Hydrological drivers of organic matter quality, mineralization and export in a tropical dam-impacted floodplain system
Roland Zurbrügg
Acknowledgements:
Stephan Suter, Bernhard Wehrli, David B. Senn Institute of Biogeochemistry and Pollutant Dynamics, ETH Zürich Eawag, Swiss Federal Institute of Aquatic Science and Technology
Moritz F. Lehmann Institute of Environmental Geosciences, University of Basel, Switzerland
Jason Wamulume, Griffin Shanungu Eawag: Das Wasserforschungs-Institut des ETH-Bereichs University of Zambia, Zambia Wildlife Authority J. Janssen, kafueflats.org Introduction
The Zambezi River Basin
o 8 riparian countries
o Rainfall 950 mm evaporation >90%
o 4 existing dams ( )
6 planned dams ( )
Kafue River Basin: o 152,000 km2
o 2 large dams built in 1970s
3/22 Introduction
The Kafue Flats
Lusaka
Kafue Kafue River NP
Itezhi Kafue Gorge Tezhi Dam Dam 6,500 km2 NP
4/22 Introduction
Upstream Itezhi-Tezhi dam (closed 1978)
5/22 B. McMorrow Introduction
Kafue River in the Kafue Flats
6G./22 Shanungu Introduction
The Kafue Flats
Lusaka
Kafue Kafue River NP
Itezhi Kafue Gorge Tezhi Dam Dam 6,500 km2 NP
800
)
o Seasonal flooding -1
s 600
3
o Dams changed flooding patterns 400
o Affected plant and wildlife ecology 200
(m Discharge
o No biogeochemical evidence 0 Oct Dec Feb Apr Jun Aug Oct
7/22 (from Mumba & Thompson 2005)
Introduction
Importance of tropical floodplain ecosystems
o Floodplains = high-value ecosystems Flood pulse concept
habitat, water supply, flood mitigation, food production Junk et al. 1989
o Important reactors for C and nutrient turnover
o Hydrological exchange: crucial process o Biogeochemistry o Ecological functioning
o Dam impact on exchange?
Bayley, 1995 / epa.gov
8/22 Introduction
Research objectives
1. Hydrological drivers Quantify the hydrological exchange between Kafue River and floodplain. Related to dam operation?
2. Mineralization Effects of river-floodplain exchange on the dissolved oxygen regime
3. Organic matter quality and export Effects of lateral exchange and dam operation on fluxes and quality of organic C and N
9/22 Approach
Sampling strategy
May 2008 Oct 2008 May 2009 May 2010 2000
) -1 1500
s
3
1000 dam release
Q (m 500
0 981 980 979 978
Stage (m a.s.l) Stage
Jan Jan Jan Jan Jan Jan
Sep Sep Sep Sep Sep Sep Sep
May May May May May May 2006 2007 2008 2009 2010 10/22 River-floodplain exchange and dissolved oxygen
Dissolved oxygen (DO)
300 o Steep DO decline over 40 km M) 250
200 o Low DO levels for 150 km 150
100 May 2008 50 May 2009 o Floodplain DO <15 µM Dissolved oxygen ( oxygen Dissolved May 2010 0 0 100 200 300 400 km from ITT dam Hypotheses:
. Inflow of low-DO water
. Injection of labile OM to river
. Exchange with the floodplain
11/22 River-floodplain exchange and dissolved oxygen
Discharge (Q) and natural tracers
May 2010 - flooding season
) 800
-1
s
3 3 600 1 1 steep Q decline 400 Tributaries 200 River o ~80 % loss to floodplain
Discharge (m 0 o no outflows detected 0 100 200 300 400 190 Electrical ) km from ITT dam -1 180 conductivity 2
S cm 170 160 2 increase in tracers at
EC ( 150 constant Q (DO decline) -2 δ18O -3 gain in Q after 300 km and O (‰) -4 3 18 tracer increase -5 (evaporation)
0 100 200 300 400 12/22 km from ITT dam River-floodplain exchange and dissolved oxygen
Channel morphology
) Reduction in channel cross section
2 2000 1 1 2 3 water forced into the floodplain 1500
1000 Flow and transect area 500 2 Transect area (m constant 0 100 200 300 400 km from ITT dam River channel expansion 3 inflow of floodplain water
0 5 10
depth (m) depth 15 0 50 100 150 200 0 50 100 150 200 250 300 350 channel width (m) 1 2 3
13/22 River-floodplain exchange and dissolved oxygen
Tracer mixing model: δ18O
flooding season No exchange -2.5 1 800
)
)
-1 -1 1
s s
3 3 Q 600 2 3 -3.5 2 Intense exchange at constant flow
O (‰) 400 water from O (‰)
18 18 upstream reservoir
200 18 floodplain -4.5
Discharge (m Discharge -4.5 Discharge (m Discharge δ O water 3 >80 % of discharge from floodplain 0
dry season -2.5 800
)
)
-1 -1 18 Mass balance calculations:
s s δ O
3 3 600 Constant flow, -3.5 Lateral exchange DO decline
O (‰) 400 O (‰)
18 limited exchange 18 Q 200 -4.5
Discharge (m Discharge -4.5
Discharge (m Discharge 0 . Seasonal variations? 0 100 200 300 400 km from ITT dam . Role of upstream dam?
14/22 River-floodplain exchange and dissolved oxygen
River-floodplain exchange over longer time scales
o Comparison with data since the 1960s
o FE = measure of
Qin Qout Qin Qout river-floodplain exchange
Fractional exchange ratio FE: Fractional exchange ratio FE: FE < 0
FE > 0
15/22 River-floodplain exchange and dissolved oxygen
River-floodplain exchange over longer time scales
natural hydrology 1962-1971 0.8 o Upstream: outflows from Oct-May 0.4
FE 0.0 -0.4 o Downstream: consistent inflows -0.8
Oct dam-impacted hydrology 1978-2010
Apr
Oct
Apr
Jun
Jun
Feb
Aug
Dec Feb
Aug Dec o Similar seasonality 0.8 0.4 0.0 o Reduction in FE amplitude
FE -0.4 -0.8 Dams have reduced river- floodplain exchange by 50%
Oct
Apr
Oct
Apr
Jun
Jun
Feb
Feb
Aug
Dec
Aug
Dec
16/22 River-floodplain exchange and dissolved oxygen
Conclusions
o River-floodplain exchange: dominant hydrological driver
o Flooding season: >80% of water passes through floodplain
o Driven by channel morphology
o Beyond current concepts
o Impacts on DO regime of the river
o 50% reduction by dam operation
Effects on source and fate of organic C and N
in the Kafue River?
OM OM OM from from reservoir floodplain Hypothesis: Large change in organic matter quality river distance
17/22 Organic C and N
Carbon and nitrogen speciation
flooding season
400 1 2 3 M) o Along sections of high exchange: 300 200 DOC increase, POC decrease 9% POC 91% DOC 100
OC conc. ( 0 30 o High contribution of DON M) 19% PON 20 o Low (<2 µM) DIN concentrations 10 75% DON 6% DIN
N conc. ( 0
0 100 200 300 400 Loads, source and quality of OM? km from ITT dam
18/22 Organic C and N
Export of OC and ON
dryflooding season season
) ) -1 -1 400400 × 4 OC and N loads: C × Q [t d-1] 300300 200200 POC o 4-fold increase in OC, mostly DOC
100100 DOC
OC loads (t C d OC loads (t C d 00
) ) 3030
-1 -1 × 5 o 5-fold increase in N, mostly DON 2020 PON DIN o Deficit: 1,300 t N per year 1010 DON
N loads (t N d N loads (t N d 00 00 100100 200200 300300 400400 Large OC and ON exports, kmkm fromfrom ITTITT damdam >70% mobilized from floodplain
19/22 Organic C and N
Sources of DOM and POM
flooding season
C plants -15 ITT 4 o DOM: terrestrial sedimentsa C3-plants, soils/sediments, C:N~20 -20 soils, sediments
C (‰)C C plants
13 -25 3 Emission(nm) o POM: microbial/algal DOM λ phytoplankton low δ13C measured, C:N~9 -30 POM λ Excitation (nm) 0 10 20 30 6.0 C:N ratio o Spectroscopy: terrestrial origin humic/fulvic acids 4.0
N (‰)N 15 o Constant δ N-DON, high N2-fixation? 15 2.0
0.0 N2-fixation
0 10 20 30 terrestrial DOM, phytoplankton POM C:N ratio
a 20/22 Kunz et al. 2011 Organic C and N
Conclusions
o Mobilization and export of floodplain DOM DOM o Little variation in DOM composition river distance o Stable, refractory (from upstream wetlands?)
o No change during reservoir transit
terrestrial DOM: mobilized from floodplain DOM organic matter river distance
o Terrestrial POM trapped by dam Terrestrial DOM (Kunz et al. 2011) Reservoir POM
o Discharge of phytoplankton POM Reservoir DOM
POM: PP from reservoir and floodplain Terrestrial POM high dam impact
21/22 22/22