It’s springtime in the Ontario Kawarthas. That time of the year—just before May—when the forest awakens with a sudden profusion of diaphanous florescent green—the young shoots and tiny leaves of new growth. I love the spring. Each day is unique. In this part of the world, you can practically see Nature awaken from its winter slumber with the greening of photosynthesis, the rising sap of trees and the opening of dainty flowers from delicate buds.

In the streams, marshes and rivers, the same thing is happening. As snow and ice melts with increased temperature and light, winter-locked nutrients are liberated. Together these create the iconic spring bloom of algae, which in a river is mostly periphyton (attached algae; the slimy stuff you slip on when you’re wading in a stream) that consists mostly of diatoms. Add the sudden spates that accompany spring rains and flooding, and you get a kind of spring migration. This is how periphyton colonize a river downstream and create diatom forests.

Several weeks ago, I spotted early spring blooms of periphyton in a small tributary of the larger dam-regulated Otonabee River, also Peterborough’s drinking water source. The periphyton—mostly diatoms—had formed long ‘bulbs’ of brown-green slime on the rocks and sand of the stream. They trembled in the current like grass in the wind. When I tried to collect a slime ‘bulb’ to study under the microscope, it easily dislodged and escaped downstream. The second one slithered out of my hand and also escaped.

When I finally succeeded in securing a sample and observing it under the microscope, I discovered that they were indeed a collection of mostly periphytic diatoms that attach to surfaces in a number of ways: directly on their valve face (adnate); by stalks or pads (pedunculate); through pole attachment (rosettes. stellate or zigzag forms); or within a mucilaginous tube, itself attached to the substrate. Altogether, these diatoms form what I call a Diatom Forest, complete with overstory, understory, and ground cover. Many diatoms are either tube-dwelling diatoms (e.g. Cymbella, Navicula) or diatoms growing on mucilaginous stalks (e.g. Gomphonema, Gomphoneis). In the diatom ‘bulbs’ I identified the usual suspects: Gomphonema (attaches with gelatinous stalk), Cymbella (attaches with stalk or mucilaginous tube), Synedra, and Fragilaria (attach on their ends with glue-like gel).

This morning, I witnessed extensive brown-gold streaked foam collecting on the shore of the dam-regulated Otonabee River. I knew right away what it was and what was causing it: this was the result of a diatom spring bloom and migration.

I’ve often seen foam on moving water, the kind caused by crushed diatoms (single-celled algae that live in rivers and lakes). The foam is often tinged with brown, sometimes creamy-coloured, but typically has larger bubbles and a dish soap consistency. Diatom foam is a regular occurrence during the lifecycle of diatoms, which can make up a significant portion of river biota.

When subjected to great turbulence—such as when the dams along the river suddenly open—the diatoms, along with the fatty acids of associated decaying organic matter, create a ‘soap’ as their glass shells break apart and release lipids and proteins that act as foaming agents or surfactants. Air bubbles are pulled in through the turbulence to produce foam. Because the organic surfactants lower the surface tension of water, the bubbles persist at the water’s surface.  The bubbles accumulate hydrophobic substances and the dissolved organic matter stabilizes them and aggregates them as nutrient-dense foam.

Two years ago, during a large northern wildfire that brought smoke to the town, something interesting happened; the diatom foam turned into café crema:

Diatoms naturally contribute to foam observed in rivers as they grow, die and break open. This time—in late July on the weekend of air quality warnings amid a distinct campfire smell and blushing sun from the smoke of a northern forest fire—the diatom foam took on a different quality, the cafe crema look of dust and diatoms, when the northern forest fires meet the diatom forest.
The froth had turned into a mocha-coloured micro-foam that resembled the crema of a well-made espresso. Entrained by the wind that conspired with the river’s current, the film grew into caramel ropes that covered the rocks on shore in stripy shades of delicious earthy mocha cream—a synesthete’s dream. 

Was this literally smoke on the water?

I had to admit, this was foam with attitude; this was designer foam, the café crema micro-foam of my coveted flat white.

The metaphor isn’t farfetched: in fact, crema—the most prized aspect of a well-made espresso—is created through a similar process when hot water emulsified coffee bean oils and floats atop the espresso with smooth tiny bubbles.

 I collected foam samples and confirmed that the café crema froth consisted mostly of:

• detritus (decaying organic material from organisms and likely smoke particles from the northern forest fire),
• living diatoms and other algae, and
• the remains of many broken and empty diatom frustules.

The Diatom Forest and Drinking Water of the Otonabee River

Later in the summer months, Peterborough residents often start to notice taste and odor issues in their drinking water. This is caused by several volatile organic compounds (VOCs) created and released by periphyton as secondary metabolites associated with growth and reproduction or in response to age, death or environmental stresses. While not harmful, the T&O compounds are detectable even at extremely small concentrations (e.g. parts per trillion). 

The most common T&O compounds in summer Otonabee River water include two terpinoids: geosmin and 2-M19 (2-methylisoborneol), secondary metabolites produced by river algae that include the Cyanobacteria, Actinomycetes, Myxophyta, Chlorophyta and diatoms. The most likely T&O culprit in the Otonabee River appears to be three common filamentous Chlorophyta Spirogyra, Zygnema, and Mougeotia—all temporary guests of the river’s diatom forest. During the warm summer months, these filamentous green algae proceed to dislodge from their natal forest to form clouds of green froth and ‘hair’ in the river shallows. As the algal blooms grow and age and die in masses, they release geosmin and 2-M19 into the water. 

The diatom forest also provides potentially significant sources of biogenic taste and odor VOCs in a river. They do this in two ways:

• through cell damage and death: catabolic enzymes are activated during cell damage and death from grazing or other environmental stress, producing oxylipins and nor-carotenoids; these act as pheromones, grazer deterrents, teratogens, and toxins. 
• Through cell degradation: VOC release from decaying biomass by heterotrophic bacteria, fungi and other microorganisms, producing hydrocarbons, amines, terpines and sulfides. 

When diatoms slough off the top-heavy diatom forest and their frustules break apart—particularly during a diatom crash through some disruptive event (e.g. predation, disease, temperature, photolytic stress, dehydration, chemical treatment, or destructive turbulence such as a storm, dam release, etc.)—the diatoms release oxylipins and polyunsaturated fatty acid (PUFA) derivatives, among other things. Oxylipins carry a fishy, rancid, oily or cucumber odor, caused by unsaturated and polyunsaturated aldehydes (PUAs) and other alkenes derived from the fatty acids. They can also cause a ‘grassy-fruity-floral’ odor. The hydrocarbons, amines, terpenes and sulfides released by the degrading diatom mass may smell like solvent, fuel oil or gasoline, acrid burnt fat, and old tobacco.

The production and release of volatile taste and odor compounds is natural to river algae and occurs during cell metabolism, growth and eventual death and decay. It is however noteworthy that environmental stresses can escalate and intensify the release of T&O compounds.  

References:

Diatoms of North America. 2021. 

Burfeid Castellanos, Andrea M. 2018. “Ecological Factors and Diatom Diversity at Rivers of the Iberian Mediterranean River Basins: macro-scale, meso-scale and micro-scale. PhD Thesis, University of Barcelona. 233pp.

Munteanu, N. & R. Maly. 1981. “The effect of current on the distribution of diatoms settling on submerged glass slides. Hydrobiologia 78: 273-282.

Otonabee Conservation. 2018. “Otonabee Region Watershed Report Card 2018.” Otonabee Conservation.

Palmer, Marvin C. 1959. “An Illustrated Manual on the Identification, Significance, and Control of Algae in Water Supplies.” U.S. Department of Health, Education, and Welfare, Cincinnati, Ohio. 98pp.

Roemer, Stephen C., Kyle D. Hoagland, and James R. Rosowski. 1984. “Development of a freshwater periphyton community as influenced by diatom mucilages.” Can. J. Bot.62: 1799-1813. 

Ross, R., Cox, E.J., Karayeva, N.I., Mann, D.G., Paddock, T.B.B., Simonsen, R. and Sims, P.A. 1979. “An amended terminology for the siliceous components of the diatom cell. Nova Hedwigia, Beihefte 64: 513-533.

Round, F.E., Crawford, R.M. and Mann, D.G. 1990. “The Diatoms. Biology and Morphology of the Genera.” Cambridge University Press, Cambridge. 747pp.

Smolar-Zvanut, Natasa and Matjaz Mikos. “The impact of flow regulation by hydropower dams on the periphyton community in the Soca River, Slovenia. Hydrological Sciences Journal 59(5):1032-1045.

Watson, Susan and Friedrich Jüttner. 2019. “Biological production of taste and odour compounds.” In: “Taste and Odour in Source and Drinking Water: Causes, Controls, and Consequences”, Chapter 3. IWA Publishing 63-112pp.