Learning about the natural cycles that govern the health of the Salish Sea starts with peering into your local stream. There you’ll find the hidden world of benthic macroinvertebrates.

In the summer of 2019, while a communications intern on assignment for King County Department of Natural Resources and Parks, I was first introduced to the intriguing world of benthic macroinvertebrates. With the sun barely peeking out over the Cascades, I packed into a truck with two members of the County’s stream bug monitoring team and headed out to Coal Creek Natural Area in Bellevue, Washington.  Treading lightly, I followed the two young professionals into the living stream. I watched attentively as one of them used a Surber sampler, a tool reminiscent of a butterfly net, to capture bugs hiding in the rocks and pebbles. Meanwhile, the other team member used a convex spherical densiometer, a tool used to measure the forest canopy above the stream.

A Stream Team Member collects a sample of benthic macroinvertebrates using a Surber sampler tool.

Benthic macroinvertebrates collected in the sample are sifted out from rocks and other debris and put in an alcohol solution to be sent to the lab for identification.

What do all benthic macroinvertebrates have in common? They’re indispensable. Benthos (organisms that live on, in, or near the bottom of a body of water) provide essential ecosystem services that keep streams healthy. They also fill a vital spot in the stream food web. Learning about these creepy crawlers is fun and helps us to better appreciate the role animals play in keeping the Salish Sea a livable place. You need only trek as far as your local stream, and with a level of care, stop and notice the world just below the water. No special equipment needed.

Even the most ramshackle urban stream harbors life and is something to be cherished.

The way a stream bug eats also says a lot about its unique function in the freshwater food web and in what part of the stream you’re likely to find it. For example, the stonefly larvae, of which there are many different species, are often found on stones and other wood debris in the fast-moving portion of a stream ecosystem. Functional feeding groups for stream bugs include shredders, scrapers, collectors, and predators, and are best described using adjectives like cool and awesome.

Shredders and scrapers include the slender winter stonefly larvae, water pennies, and my personal favorite: caddisfly larvae. The caddisfly larva is a shredder that noshes on fallen plant debris from the tree canopy above the stream. This fastidious insect fastens a casing made of stone to protect its fleshy, developing body, reinforced with super silk that has long been the envy of the scientific community. You’ll find these little guys attached to rocks in slower moving water where plant detritus collects. Look closely and you’ll notice the larvae moving around in their stone houses — a dance to move oxygen over the gill tufts they use to breathe.

A shredder’s trash is another collector’s treasure. The detritus broken up by the shredders is consumed by the collectors, such as aquatic earthworms, dixid midges, and freshwater clams and mussels. And, in case you’re wondering, they also eat the poop of other animals. Indeed, nothing is wasted in the stream.

Many of the aquatic insect larvae mentioned above take to the wing as adults to mate, deposit eggs into the stream, and then die, completing a remarkable water-to-land transmogrification. Just think, the dragonfly cruising above your garden in the summer — picking off unsuspecting flying insects with staggering accuracy — spent most of its life in the water, most likely close to your house. Some species of mayfly live a mere day out of the water as adults and are much more concerned about sex than food, having shed the mouthpieces they used, as aquatic larvae, to eat

stream bug monitoring

As if maintaining the health of freshwater ecosystems wasn’t enough responsibility, benthic macroinvertebrates’ sensitivity to pollution makes them great biological indicators of overall stream health. Because of this, aquatic ecologists working for resource management agencies in both the US and Canada have a vested interest in monitoring the presence — or absence — of stream bugs in the streams of the Salish Sea.

Take for example, the stream bug monitoring program of the Natural Resources and Parks Department of King County, Washington. Using a sampling method called the Benthic Index of Biological Integrity (B-IBI) aquatic ecologists can measure the presence of bugs and get a pretty good idea of the overall health of a stream ecosystem. To give you a good idea of what stream bugs are up against, read this statement from the King County Stream Health Indicators webpage: “The biodiversity of a stream declines as flow regimes are altered, habitat is lost, chemicals are introduced, energy cycles are disrupted, and alien taxa invade.” Streams receive a report card, ranging on a scale of 1 through 5, based on the prevalence of three major types of stream bugs (mayfly, stonefly, and caddisfly) that are sensitive to pollution. Other indicators are measured as well, such as the presence of bugs that aren’t nearly as picky about where they live, and can make home in more polluted water spaces.

So, one important proviso when visiting your local stream: If it’s polluted, you’re unlikely to see many of the benthos discussed in this article. On a recent trip to Longfellow Creek in West Seattle, I was unable to spot any dancing caddisfly larvae, or the molted exoskeleton of stonefly larvae. I did however encounter a Signal crayfish, a Washington native, and reader beware, they pinch—they pinch hard!

regaining a sense of responsibility for the world around us

The Environmental Protection Agency estimates that in five years the human population in the Salish Sea region will reach nine million — that’s nine million of us competing for the natural resources that make the region such a desirable place to live. Freshwater is at the top of the list. We use freshwater for drinking, agricultural production, and for recreation. Competing for the same resource, and existence in a human-dominated world, are the animals that inhabit the streams, rivers, and lakes of the Salish Sea. With their compound eyes, hooked claws, and mandibles, stream bugs don’t elicit the same noble adoration as the other Salish Sea animals we revere, but they are just as deserving of our attention. Without them streams would fall into disrepair. We wouldn’t have salmon, and without salmon we wouldn’t have orcas. You get the picture.

In his book Celebrate the Solstice: Honoring the Earth’s Seasonal Rhythms through Festival and Ceremony environmentalist Richard Heinberg writes, “As consumers, we have lost our sense of responsibility — which after all, is the ability to respond: the ability to sense and align with the fundamental rhythms of nature and of life itself as it bubbles up from the core of our being. To regain our proper human responsibilities, we must begin to take more of an interest in the welfare of our immediate natural surroundings.” 

Concerning ourselves with the welfare of our immediate surroundings starts with something as simple as visiting your local stream. There, we are reminded that we share our home with other creatures who do so much for us and ask nothing in return save for the right to exist.

This photo gallery shows some other life stages of insects discussed above. It also contains one exuvium that was left behind as the adult emerged.

Click on the gallery to see it full-sized with captions.

Photos by Thomas Noland

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