The impact of 170 years of flow regulation on processes and patterns in an eastern Tasmanian river system

The flow regime of Tooms River, a tributary of the unregulated Macquarie River in eastern Tasmania, was modified in 1840 by a shallow dam in its upper reaches, resulting in reversed seasonality of high and low flows: a classic, long-term example of a river subjected to anti-drought‚ÄövÑvp, but without the confounding thermal effects of impoundment stratification. I found surprisingly large differences between sites on this regulated branch and sites on the adjacent unregulated branch of the Macquarie, which persisted for 8 km downstream despite tributary inflows. This 2-year comparative study coincided with a prolonged supra-seasonal drought, which enabled examination of the effects of reduced stream flow on both regulated and unregulated examples of confined rivers in a naturally variable climatic regime. The regulated sites remained distinct from the unregulated sites, in terms of riparian litter fall, biofilm abundance and macro-invertebrate diversity. This is one of very few studies of the long-term effects of flow regulation (e.g. see Kondolf and Batalla 2005; Sheldon and Thoms 2006b), and differs from the majority of regulated river studies in that the effects of regulation of Tooms River are not confounded by land use practises or thermal stratification of the reservoir. However, this impoundment is typical of small irrigation impoundments in Mediterranean and semi-arid climate zones. The ecological consequences of drought and antidrought‚ÄövÑvp in a region with a naturally variable flow regime may inform management of biodiversity in other regulated rivers in similar climate zones. Human population increases and climate change pressure on water supplies are likely to increase the demand for many more dams of this size and type of flow alteration (Benstead et al. 1999). The determination of the long-term effects of flow regulation is essential, so that future decisions on water allocations can be based on better knowledge of the impact on downstream ecosystem services. Prolonged regulation of the flow regime of Tooms River has had major effects. Regulation has not only reversed the seasonality of flow, but has also dramatically reduced flow variability, resulting in contraction of the stream channel immediately downstream of the dam, and the establishment of mature eucalypts close to the channel, in contrast to the shrubby riparian vegetation maintained by the variable flow regime of the Macquarie River. The timing of allochthonous litter inputs to the benthos was altered, with peak litterfall delayed until irrigation demand fell at the end of summer. In addition, there was reduced lateral connectivity, with movement of leaf litter between the riparian zone and the benthos dependent on overland flow. As predicted by other studies, biofilms were more abundant in the regulated river, but the closed canopy immediately downstream of the dam did not suppress autochthonous productivity, against expectations. Chlorophyll a analyses indicated an autotrophic system, dominated by cyanobacteria and diatoms, in contrast to filamentous algae and diatoms in the unregulated river. Macroinvertebrate communities were significantly different between rivers, but also between sites within rivers, with a depauperate fauna closest to the dam. There was partial recovery of regulation effects at the downstream regulated site, but the invertebrate fauna remained distinct to the unregulated river. In contrast to the majority of studies, there was little difference in invertebrate species richness or evenness between sites or rivers, but diversity was marginally higher at the unregulated sites. Invertebrates from the adjacent Macquarie River were expected to colonise lentic habitats in the regulated river during the drought, but this did not eventuate, suggesting that there may be significant barriers to dispersal or colonisation. Stable isotope analyses indicated a clear separation of the invertebrate food webs of the two rivers. Although both rivers had abundant leaf litter, terrestrial resources were a minor dietary component in both rivers. Vascular macrophytes dominated unregulated river diets, while cyanobacteria were more important in the regulated river. Feeding preference trials for the dominant terrestrial leaf species and for macrophytes showed few preferences for terrestrial leaves, despite large differences in toughness, C: N ratios and tannin content. Unlike other Australian and overseas studies, most invertebrates showed a clear preference for the macrophyte Triglochin procerum. This was supported by the stable isotope analyses and suggests that the prevalence of macrophytes in invertebrate diets may be underestimated in many food web studies. It is likely that similar long-term effects will eventuate in other regulated rivers in Mediterranean climate zones as irrigation dams mature.