Clear water and colorful rocks (top) contrast with periphyton covered rocks (bottom), both found in the near-shore of Lake Superior and may indicate differences in water quality. Bubbles in the bottom image are due to the high photosynthetic activity.


Most people experience the North Shore of Lake Superior along its rocky coastline and most expect to see clear waters and colorful rocks.

At times, some may notice a light brown or light green slimy covering on the rocks or even longer, dark green filaments. These shallow coastal waters are home to algae, specifically a group of algae called periphyton.

Periphyton are well-adapted at living on hard surfaces. The attached algae growing on rocks (epilithic periphyton) within the shoreline splash-zone (eulittoral zone) can play an important role in the aesthetic, and other beneficial uses of the shoreline. It is also an important component of the nearshore food web, a factor affecting the condition of benthic habitat, and a potential refugium for fecal indicator bacteria, human pathogens, and botulism in fish and waterfowl. The rapid growth ability of the eulittoral periphyton, in response to nutrient inputs, offers the potential to use this community of algae as a relatively low cost, sensitive, early indicator of localized differences in nutrient loading along lakeshores before pollutant concentrations are dispersed and diluted in deeper offshore waters.

To borrow a term from NRRI Great Lakes Researcher, Dr. Euan Reavie,
algae are ‘first responders” to water quality changes within the lake.

In 2012, NRRI researchers received a grant from the MN Lake Superior Coastal program to take a look at these periphyton communities and determine if they can be used to detect nearshore influences.

The projects main objectives were to:

  1. Establish baseline (i.e. benchmark) conditions for attached algae along the rocky North Shore Lake Superior “splash zone” by establishing a sampling protocol, determining areal periphyton biomass and species composition, and providing archival algal material for other scientists interested in the issue.
  2. Determine if periphyton can be an indicator of nutrient pollution along the MN Lake Superior shoreline.

Collecting periphyton


We collected material from 17 sites from Leif Erickson Park to Stony Point in July and August of both 2013 and 2014. Collecting periphyton involves scrubbing material from rocks in a quantitative way that allows for a determination of amount of biomass per unit area, or areal biomass. There are several ways to measure algal biomass with the easiest and least expensive being measuring dry weight. Once the material is dry you can “burn” the material at 550 °C which gives you the amount of organic matter within the sample.  Although these are inexpensive and quick techniques there is at least one problem; the material scrubbed off the rock is actually a community of algae, fungi, bacteria, and other microorganisms as well as detritus and aquatic insects. A measure of chlorophyll content of the material can often provide a more definitive measure of algal biomass.

View the Results

View the Cook County Results


The use of qualitative, visual assessments of periphyton coverage has long been used in streams and lakes. The US EPA uses a Rapid Bioassessment Protocol that includes a ranking of observed algal growth in streams. Lake Tahoe researchers also have used a ranking protocol as part of their long-term periphyton monitoring program. These methods are invaluable because of their low cost and because they are long-term studies.  As we saw in our 2013-2014 periphyton study, algal biomass can vary significantly from year to year. These long-term qualitative studies may be the best way to determine if there are actually changes in the system.

Storm sewer outlets

There are many storm sewers entering the lake along the Duluth shoreline and there is often more algal growth near these outlets. In the fall of 2012 we walked the entire Duluth shoreline from Canal Park to the Lester River mapping and photographing any pipes, culverts, or ditches we saw. If water was flowing from the outlet we measured temperature and specific conductivity. The outlets we observed were compared to those the City of Duluth stormwater utility has mapped. Some, 13 out of 45 of the pipes/culverts we observed were not on the City list of stormwater outlets. Most of these were dry and were most likely capped or disconnected outlets.

Download the full report

View the site locations with images on our interactive map.

Project Acknowledgment:

This project was funded in part under the Coastal Zone Management Act of 1972, as amended by NOAA’s Office of Ocean and Coastal Resource Management, in conjunction with Minnesota’s Lake Superior Coastal Program. Additional support came from the Natural Resources Research Institute at the University of Minnesota-Duluth, and the City of Duluth Stormwater Utility.

Other useful links:

For more detailed information about algae and aquatic plants, how to identify them, and how to measure their abundance, visit Water On the Web (WOW) -- lake modules [2+3] and [8+9], stream modules [4+5], and the Lake Access section on aquatic plants.