2015 Winter Birch Lake Dissolved Oxygen

By Brian Corcoran, Water Resources Manager, VLAWMO

Each year the Vadnais Lake Area WMO (VLAWMO) takes monthly dissolved oxygen readings on all monitored lakes within the watershed to monitor the health of the water bodies within VLAWMO.

Dissolved oxygen (DO) measures the amount of gaseous oxygen (O₂) dissolved in an aqueous solution. Oxygen gets into water by diffusion from the surrounding air, by aeration or rapid water movement, and as a waste product of photo-synthesis from aquatic vegetation.  Adequate DO is necessary for good water quality and aquatic life. 

The normal range to support aquatic life is between 4-10mg/l.  Sustained readings below 4mg/l will stress fish and can cause fish kills.  Game fish such as bass, walleye, and pan fish are less tolerant to low DO levels compared to rough fish such as carp and bullhead.  Water temperature and rotting organic matter (leaves, grass clipping, dead aquatic vegetation, runoff) also affect DO levels. Colder water can hold more DO than warm water, and bacteria consume a lot of DO to break down organic matter. 

Dissolved oxygen fluctuates throughout the year.  DO levels usually drop during the winter months as sunlight is limited and some aquatic vegetation is dying.  DO levels will rise in the spring as the ice melts and aquatic vegetation begins to grow.  As water temperatures warm during the dog days of summer, DO levels usually drop; as water cools in the fall, DO levels usually rebound until ice forms. 

The severity of a winter can really change DO levels.  Snow pack can reduces the amount of sunlight entering a water body limiting photo-synthesis; the earlier a lake freezes and snow pack builds, the lower the DO levels may get throughout the winter months.  For shallow lakes, hard, long deep freezes can also affect DO levels.  A thick ice pack will limit the amount of “swimmable” water in a shallow lake and may trap fish in certain areas of a water body.

So far no concerns, Birch Lake’s DO levels are good.