The influence of retention time and wind exposure on stratification and mixing in two tropical Australian reservoirs
Townsend, Simon A.
Archiv für Hydrobiologie Volume 141 Number 3 (1998), p. 353 - 371
published: Mar 9, 1998
ArtNo. ESP141014103009, Price: 29.00 €
The seasonal pattern of stratification and mixing, over an eight year period, was examined for two reservoirs. Darwin River Reservoir (DRR) and Manton River Reservoir (MRR) lie in adjacent catchments, 6 km apart, in the Australian wet/dry tropics. The water bodies differ principally in their retention time and wind exposure. Both reservoirs mix completely during the dry season (Jun-Aug), though the duration of holomixis was longer in DRR due to its greater exposure to wind. Thermal stratification developed in both water bodies early in the dry-wet transition (Aug), reaching maximum stability in October. The stability of stratification then decreased over November and December due to deepening of the epilimnion, erosion of the hypolimnion by the metalimnion and hypolimnetic heat gain. Between December and March, winds associated with major wet season meteorological events, such as monsoonal and cyclonic weather, large tropical storms and a rainfall depression, caused holomixis in DRR, usually lasting a few days. MRR, in contrast, did not mix due to the reservoir's shelter from the prevailing winds and the intrusion of cool inflow which strengthened the stability of stratification. A second stability maximum was reached at the end of the wet season in both reservoirs, when weather conditions had cleared and the surface waters had gained heat. During the transition between the wet and dry seasons (Apr, May), DRR experienced either alternating periods of holomixis and stratification, or a gradual breakdown of stratification, similar to that in MRR. The thermal regime of MRR is classified warm monomictic. In contrast, DRR's pattern of circulation is best described as discontinuous warm polymictic, because the reservoir mixed more than once per year, yet experienced long periods of stratification. Although subject to the same regional climate, the thermal regimes of the two reservoirs differed appreciably mainly due to their different retention times and exposure to wind. In addition to regional climatic influences, the interaction between a lake's physical environment and its climate also affect seasonal circulation. This latter influence is probably most pronounced in the tropics where the stability of lake stratification is low relative to higher latitude water bodies.