The influence of advection on water quality variation in a deep Australian impoundment

Data spanning twenty years (1961-1980), for a site adjacent to the dam wall in Lake Burragorang (34° 55' S; New South Wales, Australia), is analysed with the emphasis placed on physical and chemical (water quality) records. Lake Burragorang is a deep (105 m), dendritic (shoreline development 11.53), essentially warm-monomictic lake with a median bulk retention time of 1.8 years. Inflows are unpredictable seasonally and annually, however, and the study included years with retention times ranging from 0.5 - > 22.0 years. The aim of the investigation was to assess the long-term behavioural variation and to explain the major factors contributing to that variation. The cycles of thermal and oxygen stratification are profoundly influenced by inflow/outflow relations. The monomictic cycle is interrupted in c. 50% of years, by cold underflows which help to prevent the short period of circulation (1 - 2 months) which occurs in other years. A significant role is almost certainly played by the mid-depth outflow current, which is presumed to interfere with convective mixing deeper than c. 44.5 m (below Full Supply Level). The sub-surface off take is not used in dry years. A seasonal cycle of inflow is found for the lake, and the interchange between interflows and under low is predicted with about 80% success on the basis of 4 temperature measurements, from the two major inflowing rivers and the surface and bottom adjacent to the dam. A comparison of monthly mean profiles of temperature and dissolved oxygen from wet and dry year groups (differing in total annual inflow by approximately one order of magnitude) reveals that advective processes increase the summer heat income and help to distribute heat more deeply into the water column. Subsequently, these processes increase the rate of autumnal heat loss, yielding an almost equal winter heat content and effectively increasing the annual heat budget by 22% of the dry year budget. The downward movement of heat is found to be most closely related to the volume of water subtracted through the sub-surface offtake, rather than to the volume of influent water. Within the accuracy of profiling (6 m intervals) the wet and dry years appear not to differ with respect to the depth of the convectively mixed layer during the cooling phase (March - June). The summer maximum of Schmidt stability is lowered by only 1% in wet years, but autumnal stability is lowered by up to 33% while, in winter, stability is enhanced by > 100% of the dry year stability. The maximum Birgean wind-work increases by 100% in wet years as a result of advective effects which are outside its conceptual framework. The volumetric hypolimnetic oxygen depletion rate is doubled in wet years. Turbidity is linearly related to the volume of underflows, but the relationship is much weaker with respect to interflows which generally occasion lower levels of turbidity (for a given inflow volume) at the outflow site. Lake Burragorang is phosphorus limited and a close relationship is found between total phosphorus and chlorophyll concentrations. Inflow and outflow are considered the major contributors to the variation In Lake Burragorang's behaviour for the period 1961 - 1980, altering thermal and oxygen stratification behaviour, markedly affecting water quality, and Introducing the nutrients required to support algal growth above the usually low levels of dry years (1 - 5 mg m-3 of Chlorophyll-a). No obvious period of trophic upsurge is found for the lake, and the data presented does not indicate any significant trend towards eutrophication, though such trends will be difficult to determine against the advection induced variability of the system.