Surfacefluxcalculatedfromhydroacoustics – high variabilitywith median between 100 and 1000 mg m-2 d-1

  • Surfacefluxesfromhydroacoustics (H) andchambers (C) on same day

  • Chamberfluxeshigher – integratingoverlowfluxes


Eddy covariance

Hydroacoustics

Temporal variabilitycomparison

  • Ebullitioncorrelatedwith time ofday,

    mostlikely due towaterlevelfluctuations

Chambers


Spatiallimitationsof all methods

AdaptedfromEugster et al. (2011) Biogeosciences 8: 2815-2831


Spatiotemporalvariabilityofsurfaceebullitionfluxvia hydroacoustics

  • Surfacefluxesaveragedfrom all surveysandcontouredoverbathymetry

  • Standard deviationshowswheremost variable fluxareascanbefound


Conclusions

  • Echosoundingcanhelpefficientlysurvey larger areas

    • Expandingthespatialcoverageof traditional methodsandhelpingtoidentify large-scalevariabilty

  • Per segmentanalysisthenhelpsidentifysmall-scalespatialvariabilitywithinobservedebullitionhotspots

    • Thus aiding in futureresearchforcausesof such variability

  • Bubble sizemaybeimportantcauseforobservedvariability

    • Echosoundercalibrationsallowforbubblesizedetermination

  • Surfacefluxcalculationsaredependent on bubblesizeandthedissolutionmodel

    • Thenhydroacousticmethodscanbeappliedandcomparedwithotheratmosphericemissiontechniques


Thanksto

I. Ostrovsky, D. McGinnis, W. Eugster, D. Senn, M. Kunz, T. Diem, A. Zwyssig, M. Schurter, C. Dinkel, H. Balk, A. Rynskiy, J. Wüest, B. Wehrli

Zimbabweancrew, ZRA, BKW

Swiss National Science Foundation


CH4 emissions from Basin IV

Surface CH4

concentrations

3 other deltas = avg 3 measured

Vertical CH4

profile near dam

Ebullition emits 100x more CH4 than other pathways


L. Kariba relative to other tropical reservoirs

  • Rough upscaling shows ebullition remains most important

    • Higher than that in other tropical reservoirs

      • Dam emissions highest in other reservoirs

  • Total CH4 emission comparatively low

    • Due to dam release of CH4-poor epilimnion water

  • Total CH4 emissions from tropical reservoirs

    1Abril et al., 2005; 2Kemenes et al., 2007; 3Bambace et al., 2007; 4dos Santos et al., 2006


    Ebullition areas

    < 11% of lake area

    CH4 emission from Lake Kariba

    • Dam: 170 t y-1

    • Diffusion: 540 t y-1

    • Ebullition: 60,000 t y-1

    • Total: 10 t km-2 y-1

    • Less than all recorded tropical reservoirs


    Chap. 6 – KARIBA Acoustics


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