The impacts of farming and river regulation on billsbongs of the Southeast Murray Basin, Australia
This thesis is an investigation into the effects of farming and river regulation on billabong (floodplain lake) ecology. The study region is in the southeast comer of the Murray-Darling Basin. Billabongs comprise a significant natural freshwater lake system in the one million km2 drainage basin dominated by semi-arid conditions. The region was settled by Europeans in the middle 1800's, and early farming, activities, mainly the running of cattle and sheep, were particularly intense....[Show more] Extensive timber extraction also occurred at this time. Both these activities have probably declined to a degree since the early 1900's but are practised in the region today. Fertiliser use became widespread in the 1930's. River regulation did not begin in earnest until the construction of the Hume Dam in 1930. In the study region regulation has decreased monthly variation in river flows, mainly by maintaining unnaturally high flows during summer irrigation demand, increased daily flow variability with short-term releases, decreased the frequency of minor flooding, slightly increased flood duration, and decreased sediment loads near weirs. These and other impacts of European settlement on the regional landscape are profound, and may have an effect on billabong limnology. Concepts of disturbance ecology are used as a framework for assessing changes to the biota living in billabongs resulting from European settlement. Two complementary lines of evidence are followed, one physicochemical and one biological, and evidence for the impacts of farming and regulation is sought from both historic and present day patterns of limnology. The limnology of 43 billabongs influenced to varying degrees by farming and regulation were surveyed every 2 months over a 15 month period. The billabongs vary in mean depth from about 0.3 to 4.5 m, and significant fluctuations in depth occur, both seasonally and between wet and drought periods. Natural variation in total nitrogen, total phosphorus, turbidity, and secchi depth billabongs is marked and dominated by temporal factors; mainly fluctuations in depth accompanying seasons and droughts, but also flooding. Significant shorter-term temporal variation is also implied from the data. In contrast, natural variation in pH is minor and salinity is always low. Maximum depth is the only systematic source of natural spatial variation revealed, apart from a slight east-west shift in major ion balance; the main determinants of nutrient limnology and light environment of billabongs found in this study are temporal factors. Since natural variation in physicochemistry is dominated by relatively cyclical factors (seasons and floods), much natural disturbance in billabongs appears to be of the pulse variety, and to be moderately to highly predictable. It is likely the biota have evolved adaptations for this variation. Farming only affects phosphorus and salinity, but the effect on phosphorus is minor in comparison to natural variation, and salinity levels remain low. In contrast, river regulation has important direct effects on billabong depth, which on the decadal scale translates to billabong permanence. The indirect effects of regulation on nutrients and the light environment, from effects on depth, are likely to equal natural variation. However, most of the anthropogenic variation mimics natural variation, and the biota may already possess the adaptations to deal with it (may be 'preadapted'). The pattern of rapid depth fluctuations and summer flooding of low-lying billabongs created by regulation is 'new' to the ecosystem, at least in the current climatic regime, and the biota may not cope as well with this. Based on the skeletal remains of Cladocera in the surface sediments of 41 billabongs, farming and river regulation cause a relatively minor impact on the cladoceran fauna. Farming and regulation both affect some uncommon species, and regulation affects the overall diversity, richness and equitability of assemblages. Farming therefore has less of an effect than regulation, which is consistent with the patterns of physicochemistry observed. However, on the whole, assemblages from farmed and regulated billabongs are not very different from those in billabongs remote from farms and on unregulated river reaches, suggesting that the Cladocera are 'preadapted' to anthropogenic environmental variation by relatively high natural variation in physicochemistry. The above conclusions of no or low impact of farming and regulation on billabongs rely on billabongs remote from farms and on unregulated river reaches being unimpacted by these land use activities. This assumption is tested by examining the historical sedimentary record of billabongs. The historical record of physicochemistry in 8 billabongs was examined. Sedimentation rates are currently about 5 mm per year, but have increased by an order of magnitude since settlement. Based on the stratigraphy of sediment structure, organic matter content, and the atomic ratio Fe:Mn, redox conditions have changed in a minimum of 4 billabongs and a maximum of 6 since settlement. These include billabongs on unregulated river reaches and presently distant from farms. The direction of change is usually towards more oxidising conditions, but it is possible that the interpretation of the direction of changing redox (as distinct from the occurrence of changing redox) has been corrupted by the higher sedimentation rates following settlement. External phosphorus loading has, if anything, declined since settlement, but it is more likely that no change has occurred at all and that the patterns of phosphorus are due to the changing redox conditions. Because redox conditions have changed in billabongs considered as low-impact controls, assessments of impact based on the present day limnology of billabongs may underestimate the effects of farming and river regulation. The historical record of both Cladocera and the extent of macrophyte cover are inferred from the stratigraphy of cladoceran assemblages in the sediments, and the record of siliceous algal productivity is obtained from profiles of loosely bound silica. Aquatic macrophyte abundance in 5 of 7 billabongs examined decreases markedly at settlement, notwithstanding that taphonomic alteration of assemblages increases. The pattern is most pronounced in large, deep billabongs, and absent from small billabongs. The decline in macrophytes appears to have resulted in the demise of a group of closely related Rak or Ephemeroporus species that are codominant before settlement. Otherwise, the regional diversity of Cladocera has changed little with settlement, and possibly even increased from the introduction of exotic species. However, temporal trends in 7 species suggest they will be pushed towards regional extinction in the future. Siliceous algal productivity appears to have decreased in 4 of 7 billabongs with settlement, although the patterns may be an artefact of increasing sedimentation rates. The patterns are most pronounced in small billabongs, and due to their timing an early land use activity is again implicated. The historical data suggest that the aquatic macrophyte flora and associated fauna declined due to an early land use activity related to farming. It is likely that the change in redox occurred due to the decline in macrophytes, which lowered organic matter supply to the profundal zone. Furthermore, the persistence of depressed macrophyte levels, and continuing trends of changing relative abundance of some cladoceran species, suggest that the farming impacts are of the press variety, yet direct farming activity on the floodplain may have decreased this century. The macrophyte-free state may been maintained in large, deep billabongs, in spite of the removal of the disturbance agent, because of the existence of alternative stable states in the aquatic vegetation. Early farming may have caused the large, deep billabongs to switch from natural macrophyte dominance to natural phytoplankton dominance, as has been suggested to occur in shallow lakes in Europe. If so, it is quite likely that the decline of macrophytes in large, deep billabongs is reversible by temporarily lowering water levels in the spring time, and this would greatly benefit billabongs and the greater floodplain ecosystem. Once macrophyte beds are reestablished they should be stable unless further unnatural perturbations occur. If properly designed, attempts to restore macrophytes in billabongs can be used to test the hypothesis of alternative stable states in billabong vegetation, so that such attempts offer an opportunity to meld management and science in one project. This work demonstrates that even in ecosystems with high rates of turnover, events with return times on the order of centuries can have significant impacts on the ecosystem. Assessments of anthropogenic impacts to billabongs that do not include an historical component are likely to provide underestimates of the full impacts. While disturbance concepts appear to explain some of the patterns observed in the biota, other traits of the ecosystem (e.g. alternative stable states in the vegetation) may be required to explain the broad-scale patterns observed.
|Collections||Open Access Theses|
|b20046479-ogden.pdf||153.38 MB||Adobe PDF|
Items in Open Research are protected by copyright, with all rights reserved, unless otherwise indicated.