Mystery of the Month: July 2014

By Bill Bartodziej

Ample spring runoff and cool temperatures seemed to favor mat-forming algae.

The net in net-forming algae.

WARNING: Do not release your laundry drain lint traps into local waters or it could become ALIVE! Of course we're joking, but what is this net-forming aquatic stuff? One hint: it's not a plant.

Answer:  It's algae - and lots of it. 

You may have recently noticed a bumper crop of algae, a eukaryote, on District lakes - especially mat-forming algae.   


Surfaced mat-forming algae was a common sight in many of our lakes and ponds this spring and early summer. Algae samples were taken from several lake systems for species identification. We found that the mats were composed of five common species of green algae: Hydrodictyon reticulatum, Pithophora, Rhizoclonium hieroglyphicum, Lyngybya, and Spirogyra. These species are typically found in lakes around the metro area.


So why are certain years worse than others? Some watershed and lake managers speculate that all of the runoff that we had this spring-early summer has fueled these algal blooms. However, we really don’t have the data at this time to support this hypothesis.


We do know that most of us look at these mats as a recreational nuisance and a big eyesore. With some people, these mats trigger the thought of a “dirty lake” even though water quality may actually be stable and within normal levels for nutrients.


In terms of ecology, these mats are likely not causing adverse impacts to our lake systems. Some aquatic invertebrate species actually take advantage of this type of habitat. Ducks forage in the surfaced mats of algae. Small fish hang under the mats and seek shelter and food. 


To start to become an algae geek, you may want to inspect some of their finer qualities up close to start to gain appreciation. Here’s a short video, www.youtube.com/watch?v=XeUWulMEkV8 that provides a detailed look at one of these algae - Hydrodictyon reticulatum, nicknamed “water-net.” It lives up to its name by having a net-like structure which forms membrane-like colonies.


How could something so elegant be such a nuisance?

Casey Lake now has an abundance of net-forming algae on the surface and a diverse native plant community below.  This condition is still much better, ecologically, than having no aquatic plants and frequent blue-green algal blooms (which is what we had when carp dominated the system a couple years ago).

Most Lake Wabasso residents are looking over surfaced mats of algae today, but high-quality
native plant communities are under the mats.

Some shoreland owners equate this surfaced algae to poor water quality, even though
water quality has significantly increased in this lake system.

From a recreational standpoint, large expanses of algal mats are trouble. They make boat navigation difficult and fishing is extremely challenging around these mats. From a management perspective, recreational considerations might justify control of these mats. For example, fishing access points, the boat launch, boat docks, and the swimming beach area on Lake Phalen are cleared of aquatic plants and surfaced algae.

A thriving native plant buffer on Phalen.  In the background, the surfaced algae sits on top of Eurasian watermilfoil.  Mechanical harvesting will start in early July.  The City of St. Paul and DNR pay for most of the control.

On a positive note as we reach the heart of summer, we have seen surfaced algal mats on the decline in a few lake systems. We will definitely continue to monitor these algal mats on our lakes and report any noticeable changes as we enter the dry-hot period of summer.