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2023 - The Summer of Aquatic Vegetation

As I start this post, it is early July and I can not remember a year that has been as weedy as this year has been at this time of the year. It started early with the filamentous algae and has continued on with rooted aquatic vegetation. I happens most every year - Driftless spring creeks are incredibly productive so they grow "weeds". However, this year has been something else. The obvious question is why is this year so weedy compared to most others? And, of course, I will dig into that - eventually.

A weedy stream
A photo from July 6, 2023 - while this reach tends to get weedy, aquatic vegetation has come earlier and heavier this summer compared to most.

While many look at the weeds as a negative, I do not mind the weeds. In fact I rather like that it makes fishing a nymph or a streamer nearly impossible - which I am quite fine with. I sort of enjoy that the casting lanes are tightened and "the spot" becomes even smaller. I enjoy the challenge of it all. The casting lanes are smaller, the currents "messier", and the action all on top. And to be quite honest, if it keeps anglers away, I am OK with that, sometimes.

If you fish at all in Wisconsin or the Midwest, you know it has been a dry year and streams have been low. That is certainly a large part of the "weed" puzzle. While we have received a little rain lately, we are in "flash drought" or so says the expert that I caught on Wisconsin Public Radio from the University of Wisconsin that studies droughts. And Drought Monitor tells a similar story of this summer.

90 day precipitation departure for Wisconsin from April until early July.

It seems that the ideal conditions for aquatic vegetation - stable flows and substrates; a lack of high, flushing flows; and lots of sun and warmth - all lined up this year. The first two things on that list go together - a lack of rain means that stream flows are lower than usual and there are more places for weeds to grow. One of the main requirements for aquatic plants is stable substrates. When substrates are being moved by the current, plants are unable to root or remain attached to substrates. With flows as low as they have been this year, there is simply more stream bottom for weeds to grow into. I think there is a bit of a 2018 flood effect as well. That flood rearranged streams and their substrates and it's taken a few years and some more moderate floods to stabilize substrates in some reaches.

I think the above is a huge part of the story so far this summer. I say "so far" because by the time this post is ready to be posted, a "gully washer" may blow out much of this summer's growth and change everything (it didn't happen). Streams are dynamic, they change quickly.

Rullands Coulee hydrograph
Gaging station on Rulland Coulee in Monroe County. This stream and about 20 others are gaged in the county.

Flows this summer have been amazingly consistent - as evidenced from one of Monroe County's gaging stations. These - and the rest of the gaging stations - are available through the National Weather Service's Advanced Hyrdrological Prediction Service.

Bridge fish
View from the bridge railing - the vegetation almost mimicked a riffle dropping into a deep pool and fish are exactly where yoiu would expect them to be.

By nature, aquatic vegetation in streams is transient and ever changing. It changes over the course of the season. I won't pretend to be a botanist or know a ton about photosynthesizing organisms in streams but I've been fishing the Driftless and other places long enough to have witnessed predictable patterns. First, we anglers tend to categorize everything as "weeds" but there are not only a number of different species but different taxa as well. The "slime" you have probably experienced is generally filamentous algae which seems to dominate the stream bottom early in the season. While filamentous algae is still present, it seems to give way to rooted plants as the season goes on. I am not fully sure why but I assume a lot of it has to do with light availability, water temperature, and I am sure other factors.

A mix of rooted aquatic plants and filamentous algae.
A mix of rooted aquatic plants and filamentous algae.

Common names are often terribly useless and few are more useless than "algae". What we call algae are a huge variety of photosynthetic organisms across a great number of different taxa (polyphyletic). Because of this taxonomic diversity, we lack a great definition of algae. Probably the most common working definition is that they are photosynthetic, eukaryotic organisms that lack differentiated cells and tissues. If you understand just a tiny bit of biology, you understand just how broad this definition is. And, of course, there are tons of exceptions - things we call algae that do not meet this definition. For example, blue-green algae (Cyanobacteria) which are not technically an algae as they are bacteria and thus are prokaryotes (lack a nucleus). Algae range from pelagic phytoplankton and benthic periphyton to kelp which much more resembles a more complex plant that forms "forests". Algae are in mutualistic relationships with coral, with fungi in lichens, and in a host of other organisms. Algae are incredibly diverse and important.

Macroinvertebrates growing on aquatic macroinvertebrates.
Aquatic vegetation serves as a substrate for macroinvertebrates.

I think one of the most interesting things about aquatic plants is their evolutionary history. In our area, aquatic macrophytes are mostly angiosperms which is by far the largest group of "land plants" on earth. Angiosperms are flowering plants which are most of the plants around you right now. Your lawn, your garden, agricultural crops, and all the trees around you that are not pines/conifers are flowering plants. In the evolution of land plants, we see movement of taxa from continuously wet areas required by liverworts and mosses to plants that are able to live in drier areas until plants are able to live in deserts and tundra habitats. Adaptations like roots, stomata, vascular tissues (xylem and phloem), pollen, seeds, and fruits are shared by "higher plants" - those that have evolved to live in drier places. The interesting story about aquatic macrophytes is that returned to the water - much like whales and other cetaceans (yeah, that's right - whale evolved from land animals).

Role of Algae and Macrophytes

Photosynthetic organisms are the base of the food web. Primary producers - organisms that convert sun energy into biomass support heterotrophs like us, aquatic macroinvertebrates, and fishes. As I wrote about in the post on the River Continuum Concept, stream ecology is based upon their physical setting (position in watershed, geology, etc.) which helps determine what types of species are found in the stream itself. Aquatic macrophytes tend to be most common in the middle to lower reaches of streams or headwater streams with little canopy cover.

Small stream with little canopy cover.
A small stream through a seasonal / rotationally grazed pasture - it is generally small enough that grasses provide some canopy coverage and aquatic macrophytes are uncommon.

While I certainly can not list all of the roles of aquatic vegetation, here is a partial list of some of their most important features.

  • Base of the food web - primary producers are the energy source for heterotrophs (consumers) - which in trout streams are largely macroinvertebrates, which of course are "fish food". In general, the larger the base of the food web, the greater the biomass of predators that can be supported.

  • Habitat - aquatic macrophytes provide habitat for not only macroinvertebrates but fishes as well. Think of them as "kelp forests" in our streams - they provide a three dimensional structure and places to hide. They also help shade much of the stream keeping it colder.

Narrowed weed channel
A deep channel providing cover on both sides...better make a good, accurate cast!
  • Hydraulics and Sediment Storage- weeds help keep water levels up, trap sediments, and produce narrow but deep channels. I think one of the more amazing things this summer - with the limited rainfall we have experienced - is just how consistent stream levels have been. Visit the National Weather Service Advanced Hydrologic Prediction Service and see for yourself...

  • Consumption of nutrients - most Driftless streams have an excess of nutrients, plants tie up some of these nutrients making them less available to more noxious primary producers like filamentous algae. In other words, it's either the weeds are a ton more "slime". I'll take the weeds.

While I understand why many bemoan weeds in the summer time, they are hardly avoidable in larger streams with limited canopy coverage. You can seek out more shaded reaches that will grower fewer weeds. And sometimes downstream reaches that lack enough stream power may prevent many weeds from establishing. Of course, the extraordinary amount of weeds are largely due to low flow and lots of sun and heat so moving downtream is not much of an option. I suggest tying on a terrestrial dry fly and learning to embrace the weed growth. But set that hook and get the fish's head above the weeds before they burrow in and get away - trust me...


Bornette, G., & Puijalon, S. (2011). Response of aquatic plants to abiotic factors: a review. Aquatic Sciences, 73, 1-14.

Lusardi, R. A., Nichols, A. L., Willis, A. D., Jeffres, C. A., Kiers, A., Van Nieuwenhuyse, E. E., & Dahlgren, R. A. (2021). Not all rivers are created equal: The importance of spring-fed rivers under a changing climate. Water, 13(12), 1652.

Nichols, A. L., Lusardi, R. A., & Willis, A. D. (2020). Seasonal macrophyte growth constrains extent, but improves quality, of cold‐water habitat in a spring‐fed river. Hydrological Processes, 34(7), 1587-1597.

Wang, C., Zheng, S. S., Wang, P. F., & Hou, J. (2015). Interactions between vegetation, water flow and sediment transport: A review. Journal of Hydrodynamics, 27(1), 24-37.

Willis, A. D., Nichols, A. L., Holmes, E. J., Jeffres, C. A., Fowler, A. C., Babcock, C. A., & Deas, M. L. (2017). Seasonal aquatic macrophytes reduce water temperatures via a riverine canopy in a spring-fed stream. Freshwater Science, 36(3), 508-522.

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