Eutrophication study of Lake Daylesford, Victoria

Abstract

The occurrence of algal blooms in Lake Daylesford (area 11.6 ha; maximum depth 9 m) each summer in recent years was investigated. Surface water temperature of the lake varied from 7 to 25ºC. The lake which is fed by Wombat Creek, a small permanent stream, was cheimomictic with stratification occurring between summer and early autumn. In most years classical 'turnover' would be unlikely; rather, epilimnetic water would be flushed from the lake by inflowing creek waters approximately 12 times per year. Surface waters were always well oxygenated but the hypolimnion was anoxic between December 1976 and March 1977.

Transparency was greatest in winter and early spring and least in summer and early autumn, this reduction being caused by autochthonously produced algal matter and allochthonously derived suspended material. Incommon withmany other Australian lakes, Lake Daylesford had ashallow euphoticzone 1-2 m deep that would limit phytoplankton production. In March and June 1977, over 99% of the incident visible light was absorbed in the top 1 m of the water column.

The lake was classified as eutrophic on the basis of chlorophyll a levels. productivity, total phosphorus loadings and algal species. Chlorophyll a levels ranged from < 1 µg 1^-1 in June 1977 to 79 µg 1^-1 in February 1977; the productivity maximum was 210 mg C m^-3 day^-1 (560 mg O₂, m^-3 day^-1) in March 1977 and the estimated total phosphorus loading was 2.8 g P m^-2 year^-1. The algal community was dominated by three genera: the blue-green alga Anabaena, the euglenoid Trachelomonas and the diatom Asterionella.

Evidence suggests that reduction of the phosphorus loading to the lake will do much to eliminate the annual algal blooms. Provision of sewerage facilities for the township of Daylesford may achieve the required reduction. The lake sediments contain elevated levels of phosphorus, mainly a result of past discharges of waste material from a potato-processing factory; the contribution of this sediment phosphorus to the water column is unknown but should be investigated.