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During 1995-1996, CWR in conjunction with SKM consultants designed and constructed a destratification device in North Pine Dam, Queensland. The dam experienced high concentrations of manganese, iron, and cyanobacteria (blue-green algae). The destratification unit consisted of two 600 metre long diffusers, with 200 L/s of compressed air pumped through 68 ports spaced 17 metres apart.
Recent analysis (2002-2003) of long-term temperature and dissolved oxygen data shows the strong impact of the destratification unit in increasing dissolved oxygen concentrations at depth (FIGURE 1). A detailed analysis of phytoplankton records by Antenucci et al (2004) has demonstrated the positive impact of the destratification in reducing the rate of eutrophication, despite catchment degradation leading to a continuing increase in phytoplankton concentrations (FIGURE 2).
FIGURE 1: Monthly average dissolved oxygen concentrations as a function of time and depth before (upper panel) and after (lower panel) the implementation of the destratification system. Note the dramatic reduction in the duration and vertical extent of the anoxic region.
FIGURE 2: Average annual chlorophyll a concentration showing data from the wall (blue circles), the offtake (red triangles) and from the upstream region far from the destratification unit (red squares). Trend lines are also indicated, and show the reduced rate of increase of chlorophyll concentrations in the vicinity of the dam wall as a result of the destratification system, and the continuing increase in concentrations in the upstream regions. The continuing upward trend is due to ongoing catchment degradation and loading.
The long-term effect of artificial destratification on phytoplankton species composition in a subtropical reservoir
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