During spring and early summer lake users start to see what looks like slimy balls attached to docks, boatlifts, seawalls, or any other hard structure. Although they look like something otherworldly, these bryozoan colonies (Pectinatella magnifica) are common in lakes and streams and actually serve a valuable role in lake ecology. The structures consist of hundreds or thousands of individual zooids encased in a gelatinous matrix. The individual zooids take in water and filter out microscopic algae. Water is then ejected through a larger orifice located centrally in the colony. It has been shown that a one square meter colony of the marine bryozoan Zoobotryon verticillatum can filter over 48,000 gallons of water per day. I have not seen any of our freshwater bryozoans that large, but in aggregate I am sure we have at least that volume in our lake.

Pectinatella magnifica colony (a small one)

Bryozoans are not mobile and cannot climb out of the water. However, they are occasionally seen washed up on shore or on docks. If that is the case, simply push them back into the water to continue serving their valuable water quality role. I have heard reports that once the masses are broken up small fish will start eating the exposed bryozoans. You should also be aware that if the masses are left to dry out on the dock the smell will convince you that the best thing is to put them back in the water.

At the end of the season bryophytes produce pinhead sized statoblasts that provide the “seeds” for next summers’ colony. These small black flecks are often seen on the lake surface aggregated by the wind near the shore. They are harmless and will eventually sink to the bottom. Each bryophyte colony will send out thousands of statoblasts when it dies off so the surface can become quite thick with the small black flecks.

Pectinatella magnifica statoblast

Water Year

The term U.S.Geological Survey "water year" in reports that deal with surface-water supply is defined as the 12-month period October 1, for any given year through September 30, of the following year. The water year is designated by the calendar year in which it ends and which includes 9 of the 12 months. Thus, the year ending September 30, 1999 is called the "1999" water year. (

A “water year” runs from October 1 to September 30 to more accurately measure annual runoff in our northern hemisphere climate. If rainfall was measured on a calendar year from January 1 to December 31 the current water year would stop when a lot of precipitation from the previous year is still locked up in the snowpack. This means snow that fell in one year would be measured as runoff the next year. In using an October to September water year most of the snowpack from the previous winter will have melted by the end of September, in time to start the new water year in October.