Waterholes of Australian dryland rivers: Valuable but vulnerable

Transcription

1 Waterholes of Australian dryland rivers: Valuable but vulnerable Stephen K. Hamilton, Michigan State University (USA) & Griffith University (Brisbane) Michael Douglas, Charles Darwin University Stuart Bunn, Griffith University

2 Australia s dryland rivers Prone to spectacular floods owing to monsoonal rainfall Many rivers only flow intermittently Most of the time there may be little to no flow

3 Flow variability is greatest in dryland rivers of Australia and South Africa Summary medians (23 hydrological measures) Cooper Diamantina Mekong Mississippi Colorado Danube Rhine Burdekin Limpopo Fitzroy Vaal Darling 0 OGO NIG MEK NEV OUB SAN FRA NIL BAN DAN PET KOL ZIJ SYR SEV AMU MIS VIS SAO NTH RHI TIS LOI COL DNI SON IND KUR REA BAH STH ARK WIN YAN GOD URA RIO HUA ASS KRI DON REE AUX ODE SNA DAR VAA FIT LIM BUR DIA COO from Puckridge et al. (1998). Mar. Freshw. Res. 49, 55-72

4 Most Australian river systems flow through semiarid or arid landscapes Multiple channels are common Cooper Creek shown here

5 What is a waterhole? Usually refers to deeper segments of river channels Water persists through the dry season Tend to occur at points of flow convergence or constriction Scouring deepens and broadens the channel May form at natural barriers Billabong is used variably and may refer to other floodplain lake types

6 Waterholes are the most frequently inundated water bodies of the river-floodplain system Isolated pans Terminal lakes (e.g. Lake Eyre) River water level (flow) Overflow lakes and swamps Waterholes in furthest downstream reaches Waterholes along distributary channels Main-channel waterholes

7 Limnological features of waterholes - I Periodically flushed by river water Usually contain fresh water Flood waters often are dilute (~ μs/cm) Evapoconcentration can cause salinity to develop Broad shallow lakes on floodplains often become saline Waterholes usually do not (unless there is saline groundwater)

8 Limnological features of waterholes - II Long, narrow, often incised and lined with gum trees Not usually deep enough for persistent thermal stratification (i.e., < 4 m) Diurnal stratification typical

9 Limnological features of waterholes - III High clay turbidity is common Fine particle size (median <1 μm) Anthropogenic influences? Limits aquatic primary production and nutrient uptake Promotes diurnal stratification Seals against exfiltration Already filtered through 0.45-μm membrane!

10 Hydrology of waterholes during the isolation phase Cooper Creek waterholes: Evaporation was slower than expected Tracer study indicated no significant groundwater inputs Results supported by work at other sites Yet some waterholes are probably groundwaterdependent

11 Estimated persistence times of Cooper waterholes Each cluster of 3 bars represents one waterhole and 3 tracers All 12 waterholes would normally not dry between flows (up to 21 months) 7 of the 12 waterholes unlikely to persist beyond 2-3 years between flows 2 of the 12 persist much longer and may be more important as refugia 18 O Na Cl { One of 12 waterholes Months to reach 10% of bankfull volume Hamilton et al Limnol. Oceanogr.

12 Waterholes serve as refugia for diverse aquatic biota

13 Aquatic productivity in waterholes Algae appear to support most aquatic metabolism Benthic algae along edges may be particularly important Phytoplankton can also be abundant in isolation phase Macrophytes often absent Frequency Chlorophyll-a in waterholes (Apr-May 2002) Upper class limit (ug/l)

14 Food webs in waterholes are based on algae 3,650 Gross Primary Production (P) Respiration (R 24 ) (mg C m 2 day -1 ) P:R = 1.9 1,960 P:R = Stable isotope signatures Bunn et al. (2003) CPOM: FPOM: Riparian Smelt Yabbies Yellowbelly Snails Shrimp Gudgeons Algae Benthic algae = 70-80% consumer biomass

15 Waterholes as ecological and cultural assets Aquatic biota concentrated as floods recede (subsidies) Aquatic refugia Support riparian forest Important water source for humans, livestock, wildlife Highly valued by people Harvest of resources Indigenous cultures Ecotourism

16 Vulnerability to human impacts Alterations in the natural flow regime (dams, weirs, diversions, capture or extraction of water) Reduced floodplain contact Erratic water level changes, increased low flows Reduced aquatic production? Sedimentation in absence of scouring flows Livestock trampling Climate change Altered hydrology Sea level rise

17 New research on tropical rivers including waterholes TRaCK Tropical Rivers and Coastal Knowledge Addresses rivers, floodplains, waterholes Develop knowledge base for sustainable use of water resources Physical, ecological, and social dimensions to be considered

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20 Water holes - refugia