Very low water levels, cyanobacteria until October, phosphorus and sediments to keep track of. Is the lake’s health threatened? 

Low water levels 

The summer of 2021 was one of record-breaking dryness for the region. Average and real amounts of precipitation from May to September were the weakest since 1961, resulting in lower water levels in the lake. How then can we explain that they were even lower in 2013 and 2016, when there was more rain? Anaïs Renaud, a biologist with Renaissance lac Brome, responds: “The Town of Brome Lake seems to have managed the dam better by letting less water pass through and only opening the sluice gates in response to rainfall.” 

Cyanobacteria until October 

As many will have noticed, cyanobacteria were visible as of mid-august and were more abundant in October. Why? “The response is simple,” says Renaud. “The warm weather lasted longer this fall, thereby prolonging ideal conditions for the growth and proliferation of blue-green algae.” 

More minerals in the water 

In 2021, concentrations of phosphorus and suspended matter in tributaries increased. We know that phosphorus is principally introduced into water courses during rainfall and by shoreline erosion. Where then does this extra phosphorus comes from if we had less rain in 2021? Two factors seem to account for this: heavy rains that fell over short periods and that then flowed towards water courses without being absorbed by the ground and the very low water levels which facilitated the re-suspension of sediments in the water column. 

A fragile equilibrium 

The year 2021 showed us that the health of the lake is still fragile and that its equilibrium can easily be disturbed. What is to be done? Here are a few suggestions:
• Avoid the overflow of sewers and septic systems;
• Avoid the use of fertilizers and products rich in phosphate;
• Continue revitalizing the shores of the lake and its tributaries; 

• Do not use motor boats in shallow zones or near the shores, and respect speed limits. 

Renaissance lac Brome remains vigilant 

In 2021, Renaissance lac Brome’s teams worked on a range of major projects designed to prevent any further deterioration in the lake’s health, notably:
• An inventory of rusty crayfish populations in the tributaires; 

• A follow-up on the quality of the lake water and its tributaries; 

• A characterization of the shores of the Quilliams strea 

Translation: Alexandre Hackett