Primary Producers (algae)
in Lake Superior Streams

If you look into a cold, clear, fast-flowing stream, you'll notice that you can't see any plants. Does this mean that nothing lives there?

Local streams usually flow much too fast to allow large plants to live in them. And most algae is washed away quickly. However, you may see long, very thin strands of green waving downstream from their attachment to a rock or submerged branches. If it feels sort of "ropy" and not slimy, then these long strands are probably an alga called, Cladophora, that likes turbulence. But most of the plants in the stream won't be visible to you; that is, unless you know what to look for. Cladophora also grows naturally on the rocky shoreline of Lake Superior and where there are excess nutrients (eutrophication) can grow to produce massive amounts of organic material and become a huge problem such as at Lake Michigan the past few years. Something's Amuck provides more detailed information. Additionally, Blue-Green Algae can cause serious health problems in pets and humans exposed to it. When in doubt, stay out!

There has been some concern about increasing algal growth in North Shore streams. Check out this 2003 report for a qualitative assessment on increasing periphyton in some local streams.

Anyone who has ever tried to walk across a stream knows that the rocks under the water are slippery. If you were to scrape a bit of that slime off and put it on a glass slide, and look at it under a microscope, you would see a community of diatoms, blue-green, and green algae, single-celled animals, bacteria, fungi and lots of organic matter. This is what makes the rocks slippery. It so happens that these plants are the favorite food of many of the stream's herbivores!

Diatoms of many different types cover all the wet surfaces of the stream: the rocks, the leaves and sticks and logs that have fallen in, and various kinds of debris. In fact, the streambed is coated with plants!

In most small streams, however, often the greatest contributor of plant food of all is the riparian zone: the margins along the stream that are filled with vegetation. These plants, like all plants, drop their leaves--which fall into or are washed into the stream. This is allochthonous matter (from outside the stream), as opposed to autochthonous matter (from inside the stream, like algae and diatoms). These leaves can't make oxygen, since they are dead, but they provide food to the creatures in the stream. Not only the leaves themselves can be eaten, but also whatever bacteria or fungus is covering the leaves, rotting them. It is this bacteria and fungus that is what crayfish are really after when they eat decaying plant matter.

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.