Professor Graham Harris, ESE Systems
prepared for the 2006 Australian State of the Environment Committee, 2006
Although the data are sparse, there is evidence that habitat modification and land use change have altered the stream metabolism of inland waters. Increases in aquatic nutrient levels and productivity, as well as a change in the structures and metabolism of biofilms, have changed the sources and availability of carbon and other major nutrients in rivers. More of the carbon supplying stream metabolism now comes from more readily available sources from instream production, and less comes from relatively refractory catchment and riparian sources. These changes are consistent with the change of state from clear to more turbid conditions. All the changes seem to favour increased algal blooms, greater sediment respiration and oxygen demand, and more frequent periods of anoxia in the bottom waters of pools, weirs and dams. Loss of riparian vegetation also favours this same sequence of interactions, particularly in upland streams.
The construction of large numbers of dams and weirs has led to the impoundment of large volumes of water. Through the resulting increasing water residence times, particularly in major river basins, the more easily decomposed sources of carbon are removed and oxygen is stripped from the water. This has led to increased nitrogen stripping through denitrification and a lowering of the nitrogen:phosphorus ratio; further favouring the growth of cyanobacterial blooms. Nitrogen cycling in rivers has been further modified by the construction of impoundments because the cold water that is discharged downstream from the bottom waters of dams and weirs is low in oxygen and rich in ammonia from decomposition of organic matter in those waters. Large dams act as sources of ammonia for downstream ecosystems.