Cyanobacterial (Blue-Green Algal) Blooms in Lake Hume

"February 2006".

Project Number: Development of a Conceptual Model for the Development and Maintenance of Blue-Green Algal Blooms in Lake Hume Including the Preparation of a Draft Monitoring Plan for the Dam - M/BUS/94.

MDFRC item.

42 pages.

Between 2002 and 2004 there were a number of high alert blue-green algal (cyanobacterial) blooms in Lake Hume (i.e. >15,000 cells/ml). This report examines the factors that might be controlling blue-green algal blooms in the Lake, with an intent to develop a conceptual model that links the incidence of blue-green algal blooms to lake or catchment condition. Although many of the factors that are thought to promote blue-green algal blooms occur in Lake Hume, actual blue-green algal blooms aren’t very common. There have been a few recorded incidences of blooms in the lake – e.g. immediately following commissioning of the dam, in 1973, 1981-2 and 2002-04. The last three periods coincided with severe droughts in southeastern Australia. The available data are insufficient to be able to unequivocally assign causation of blue-green algal blooms in Lake Hume. However we propose a model that links sediment chemistry in the lake to bioavailable N:P ratios in the surface waters. Under non-drought conditions we suggest that the surface N:P ratios favour algae other than blue-green algae. Under drought conditions the N:P ratio decreases, leading to bluegreen algal blooms. We propose a monitoring and research strategy to test this model for Lake Hume. Specifically: - We recommend fortnightly depth-integrated samples be taken within the mixed layer for at least two fixed sites in the dam using the methodology and data reporting outlined in Matveev and Matveeva (2005); We recommend that the loads of phosphorus (total and filterable reactive), nitrogen (total, ammonia and oxides of nitrogen) and carbon (dissolved organic carbon and fine particulate organic matter) entering and leaving the lake are determined; We recommend that the phosphorus and nitrogen species (see above) and dissolved oxygen are monitored at least 4 sites within the lake (Murray, Mitta Mitta and Bowna arms and near the wall) both above and below any thermocline, on a fortnightly basis; We recommend that a three dimensional hydrodynamic model of Lake Hume should be developed In addition, we also make a number of suggestions regarding future research to fill identified knowledge gaps.