Is turbidity in a shallow Tasmanian lake controlled by wind or by the extent of macrophyte coverage?

Historically, Woods Lake in the Central Highlands of Tasmania has been considered a turbid (normal range 20 – 40 NTU) and meso-eutrophic lake. Woods Lake was impounded in 1962, largely to supply irrigation water to downstream agriculture. The turbidity has been a concern, since this lake contains a significant population of the threatened Galaxias tanycephalus. However, Crook (1995) showed that high turbidity was persistent and strongly linked to low water levels and strong winds. This correlated with small particle size and high phytoplankton biomass. Macrophyte coverage was limited to sheltered areas (Hydro Tasmania, unpublished). A series of sediment traps were used to determine sediment resuspension rates at a range of depths. Initial results indicate that resuspended sediment is largely composed of organic material and rapidly falls out of the water column. Maintenance of higher water levels and expansion of macrophyte beds across the lake may have reduced sediment resuspension. Chlorophyll a concentrations were generally low during the study period. Nitrogen and phosphorus concentrations indicate that the lake is oligotrophic. Our conclusion, based on the sediment trap data, is that turbidity levels in the lake are no longer a concern, unless there are major changes to macrophyte persistence or water levels.