Pollutants settle in Great Lakes more quickly in winter than summer

By working out the rate at which algae becomes phosphorus deficient, researcher Russel Cuhel concluded that algae must circulate in the Great Lakes during winter and spring at least every two to five days. Therefore, circulating water carries contaminated particles to the lake bottom in just a few days during winter or spring, about two to four times faster than they would fall in still water.

Pollutants like PCBs and DDT often enter the Great Lakes attached to particles flowing down rivers or falling out of the atmosphere. Understanding what happens to these contaminants after they enter the lakes requires, in part, determining how long small particles remain suspended in the water before they settle out on the bottom. Wind blowing across the Great Lakes makes water circulate in patterns that make this a difficult question to answer.

“Physical oceanographers have torn their hair out for decades trying to get at the actual rate at which water moves vertically in a windy situation,” said Cuhel.

Using algae, Cuhel devised a partial solution. During the winter and spring, Lake Michigan circulates from top to bottom, and algae ride along, changing their biochemistry as they go from light to dark zones. Cuhel showed that when water circulates from top to bottom in the lake, algae obtain phosphorus only when they are near the bottom.

Cuhel also realized that the algae must circulate often enough to obtain phosphorus at the same rate that they use it up. Cuhel measured this rate in his laboratory and found that algae showed signs of phosphorous deficiency in two to five days. From this Cuhel concluded that algae must circulate in Lake Michigan during winter and spring at least every two to five days. Therefore, in winter or spring, circulating water carries contaminated particles to the lake bottom in just a few days, about two to four times faster than they would fall in still water.