When the River Runs Dry: The Ecology of Dry River Beds Author Steward, Alisha Louise Published 2012 Thesis Type Thesis (PhD Doctorate) School Griffith School of Environment DOI https://doi.org/10.25904/1912/3847 Copyright Statement The author owns the copyright in this thesis, unless stated otherwise. Downloaded from http://hdl.handle.net/10072/366740 Griffith Research Online https://research-repository.griffith.edu.au When the River Runs Dry: The Ecology of Dry River Beds Alisha Louise Steward Bachelor of Science (Honours) Australian Rivers Institute Griffith School of Environment Science, Environment, Engineering and Technology Griffith University, Nathan, Queensland, Australia Submitted in fulfilment of the requirements of the degree of Doctor of Philosophy July 2012 Abstract Temporary rivers and streams that naturally cease to flow can be found on every continent. Many others that were once perennial now also have temporary flow regimes due to the effects of water extraction or changes in land-use and climate, while others that used to run dry no longer do so due to water releases and waste water discharges. The dry beds of temporary rivers are an integral part of river landscapes and have: a role as seed and egg banks for aquatic biota; a unique diversity of aquatic, amphibious and terrestrial biota; a role as dispersal corridors; as temporal ecotones linking wet and dry phases; and as sites for the storage and processing of organic matter and nutrients. They also have a societal values, such as significance in human language and culture; agricultural uses; sources of sand and gravel for building purposes; and as places for recreation. ‘Traditional’ conceptual models of riverine ecosystem structure and function do not consider the dry phase. As a consequence, these models are incomplete and are thus not fully applicable to many parts of the world where temporary rivers are common and the dry phase is significant. For this thesis, I developed conceptual models of temporary river ecosystems that illustrate both wet and dry phases. I then investigated whether the invertebrate fauna of dry river beds were subsets of the riparian zone, or a unique assemblage. I monitored the temporal changes in invertebrate assemblages to determine how they responded as a river bed dried. Invertebrates were selected as they are ubiquitous, easy to collect, and can rapidly colonise newly-created habitats following events such as floods or droughts. I then examined the contribution of algae and terrestrial sources to dry river bed food webs using stable isotope analysis, and I conducted experiments using dead fish to i examine the potential rate of uptake by terrestrial consumers of aquatic material that may become available as a river dries. Dry river beds can be physically harsh places, and often differ substantially in substrate, topography, microclimate, and inundation frequency when compared to adjacent riparian zones. In this study, dry river beds experienced higher maximum and lower minimum temperatures than riparian zones, with some summer surface temperatures above the thermal tolerance of most eukaryotic organisms (60 °C). To investigate the geographical and climatic breadth of the prediction that dry river beds harbor unique invertebrate assemblages, sites were sampled in catchments with a diversity of substrate types, climate characteristics, and river flow regimes in Australia and Italy. The findings showed that dry river beds can be colonised by diverse assemblages of terrestrial invertebrates, which differ significantly in their composition to that of adjacent riparian assemblages, highlighting dry river beds as unique habitats. As a river bed dries, the invertebrate assemblages become more ‘terrestrial’ over time, with a shift from an aquatic-dominated assemblage to a terrestrial-dominated one; yet, dry river bed assemblages were found to remain unique from those in adjacent riparian zones, and are therefore not simply a subset of riparian assemblages. Invertebrate assemblages sampled in and near a drying river bed in south-east Queensland, Australia, were more similar to each other than to those sampled further into the riparian zone. Even after a river bed had been dry for four months, the terrestrial invertebrate assemblage did not converge with the assemblages found throughout the riparian zone. A drying river can mean certain death for many aquatic taxa, particularly those which do not have desiccation-resistant stages or the ability to move to water elsewhere. Dead ii and dying aquatic biota stranded in dry river beds can become food for terrestrial consumers, with some biota representing a higher quality of food than others. Terrestrial consumers were found to have a low reliance upon filamentous algae as a food source, with some filamentous algal material remaining four months after the river bed had dried. Benthic algae have been shown to drive productivity in some aquatic systems. Stable isotope analysis revealed that algae were not subsidising the terrestrial food webs associated with dry river beds. Although not consumed, it appeared that dried filamentous algal mats played a role in providing a sheltered micro-climate for terrestrial invertebrates, and a drying refuge for some aquatic invertebrates. There was no evidence of terrestrial primary consumers eating material of aquatic origin; however, there was evidence of a brief ‘clean-up crew’ of terrestrial secondary consumers eating biota of aquatic origin following the drying of an isolated pool. Experiments using dead fish placed onto dry river beds showed that these can be a short-term food source for a range of terrestrial consumers, including mammals, birds, invertebrates, and microorganisms. In dry river beds in both south-east Queensland, Australia, and northern Italy, dead fish that were available to all possible consumers were typically consumed within one to four days. The dry phase of temporary rivers, and therefore dry river beds, is often associated with negative connotations by the public (Boulton et al. 2000), as they do not contain surface water that is useful to people, and they are present during droughts, which have detrimental consequences for people and ecosystems. Dry river beds have even been referred to as ‘biologically inactive’ or biological deserts (Stanley et al. 1997). However, this is certainly not the case: they do support a range of important human and ecological values. Unfortunately, they have been largely ignored by aquatic and iii terrestrial ecologists, probably because they are perceived to be outside the domain of their respective disciplines. In light of their values, dry river beds need to be recognised and fully integrated into river management, monitoring programs, policy, and planning. Catchment-wide assessments of riverine health that ignore dry river beds are often unrepresentative of many riverine landscapes throughout the world. Dry river beds could be monitored and assessed using biological indicators, in the same way that indicators are currently used to assess aquatic ecosystem health. The results of this study provide a way forward for the recognition of dry river beds as habitats in their own right. Dry river beds must now be recognised as unique habitats in biodiversity surveys, included in catchment management plans, targeted in biological monitoring programs of river and catchment health, and ultimately, protected from future threats to their human and ecological values. iv Table of contents Abstract .............................................................................................................................. i List of figures .................................................................................................................. ix List of tables ................................................................................................................... xv List of appendices ......................................................................................................... xvii List of abbreviations and acronyms ............................................................................. xviii Statement of originality ................................................................................................. xix Acknowledgements ........................................................................................................ xx Chapter 1: General introduction ....................................................................................... 1 1.1 Terrestrial limnology: when the river runs dry ...................................................... 3 1.2 Values of dry river beds......................................................................................... 6 1.2.1 Societal values of dry river beds...................................................................... 6 1.2.2 Ecological values of dry river beds ................................................................. 8 1.3 Temporary rivers and connectivity in the landscape ........................................... 12 1.4 Aquatic-terrestrial linkages in temporary rivers .................................................. 14 1.4.1 Aquatic-terrestrial subsidies .......................................................................... 14 1.5 Riverine conceptual models................................................................................. 16 1.5.1 ‘Traditional’ models ...................................................................................... 16 1.5.2 Models relevant