Duluth Streams Periphyton

Superior Streams Algal Assessment

Duluth’s high-quality trout streams are sensitive to urbanization and rural development. These streams are subject to increasing water temperatures, decreasing aquifer recharge, and increasing water, sediment and nutrient runoff. Climate change predictions for the Great Lakes region indicate the potential for these impacts to be exacerbated via an increase in the frequency of intense storm events and decreased precipitation in the summer.

Understanding how these changes affect stream health is key in developing protection and restoration activities. Stream health is evaluated using physical, chemical, and biological measures. The aquatic biological community, primarily aquatic macroinvertebrates and fish, is most often used to determine stream health, but algae (aka periphyton) are also used.

Periphyton, the algae that grow on surfaces such as rocks, wood and plants, are useful indicators of stream health because they are sensitive to many environmental stressors and respond rapidly to changes in nutrient concentration.  Because periphyton has the ability to grow rapidly in response to nutrient inputs this community can potentially be used as a relatively low-cost, sensitive early-indicator of nutrient loading.

Currently large rivers in Minnesota are assessed for water column chlorophyll but this is not feasible for shallow, 1st and 2nd order streams including Duluth area trout streams. Minnesota currently has proposed a periphyton water quality standard for wadeable streams focused on nuisance conditions such as “undesirable slime”. Specifically the standard says levels of periphyton growth should not exceed 150 mg chlorophyll-a (chl-a)/m2 and not to exceed one-third (1/3) of the stream width.

Traditionally, the quantity of algae growing in a stream is measured by collecting material from substrates such as rocks, wood, and sand sometimes even plants. The sample collected is then taken to a laboratory where the chlorophyll and organic matter within that material is measured. This method gives the amount of chlorophyll or organic matter per unit area, typically in milligrams (or grams) per square centimeter or square meter.

These laboratory methods have been combined with Visual Assessment (VA) techniques developed and well tested in New Zealand and Montana for unproductive streams similar to North Shore Superior tributaries. This method involves estimating the amount of the stream bottom that is covered by visible algae as well as estimating average filament length and mat thickness. VA is much quicker and cheaper allowing more frequent assessments during the growing season.

One of the goals of our project was to determine if the VA methods can be used in Duluth Area streams as a more efficient way to assess stream health. In 2018 and 2019 our project measured algal biomass in several Duluth trout streams using these two methods.  Our primary goal was to understand what 150 mg chlorophyll per m2, the draft MN standard, looked like in these trout streams.

Field Sampling (What We Did)

All of the sampling locations were located in riffle/runs with cobble or small boulder rocky substrates. The method we used to sample periphyton required the removal of rocks from the stream, placing them into a shallow pan then scrubbing a small circle (40 mm diameter) with a brush to remove periphyton.  Samples were then rinsed into a brown bottle, kept cool, and returned to the laboratory and processed for subsequent determination of chlorophyll and organic matter content. We also measured stream temperature, specific conductivity, and dissolved oxygen and also collected water samples for nutrient analyses.

Map of Sampling Sites

Sampling Map

Visual assessments were conducted using the categories in the USEPA Rapid Bioassessment Protocols for Use in Wadeable Streams and Rivers.

Table 1. Categories used to describe periphyton growth.
no microalgae present
algae present but not visible
<1 cm
1 - 5 cm
5 - 20 cm
> 20 cm

Three transects across the stream channel were made and we estimated percent algal cover and color, and substrate type, and measured stream channel width.

Results Summary (What We Found)

The full results of this study can be found in our final report but here are some of the highlights. Overall, in both 2018 and 2019 we found that maximum periphyton growth based on chlorophyll measurements occurred in the late spring/early summer (late May into June). A good example was growth in Chester Creek along Toledo St. 


Growth this time of year was particularly apparent because the algal community mostly consisted of bright green filamentous forms.  As the season progressed these filaments became covered with a dense growth of diatoms which make the algae appear brown. Dense growth in the early part of the growing season makes sense because nutrients from the spring snow melt have been available and sunlight levels are increasing.

We also noticed that periphyton coverage was very dependent on substrate type and also variable across the stream channel.

Substrate stability is important because the larger rocks stay in place during high flows. We also noted that, not surprisingly, large boulders and bedrock can support dense algal growth. Unfortunately these substrates are much more difficult to sample quantitatively.

We also made a couple of other observations worth noting.  One being that benthic invertebrate grazing can dramatically affect periphyton biomass, particularly in late summer.

The second is that we did not see any noticeable growths of blue-green algae though we did observe some Phormidium and Oscillatoria filaments under the microscope.  The cyanobacteria, Phormidium has been known to cause toxic or harmful algal blooms (HABs) in some cases. Note that just because the species is present does not mean they are producing toxins.

We collected 148 individual samples total over the growing seasons (June-August in 2018 and May-July in 2019) from 9 streams.  Of those only 13 or 9% exceeded the MN draft chlorophyll standard of 150 mg/m2. If we look at average values for the streams, only 3 sites exceeded the standard and covered more than 1/3 of the channel.
The visual assessment method we used appears to not work well in the Duluth streams, at least in those sites we assessed. We could find no correlation between the VA categories we recorded and the measured chlorophyll and organic matter.  Essentially the primary reason for this is that these streams do not experience the wide range of periphyton biomass those researchers in other areas have seen. Studies in other parts of the world where the VA method is often used see much more biomass. Areal chlorophyll in some New Zealand streams range from 20 to 580 mg/m2 (Kilroy, 2013) while in Montana the range reported is 44 to 1,276 mg/m2 (Suplee, 2009). In our study the average value was 46 mg/m2 with a range from 1 to 249 mg/m2.

Our results show both methods have disadvantages. The more quantitative measures of biomass show a great deal of seasonal and spatial variability while the VA methods do not correspond well with the low overall low biomass encountered. Though the typical VA method as used here may not correspond well to measured biomass values it may be valuable as a means to track algal growth over time.  Periphyton characteristics routinely recorded at the same location (mat thickness and color, filament length, and estimated channel coverage) are observations that can be made frequently at very little cost and provide a great deal of information.  Potentially, it may be possible to train volunteers to use a modified VA method at the Crowd Hydrology sites located throughout the Duluth area to track periphyton growth over time.


Kilroy, C, DJ Booker, L Drummond, JA Welch, and TH Snelder. 2013. Estimating periphyton standing crop in streams: a comparison of chlorophyll-a sampling and visual assessments. New Zealand J Marine and Freshwater research 47(2):208-224. 

Suplee, M.W., V. Watson, M. Teply, and H. McKee. 2008. How green is too green? Public opinion of what constitutes undesirable algae levels in streams. J. Am. Water resources Assoc. 45 (1):123-140.

Project Acknowledgment:

This page was prepared by NRRI using Federal funds under award NA17NOS4190062 from the Coastal Zone Management Act of 1972, as amended, administered by the Office for Coastal Management, National Oceanic and Atmospheric Administration (NOAA), U.S. Department of Commerce provided to the Minnesota Department of Natural Resources (DNR) for Minnesota’s Lake Superior Coastal Program. The statements, findings, conclusions, and recommendations are those of the author(s) and do not necessarily reflect the views of NOAA’s Office of Coastal Management, the U.S. Department of Commerce, or the Minnesota DNR.

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.