Ecological responses to the initial filling of an enlarged temperate reservoir: Cotter Reservoir (ACT, Australia)

  • Sally Hatton

Student thesis: Doctoral Thesis


The impoundment of rivers is one of the greatest threats to freshwater ecosystems and biodiversity worldwide. With increased population growth and climate change it is inevitable that more dams will be built in the future in many regions to supply domestic water and for the production of electricity. There is a remarkable lack of understanding surrounding the ecological mechanisms that occur during the filling of new reservoirs. This thesis provides an investigation of some of the processes that occur during the initial filling phase of an enlarged temperate upland reservoir. Cotter Reservoir on the Cotter River,Australian Capital Territory (ACT),was recently enlarged to twenty times its previous capacity,which provided a unique opportunity to investigate the ecological processes that occur during the filling of a temperate reservoir. The best-documented phenomenon of reservoir filling is trophic upsurge,where increased nutrients in the water column are transferred up the food chain.. The first phase of trophic upsurge is the release of nutrients from freshly inundated vegetation and soil into the water column. It is known that significant amounts of nutrients are released into the water column during the filling phase of reservoirs in tropical systems. The release of nutrients from the dominant vegetation in the inundation area of Cotter Reservoir and the changes in the concentration of total nitrogen (TN) and total phosphorus (TP) in the water column of the reservoir during the initial filling phase were investigated. It was hypothesised that the filling of ECR would result in a significant increase in nutrients that would be released from the dominant types of in-undated vegetation. The results indicated that eucalypts and pine,the two dominant vegetation types in the inundation zone,release the most nutrients of all vegetation types sampled. The initial nutrient release happens quickly and is sustained over a 60-day period. During the filling of the enlarged reservoir the long-term water quality data shows peaks in TN and TP concentrations during the initial reservoir filling. This study also examined long term water quality data from two other reservoirs. These data sets allowed the investigation of the extent of trophic upsurge following re-filling after extreme drawdown during drought. Reservoirs investigated were Corin Reservoir,also on the Cotter River and Googong Reservoir,near-by on the Queanbeyan River. This study shows that they also experience nutrient enrichment during re-filling events,with peaks in nutrient concentrations during filling periods. It was hypothesised that with an increase in the nutrients in the water column,there would be a concomitant increase in densities of aquatic invertebrates. The changes in the densities of microinvertebrates and macroinvertebrates that occurred as Cotter Reservoir began to fill were investigated. Microinvertebrates and macroinvertebrates in two other reservoirs on the Cotter River were also examined during the period of the study. The results indicate that the largest contributing factor to changes in aquatic invertebrates over the course of the study was changes in season. But some differences in the microinvertebrate communities during filling, with an increase in densities were detected. Conversely,slight decreases in the density of macroinvertebrates were observed during the study in Cotter Reservoir. In the two other reservoirs strong seasonal variation of aquatic invertebrates were detected during the study, but no trends were found. The predicted increases in the density of aquatic invertebrate abundance in the filling reservoir were probably not seen because the reservoir filled rapidly. Because increases in nutrients in the water column were found in Cotter Reservoir,there will likely be an increased density of aquatic invertebrates after the filling is completed and habitats stabilise. Before the reservoir began filling,the dominant food resource for fish in the reservoir was decapods. Prior to this project there was concern that Decapods,an important fish food resource,would be displaced by rising water levels as their habitat was flooded and that there would be changes in fish food resources and diet items during the filling period of the reservoir. Because of this concern,this study was focused on aquatic invertebrates as a fish food resource. The diet changes of an endangered endemic fish (Macquarie perch) and an introduced fish (rainbow trout) during the filling of Cotter Reservoir were investigated. It was hypothesised that a change in fish diet would occur during the initial filling period of the newly enlarged reservoir,with a predicted increase in terrestrial items and zooplankton in the stomach contents of both Macquarie perch and rainbow trout. The temporal changes in rainbow trout diet in two other reservoirs on the Cotter River were also investigated. The present results show that fish diet in the filling reservoir initially changed from aquatic invertebrates to terrestrial items,particularly terrestrial Oligochaetes,displaced by rising water. By mid-filling there was a shift to aquatic invertebrates (particularly Decapods) and an increase in piscivory in rainbow trout. In the other two reservoirs,rainbow trout diet was also dominated by terrestrial items across sampling periods,however these were flying insects,which were likely displaced by wind from surrounding forest. Finally,the overall changes in the food web during the filling of the enlarged reservoir were investigated. It was hypothesised that (1) pre-enlargement that aquatic resources (primarily submerged and emergent macrophytes) were the major basal food resource and this would shift to terrestrial sources during filling,(2) during reservoir filling,invertebrates and fish within the reservoir would assimilate more material derived from terrestrial production sources,and (3) there would be an increase in trout piscivory. Changes were detected with the use of stable isotope analysis (SIA) of the food web of the filling reservoir. In line with the dietary study,the SIA results indicate that terrestrial inputs became more important during the filling of the reservoir. Most importantly the results from this study suggest that native and exotic fish species are occupying same trophic level,which may have conservation consequences. Finally,this thesis outlines the contribution that the study has made to filling gaps in our understanding the ecological changes that occur during the filling of temperate reservoirs. With more dams being constructed in the future the study highlights the potential to use reservoirs as refuges for threatened aquatic species. The key management implications of this research include the potential of trophic upsurge during the initial filling phase of temperate reservoirs and how this may impact the persistence of nutrients in the system beyond filling and the implications for organisms residing in these systems. This study provides evidence that further work is needed to increase our understanding of the ecological effects of the filling of reservoirs.
Date of Award1 Jan 2016
Original languageEnglish
Awarding Institution
  • University of Canberra

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