Secrets of the Credit River
guest post by Nina Munteanu
I began my limnology career teasing out the secrets of stream life as a grad student at Concordia University, Quebec. My master’s research focused on several rural and urban streams in the Eastern Townships, not far from where I grew up.
Later, as a limnologist for various environmental consulting companies in British Columbia, I used stream macro-benthos communities—the critters that live on the stream bottom—as indicators of environmental impact from industrial discharges, agriculture and municipal development.
Macro-benthos are bottom-dwelling life you can see with the naked eye. They’re mostly made up of aquatic worms and juvenile stages of insect species (benthic invertebrates). Many of these insects start with an aquatic phase (often called nymphs or larvae) in which they voraciously feed and which lasts from several months to several years; they then emerge as adults to live briefly (from days to weeks) to mate and create new life.
Mayfly, stonefly and caddisfly larvae are commonly found in clean flowing streams; slower moving and polluted or turbid streams contain more worms, midges and amphipods.
Adult mayflies, stoneflies and caddisflies don’t feed. In fact, they don’t have usable mouthparts or digestive systems because they don’t need them—they don’t live long enough. The adult female mayfly (Dolania Americana) lives a brief five minutes. Once she emerges as an adult, she flies in a swarm of other mayflies, mates in flight and lays her eggs then dies and falls back into the water as food for fish, frogs, and other aquatic life. The stonefly (Gripopterygidae) larva, which clings to the underside of rocks and debris in fast flowing water, takes from one to three years to mature but once emerged will live from 1-4 weeks before dying.
Stream ecologists identify benthic invertebrates by their form, but they also recognize them by how they feed; how they feed is largely determined by where they are in the stream and what is around them:
-Shredders use scissor-like mouths to cut and shred apart coarse particulate matter. These include amphipods, mayflies, stoneflies, midges, and some caddisflies.-Collector–gatherers (e.g., worms, nematodes, crustaceans, and gastropods) use their broom-like mouths to sweep in fine and ultra-fine organic matter.
-Grazers or scrapers, such as beetles, mayflies, and stoneflies, feed on attached algae and biofilms. Their mouths chisel against periphyton (attached algae) on rocks, wood debris and aquatic plants.-Filtering collectors, such as blackfly larvae, use their finely spun nets to collect suspended fine organic matter, which can include phytoplankton (floating algae). The caddisfly larva Arctopsychegrandis builds a rough house made from twigs, leaf fragments, and small pebbles and spins silk nets across its “door” to capture organic matter that flows in.-Predatory benthic invertebrates, such as damselflies and dragonflies, have piercing mouthparts that act like a straw, allowing them to suck the liquid nutrients from their prey without having to chew or shred it. The dragonfly uses a hyper-thrust mechanism to give it a speed-boost as it chases prey. The dragonfly ejects water from its anal opening for a quick burst of speed; it’s just like a jet propulsion system.
Since 1909, when scientists Kolkwitz and Marsson showed that benthic invertebrates had specific tolerances to organic enrichment and other sources of pollution, ecologists have used these communities to study impacts to stream health from chemical pollution, flow disruption and habitat destruction. The EPT Richness Index was developed, based on the knowledge of certain pollution-intolerant groups. EPT stands for Ephemoroptera (mayflies), Plecoptera (stoneflies) and Tricoptera (caddisflies) and the index corresponds to their percentage in the stream. EPT benthos will disappear in areas of poor water quality, organic enrichment, low oxygen, and high metal levels.
I recently tested this in several ad hoc field trips I made with my naturalist friend Merridy Cox along the Credit River in Ontario. We started our explorations with the lower Credit River watershed, located in the urban setting of Mississauga, Ontario. We sampled the river and a few small tributaries in Riverwood Park, a few kilometres from where the river empties into Lake Ontario.
Originally named “trusting creek” (Missinnihe) by the Mississauga First Nation people, the salmon-bearing Credit River drains some 860 km 2 of Ontario and flows 90 km from its source at Orangeville, over the Niagara Escarpment, through several suburbs, and into Lake Ontario at Port Credit.
Great efforts have been made to restore and maintain the health of the Credit River and its watershed, mostly through the work of the Credit Valley Conservation Authority, together with the provincial and various municipal governments. While the water quality of the lower river is considered generally fair to poor, the river is partially saved by its gradient and turbulent flow. The length of the Credit River, up to very close to its mouth, rushes with the sound of a great storm. It tumbles and gurgles over rocks, capturing oxygen from the air; it scours gravel beds and cuts swirling eddies and creates undercut banks for foraging fish. The habitat is complex and life thrives here. Green algae cling to smooth boulders as water shears over them into pools of bubbling water. Water striders skate on the water surface in calmer backwaters. A cursory sampling of rocks in the river revealed a diversity of macro-benthic organisms. I spotted several species of mayfly, including rock-clinging Heptagenids (flat-headed mayflies) and the stone-building caddisfly Glossoma, all indicators of well-oxygenated turbulent flowing waters.
About 500 m from where we had sampled in the Credit River, we investigated a small tributary in the forest that led into the river. The creek obviously drained storm water runoff from the streets above; and, while the water was clear and contained riffles with a good flow, I found no macro-benthos on the rocks. Only blue-green algae populated the shoals. This was not surprising, given that storm water and street runoff generally contain contaminants (e.g., chlorides, heavy metals, organics, and oxygen-depriving materials) that the susceptible EPT organisms can’t tolerate.
What struck me was the deceptive nature of this contamination. Most of us, when we think of polluted water, envision a turbid stagnating watercourse with visible garbage, bubbling with toxic algae. The pollution in this tributary was invisible; so was the life. It reminded me that the face of pollution varies and ranges from the obvious (as with most organic enrichment) to the insidiously subtle (as with heavy metal contamination or acid rain).
Water holds many secrets; some good, some not so good.
Water is an introvert.
All illustrations by Kerste Voute and Nina Munteanu
Photos of Nina by Merridy Cox except one of Nina and swan by John Stewart of Mississauga News.
Photo of Merridy Cox by Nina Munteanu
All shots taken at the Upper Credit River, Ontario.
A version of this post complete with references is available at Nina Munteanu's website at this link.
Our guest blog writer Nina Munteanu is a Canadian ecologist / limnologist and novelist. She currently lives in Toronto where she teaches at the University of Toronto and George Brown College. Her non-fiction book “Water Is…” was selected by Margaret Atwood in the New York Times ‘Year in Reading’ and was chosen as the 2017 Summer Read by Water Canada. Her novel “A Diary in the Age of Water” will be released by Inanna Publications in 2020.