warming and stratification changes in lake kivu, east africa

137 Pages·2013·3.03 MB·English

W
ARMING AND STRATIFICATION CHANGES IN
L K , E A
AKE IVU AST FRICA
A THESIS
SUBMITTED TO THE FACULTY OF THE GRADUATE SCHOOL
OF THE UNIVERSITY OF MINNESOTA
BY
ARTHUR ALLEN AABERG
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS
FOR THE DEGREE OF
MASTER OF SCIENCE
SERGEI KATSEV
JULY 2013
(cid:13)c Arthur Allen Aaberg 2013
Acknowledgements
I am grateful to my advisor, Dr. Sergei Katsev, for his guidance and support – as well
as the opening he had for me to start on this project my first summer as a graduate
assistant. I would also like to thank Professor Thomas C. Johnson for the discussions
I had with him and for allowing me to borrow several books during the data analysis
phase of my research. I’d like to thank Dr. Sean Crowe for the help he provided in the
fieldinRwanda,aswellastheassistanceoftheboatcrewmembersthathelpedwherever
they could while providing us with transportation on Lake Kivu. I also thank Dr. Jay
AustinforthediscussionsI’vehadwithhimandtheknowledgehisclassesprovidedme
with, as well as for having space to conduct lab testing in his research lab at the Large
Lakes Observatory. I also thank Sergio Contreas for the encouragment he has provided
me with during discussions I’ve had with him. I am also thankful for the dissucussions
and support that Matt Kistner, Rozhan Zakaria, Messias Macuiane, Daniel Titze, Blair
Elliott, Hari Chapagain, and the other students at the LLO and those on campus have
providedme,as wellas forthefriendshipsformedin mytimeat UMD.
ThisresearchwasfundedbytheUMNOIPSeedGrant,theUMDVCAA/Dean’sOf-
ficegrantforResearchinEnergyandEnvironment,andSergeiKatsev’sstart-upfunds. I
i
am thankful to the Universityof Minnesota,Duluth PhysicsDepartment summerschol-
arship and the OIP grant for providing me with funding support for summer research
andtheMylanRadulovichGraduateFellowshipprovidedbytheDepartmentofPhysics
at theUniversityofMinnesota,Duluth.
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Abstract
To investigate changes in the temperature and stratification structure in Lake Kivu, we
haveinstalledastringoftemperaturerecordersandperformedCTDcasts. Theobtained
data havebeen compared to historicalprofiles and the heat budget for thelake was ana-
lyzed.
Lake Kivu is a meromictic lake characterized by an anomalous temperature distri-
bution with a temperature minimum close to the base of the seasonally mixed layer.
Warming rate at the depth of the temperature inversion is consistent with the historical
warming rate of the surface layer of ∼0.14 ±0.02 ◦C per decade. Deep waters (greater
than 350 m) exhibit variability in temperature and are currently warming at a rate of
∼0.04 ±0.02 ◦C per decade based on the increase in heat content since the 1970’s and
the increase in temperature seen in the deepest measurements between our 2011 and
2012profiles. ThemonimolimnionofLakeKivucannotbeconsideredtobeinasteady
state.
Thedepthofwind-inducedsurfacemixingduringthedryseasonvariessignificantly
betweenyears. Mixingto80m(thepresentdepthofthetemperatureinversion)requires
continuous winds blowing from the south at 9–10 m s−1, whereas typical wind speed
maximaarearound5–6ms−1 andcapableofmixingtoaround65mdepth. Occasional
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stronger winds cause episodic mixing closer to the inversion which removes heat, but
thisdoesnothappenonaregularbasis. Asthetemperatureinversioninrecenthistorical
profiles has been as shallow as 65 m, mixing to the inversion depth is possible during
years with stronger than average winds. With heat diffusing towards the temperature
inversion from both above and below, the temperature at the inversion will continue to
rise,resultinginareduced transportofheatoutofthedeep watersthatwillincreasethe
rateat whichthewatercolumniswarming.
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Contents
Page
Acknowledgements i
Abstract iii
Contents v
ListofTables ix
ListofFigures x
1 Introduction 1
1.1 ObjectivesofthisWork . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2 Hypotheses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2 Background 5
2.1 LakeKivu: General Information . . . . . . . . . . . . . . . . . . . . . 5
2.2 Stratification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.2.1 Inversionintemperatureprofile . . . . . . . . . . . . . . . . . . 9
2.3 ComponentsoftheHeat and WaterBudgets . . . . . . . . . . . . . . . 10
2.3.1 ThewaterbudgetofLakeKivu . . . . . . . . . . . . . . . . . . 11
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3 Methods 13
3.1 Samplingandmeasurements . . . . . . . . . . . . . . . . . . . . . . . 13
3.1.1 CTD Profiles . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
3.1.2 Array oftemperaturerecorders . . . . . . . . . . . . . . . . . . 17
3.2 Correctionsto Temperature, Conductivity,and Pressure . . . . . . . . . 18
3.2.1 Conductivitycorrections . . . . . . . . . . . . . . . . . . . . . 18
3.2.2 Salinityand DensityProfiles . . . . . . . . . . . . . . . . . . . 20
3.2.3 Pressureto DepthConversion . . . . . . . . . . . . . . . . . . 24
3.3 Heat fluxesaccros thelakesurface . . . . . . . . . . . . . . . . . . . . 26
3.3.1 Heat lossduetoevaporation . . . . . . . . . . . . . . . . . . . 26
3.3.2 Long-waveThermalRadiation Heat Fluxes . . . . . . . . . . . 27
3.3.3 Heat FluxDueto SensibleRadiation . . . . . . . . . . . . . . . 29
3.3.4 SolarRadiationContributionsto theHeat Budget . . . . . . . . 29
3.4 MeteorologicalData . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
3.5 Dissolvedgasmeasurementsinsiliconetubing . . . . . . . . . . . . . . 31
3.6 Simulationofalimniceruptionin lab . . . . . . . . . . . . . . . . . . 34
4 Results 39
4.1 Temperaturedistributionsinthewatercolumn . . . . . . . . . . . . . . 39
4.1.1 Temperatureprofileinversion . . . . . . . . . . . . . . . . . . . 39
4.1.2 Positionsofthermoclines . . . . . . . . . . . . . . . . . . . . . 40
4.2 Lateral variabilityintemperaturedistributions . . . . . . . . . . . . . . 41
4.2.1 Inflows . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
4.3 Temporalvariabilityinthetemperatureprofiles . . . . . . . . . . . . . 46
4.3.1 Variabilitywithinthemixolimnion . . . . . . . . . . . . . . . . 46
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4.3.2 Temperaturevariabilityinthedeep waters . . . . . . . . . . . . 50
4.4 ConductivitydistributioninLakeKivu . . . . . . . . . . . . . . . . . . 51
4.4.1 Spatial variabilityin watercolumnconductivity . . . . . . . . . 53
4.4.2 Temporalvariabilityin watercolumnconductivity . . . . . . . . 55
4.5 Oxygendistributionsin thewatercolumn . . . . . . . . . . . . . . . . 56
4.6 Gas pressuremeasurementswithsiliconetubing . . . . . . . . . . . . . 57
4.6.1 Calibrationofsiliconetubing . . . . . . . . . . . . . . . . . . . 57
4.6.2 Field measurementsofgas pressures . . . . . . . . . . . . . . . 59
4.7 Limniceruptionsin lab . . . . . . . . . . . . . . . . . . . . . . . . . . 63
5 Discussion 65
5.1 Comparisonsofhistoricaltemperaturedistributionsinthewatercolumn 65
5.1.1 Decadal scalewarmingat thedepthoftemperature
inversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
5.2 Heat contentand temperaturechanges in thewatercolumn . . . . . . . 74
5.2.1 Heat contentchanges inthemixedlayer . . . . . . . . . . . . . 74
5.2.2 Heat contentchanges belowthetemperatureinversion. . . . . . 75
5.2.3 Implicationsofwarming . . . . . . . . . . . . . . . . . . . . . 76
5.3 WarminginLakeKivuversusothertropicallakes . . . . . . . . . . . . 77
5.4 DoubleDiffusionand itsVariability . . . . . . . . . . . . . . . . . . . 78
5.4.1 Variabilityin thedepth oftemperatureinversion . . . . . . . . . 82
5.4.2 Temperatureandenergyconservationrequirementsatandabove
thetemperatureinversiondepth . . . . . . . . . . . . . . . . . . 85
5.5 StabilityoftheWaterColumn . . . . . . . . . . . . . . . . . . . . . . 88
5.6 Localweather averages andheat budget . . . . . . . . . . . . . . . . . 93
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5.6.1 Weatheraverages in LakeKivuregion . . . . . . . . . . . . . . 93
5.6.2 ComponentsoftheHeat Budget forLakeKivu . . . . . . . . . 94
5.7 Pycnoclinemovements . . . . . . . . . . . . . . . . . . . . . . . . . . 101
5.7.1 Historicalcomparisonsindicateverticalmovementin the
boundariesofthepycnoclines . . . . . . . . . . . . . . . . . . 101
5.7.2 Temporalvariabilityexpected dueto internalwaves . . . . . . . 102
5.8 Effectsfrom conductivityand temperatureofinflows . . . . . . . . . . 108
5.9 Gas PressureMeasurementsinSiliconeTubing . . . . . . . . . . . . . 109
5.10 Limniceruptionsin lab . . . . . . . . . . . . . . . . . . . . . . . . . . 112
6 Conclusions 114
References 117
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