Hydrological drivers of organic matter quality, mineralization and export in a tropical dam-impacted floodplain system

Transcription

1 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 University of Zambia, Zambia Wildlife Authority Eawag: Das Wasserforschungs-Institut des ETH-Bereichs

2 J. Janssen, kafueflats.org

3 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 km 2 o 2 large dams built in 1970s 3/22

4 Kafue River Introduction The Kafue Flats NP Lusaka Itezhi Tezhi Dam Kafue Gorge Dam 6,500 km 2 NP 4/22

5 Introduction Upstream Itezhi-Tezhi dam (closed 1978) 5/22 B. McMorrow

6 Introduction Kafue River in the Kafue Flats 6/22 G. Shanungu

7 Kafue River Introduction The Kafue Flats NP Lusaka Itezhi Tezhi Dam Kafue Gorge Dam 6,500 km 2 NP o Seasonal flooding o Dams changed flooding patterns o Affected plant and wildlife ecology Discharge (m 3 s -1 ) /22 o No biogeochemical evidence 0 Oct Dec Feb Apr Jun Aug Oct (from Mumba & Thompson 2005)

8 Introduction Importance of tropical floodplain ecosystems o Floodplains = high-value ecosystems habitat, water supply, flood mitigation, food production Flood pulse concept Junk et al o Important reactors for C and nutrient turnover o Hydrological exchange: crucial process o o Biogeochemistry Ecological functioning o Dam impact on exchange? Bayley, 1995 / epa.gov 8/22

9 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

10 Approach Sampling strategy 2000 May 2008 Oct 2008 May 2009 May 2010 Sep Jan May Sep Jan May Sep Jan May Sep Jan May Sep Q (m 3 s -1 ) Jan May Sep dam release Stage (m a.s.l) /

11 River-floodplain exchange and dissolved oxygen Dissolved oxygen (DO) Dissolved oxygen ( M) May 2008 May 2009 May km from ITT dam o Steep DO decline over 40 km o Low DO levels for 150 km o Floodplain DO <15 µm Hypotheses: Inflow of low-do water Injection of labile OM to river Exchange with the floodplain 11/22

12 River-floodplain exchange and dissolved oxygen Discharge (Q) and natural tracers EC ( S cm -1 ) Discharge (m 3 s -1 ) 18 O ( ) May flooding season Tributaries River Electrical conductivity δ 18 O 1 2 km from ITT dam steep Q decline o o ~80 % loss to floodplain no outflows detected increase in tracers at constant Q (DO decline) 3 gain in Q after 300 km and tracer increase (evaporation) 12/ km from ITT dam

13 River-floodplain exchange and dissolved oxygen Channel morphology Transect area (m 2 ) Reduction in channel cross section water forced into the floodplain Flow and transect area constant km from ITT dam 3 River channel expansion inflow of floodplain water depth (m) channel width (m) /22

14 River-floodplain exchange and dissolved oxygen Tracer mixing model: δ 18 O Discharge (m 3 s -1 ) flooding season δ 18 O Q water from upstream reservoir floodplain water O ( ) No exchange Intense exchange at constant flow >80 % of discharge from floodplain dry season Discharge (m 3 s -1 ) δ 18 O Q Constant flow, limited exchange O ( ) Mass balance calculations: Lateral exchange DO decline km from ITT dam Seasonal variations? Role of upstream dam? 14/22

15 River-floodplain exchange and dissolved oxygen River-floodplain exchange over longer time scales o Comparison with data since the 1960s Q in Q out Q in Q out o FE = measure of river-floodplain exchange Fractional exchange ratio FE: Fractional exchange ratio FE: FE < 0 FE > 0 15/22

16 River-floodplain exchange and dissolved oxygen River-floodplain exchange over longer time scales FE natural hydrology o Upstream: outflows from Oct-May o Downstream: consistent inflows FE Oct Dec dam-impacted hydrology Feb Apr Jun Aug Oct Dec Feb Apr Jun Aug Oct Dec Feb Apr Jun Aug Oct Dec Feb Apr Jun Aug o Similar seasonality o Reduction in FE amplitude Dams have reduced riverfloodplain exchange by 50% 16/22

17 OM 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? Hypothesis: Large change in organic matter quality OM from reservoir river distance OM from floodplain 17/22

18 Organic C and N Carbon and nitrogen speciation OC conc. ( M) flooding season % POC 91% DOC o Along sections of high exchange: DOC increase, POC decrease N conc. ( M) % PON 75% DON 6% DIN o High contribution of DON o Low (<2 µm) DIN concentrations km from ITT dam Loads, source and quality of OM? 18/22

19 Organic C and N Export of OC and ON OC loads (t C d -1 ) N loads (t N d -1 ) dry flooding season season POC 100 DOC PON 4 5 DIN 10 DON km from ITT dam OC and N loads: C Q [t d -1 ] o 4-fold increase in OC, mostly DOC o 5-fold increase in N, mostly DON o Deficit: 1,300 t N per year Large OC and ON exports, >70% mobilized from floodplain 19/22

20 λ Emission (nm) Organic C and N Sources of DOM and POM flooding season 13 C ( ) ITT sediments a phytoplankton C 4 plants soils, sediments C 3 plants DOM POM o DOM: terrestrial C 3 -plants, soils/sediments, C:N~20 o POM: microbial/algal low δ 13 C measured, C:N~9 15 N ( ) C:N ratio o o λ Excitation (nm) Spectroscopy: terrestrial origin humic/fulvic acids Constant δ 15 N-DON, high N 2 -fixation? 0.0 N 2 -fixation C:N ratio terrestrial DOM, phytoplankton POM 20/22 a Kunz et al. 2011

21 DOM DOM Organic C and N Conclusions o Mobilization and export of floodplain DOM o Little variation in DOM composition o Stable, refractory (from upstream wetlands?) river distance o No change during reservoir transit DOM: mobilized from floodplain terrestrial organic matter river distance o Terrestrial POM trapped by dam (Kunz et al. 2011) o Discharge of phytoplankton POM Terrestrial DOM Reservoir DOM Reservoir POM POM: PP from reservoir and floodplain Terrestrial POM high dam impact 21/22

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