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The Divisa, Blang and Salto chain of reservoirs form the Salto System of hydroelectric plants, located in Sao Francisco de Paula, Rio Grande do Sul, Brazil. In the summer of 1999, a toxic algae bloom originated at Blang reservoir, contaminating the water supply reservoirs of six municipalities downstream and required special water treatment methods. This event led to the investigation of the possible sources of nutrients, particularly phosphorus, identified as the limiting nutrient. These reservoirs are located in a headwaters region with a humid subtropical climate, rural land use pattern and low demographic density. In the period between 2001 and 2003, monitoring demonstrated good water quality, with oligotrophic and mesotrophic characteristics. The investigation of the possible causes of water fertilization considered external sources (mainly non-point sources) and the internal source represented by the organic, nutrient-rich bottom sediment. The present work seeks to characterize the physical-chemical stratification of the water column and investigate the possibility of hypolimnetic water circulation under stratified conditions. In order to carry out the study, vertical profiles of parameters that indicate the physical-chemical stratification were obtained, such as temperature, dissolved oxygen and redox potential, in addition to total dissolved phosphorus and turbidity from the surface to the hypolimnion and chlorophyll-a in the photic zone. The circulation of hypolimnetic water was examined through bathymetry and dimensionless numbers that reflect the stability of the stratified body of water. Results indicate that hypolimnetic water circulation is not likely to occur under stratification, because only extreme climate events could generate winds strong enough to invert the liquid mass. The circulation of metalimnetic waters is a more likely possibility, as moderate winds could ascend the thermocline. However, the transfer of dissolved phosphorus from the hypolimnion to the reservoirs downstream could occur when the reservoir bottom gates are open to regulate discharges, which is a frequent occurrence during drought periods. Key-words: physical limnology, cascade reservoirs, phosphorus, algae bloom.

Circulação da Água Hipolimnética no Reservatório Divisa do Sistema Salto (RS)