From spring to fall there are a lot of aquatic plants to be found in our watershed, some that are beneficial to water quality and others that are not. Algae is commonly known to cause problems for water quality. One native plant that is often mistaken as algae is duckweed! Duckweed is a floating-leaf aquatic plant often found on the water surface of ponds, quiet backwaters of lakes and streams, even in inlets and ditches at times. They favor nutrient rich environments so places that have more phosphorous and nitrogen are more likely to have duckweed present. It is a native aquatic plant so it is alright if you find it in your local area, but it is also aggressive and can overcrowd a smaller calm waterbody (unlike larger more open areas that are stirred up by wind and waves).
There are two types of duckweed; greater duckweed and lesser duckweed. They consist of a round leaf or clusters of leaves an 1/8th to a 1/4th of an inch in width with roots hanging below. They are free floating but cluster together.
Duckweed
Algae bloom is on the left and duckweed is on the right.
Duckweed is good for creating a healthy habitat as it supports micro invertebrates that fish consume and food for waterfowl and marsh birds. However, it can occasionally overcrowd a smaller still waterbody blocking sunlight, which can keep plants below from producing oxygen. Lack of sunlight and oxygen can cause dying off of plants and fish over a long period of time. If you have a waterbody experiences these issues, you can remove duckweed, but it is difficult to do and requires a permit from the DNR. Examples of physical and chemical removal techniques can be found in an article by Professor Lembi from Purdue University:
https://www.btny.purdue.edu/Pubs/APM/APM-2-W.pdf
Aquatic Plant regulations and how to apply for a permit can be found here:
http://www.dnr.state.mn.us/shorelandmgmt/apg/regulations.html