Slip sliding across a stream bed can be surprising when the same types of rocks and gravel provide a solid gripping surface when climbing up the dry shoreline. Entire food webs grow on the surface of the bedrock found in lakes and streams. Bottom-dwelling benthic organisms receive much less attention than their water column dwelling counterparts but are very important and diverse residents of their habitats.
The slimy, slippery coating of rocks is mostly attributed to layers of algae and bacteria. Different algal species dominate under different flow and light conditions meaning fast flowing rocky stream sections have very different growth than pooling water or lake environments. Shiny rocks in fast moving waters are deceptive, looking like brightly colored wet rocks. Almost invisible, but dangerously slippery, the rocks in high flow, shallow areas are home to a thin layer of algae called Diatoms. Like other algae, diatoms are single-celled and photosynthetic but unique because they build a casing of transparent silica that can tightly glue onto the rock surface. Walking on rocks coated with diatoms can literally be compared to skating across a surface of silica glass. While just a glassy sheen to our eye, diatoms are beautifully symmetrical with ornate glassy tests that have provided inspiration formicroscopic art for hundreds of years.
In slower flow reaches and lakes, algae are less likely to be dislodged by currents, so rocks contain more different types of algae, some encrusting and others long and stringy. Diatoms, green algae, bacteria, and small plants grow together in a layer called periphyton. Periphyton is the base of many benthic food webs.
The creatures eating periphyton are almost as diverse and charismatic as the algae they consume. Commonly called waterbugs, researchers call them macroinvertebrates. This group is mostly composed of the larval stages of mayflies, stoneflies, caddisflies, and other winged insects that spend most of their lives underwater. These insects employ strategies like scraping algae off rocks, chewing or filtering filaments, and building casings or webs to trap particles from the water. Caddis flies hold a special place in my heart for their industrial building capabilities. While the actual larvae looks a lot like that of other insects, caddis fly larvae build complex casings and different taxa utilize different materials. In Northern Wisconsin waters, it is common to find species that create their tubular homes out of pine needles, twigs, sand, or woven silk. Casings serve as camouflage, protection, or function to attach species to rocks or help filter food particles from the water column. A single stone can contain multiple species and hundreds of individual caddis flies.
Predators like dragonfly larvae, giant water bugs, and larval Dobson flies called hellgrammites flitter among rocks feeding on stationary macroinvertebrate prey. With jaws that jut out to half their body length to ambush prey, dragonfly and damselfly larvae are common benthic predators in streams and shoreline lake regions. Giant water bugs inject prey with a solution that liquifies their insides and then use a siphon to drink up the juice. And, hellgrammites have hardened jaw structures that can slice through prey (as well as a curious child’s finger!). If larger in size, all of these invertebrate predators would be fodder for gripping nature documentaries or suspenseful horror films.
Fish and birds are top predators in these systems and act to connect the benthic periphyton food web to open water habitats, other stream reaches, and shoreline terrestrial environments.
Often overlooked as slippery slimy ooze, the participants in the periphyton food web are vital and impressive members of our complex aquatic systems.
