[Ever wonder what makes foam on the surface of streams you know aren't polluted? You'll learn all about this natural foam, in today's post by Nina Munteanu]
On a late July morning, I was on my daily walk
along the Otonabee River in the Kawarthas when I noticed a
yellow-brown creamy film on the water lapping onshore. In some places
it looked like left over dishwater, but in others, it resembled
cream. The multi-hued film swirled like flowing paint in an Emma
Lindstrom artwork. It was the “eddy shedding” patterns of
Saturn’s north polar atmosphere or Jupiter’s Great Red Spot.
Otherworldly.
But it didn’t end there…
Later in the day, the froth 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.
At
the mouth of one little tributary to the river, foam with hints of
caramel rose in a cloud of froth where the runoff tumbled into the
Otonabee River.
“Crema”
Micro-foam on Water Surface
My first thought was crushed diatoms. Diatoms are
often the main component of periphyton (attached algae) the dominant
micro-community in a riverine ecosystem that provide an important
food source and home to a diversity of other life.
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Diatom shapes diagram from AQUALITA public website |
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Diatoms
are single-celled algae enclosed in shells made of nanopatterned
silica and organic compounds called frustules. The glass frustules
are hard but porous and etched in decorative markings or bands (rows
of pores or alveoli to form striae and interstriae) that are used in
identifying the more than 20,000 species. The silica shell protects
the diatom from predators and acts as a ballast; the pores give the
diatom access to the external environment for things like nutrient
uptake, waste removal, and mucilage secretion so the diatom can
attach itself to a substrate.
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. Like I said: café
crema. This 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
emulsifies coffee bean oils and floats atop the espresso with smooth
little bubbles.
Given
that the Otonabee River is regulated by several hydro-generating dams
and locks to control its flow and provide electricity, it’s not
surprising that many of the diatoms in the foam are periphyton that
have sloughed off a submerged surface in the sudden turbulence caused
by the dam release. You’ve seen periphyton; the slippery brownish,
oily-looking felt mats that cover the cobbles and rocks of streams
and rivers. Sometimes, they grow bright greenish ‘hair’, most
likely one of the three most common filamentous green algae
(Spirogyra,
Zygnema,
and Mougeotia),
all of which I’d already observed growing on the river banks in the
spring.
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Algae filaments |
Periphytic
diatoms are often either tube-dwelling diatoms (e.g. Cymbella,
Navicula)
or diatoms growing on mucilaginous stalks (e.g. Gomphonema,
Gomphoneis).
Tube-dwellers and stalked diatoms tend to dominate benthic
environments of altered hydrological and thermal regimes downstream
of dams. These mucopolysaccharide materials also ‘glue’
everything together into a dense micro-foam.
Excited
to see if I was right in my initial assessment, I returned with a
small glass jar and sampled some of the tan-brown scum-froth then
returned home to the microscope. And I confirmed that most of the
scum was a combination of:
detritus
(decaying organic material from organisms),
living
diatoms and other algae, and
the
remains of many broken diatoms
The
Diatom Forest
Diatoms
adhere to submerged surfaces through mucilage produced between the
frustule (the diatom’s outer shell) and its substrate. The form of
the mucilage varies and includes gelatinous stalks, pads and tubes.
For instance, Cymbella
and Gomphonema
produce long stalks that attach directly to the surface, allowing
them to form a swaying canopy over the lower tier of cells such as
Fragilaria vaucheriae,
Synedra radians
and Cocconeis placentula
(think overstory and understory of a terrestrial forest or a marine
kelp forest).
As
part of the understory layer, diatoms such as
Fragilaria vaucheriae and Synedra
radians attach to the surface at one
end (apical) of their rod-shaped frustules using a mucilaginous pad
to form “rosettes” that resemble spiky understory shrubs.
Just
like trees, the canopy-forming stalked diatoms effectively compete
for available light and nutrients in the water with their vertical
reach. They provide the ‘overstory’ of the diatom forest’s
vertical stratification. These tree-like diatoms also provide an
additional surface for other diatoms to colonize (e.g. tinyAchnanthes
settle on the long stalks of Cymbella,
just as lichen does on a tree trunk).
The
stalked diatom forest acts like a net, trapping drifting-in
euplankton, such as Pediastrum sp. and
Fragilaria spp., which then decide to
stay and settle in with the periphyton community. The mucilage
captures and binds detrital particles in both lower and upper stories
of the diatom forest; these, in turn, provide nutrients for the
diatom forest and additional surfaces for colonization. In their work
with periphyton communities, Roemer et al. (1984) found
several diatoms (e.g. Diatoma
vulgare, Fragilaria
spp. Stephanodiscus minutula)
entangled in the complex network of cells, stalks, and detritus of
the diatom forest’s upper story. They also found rosettes of
Synedra radians—like
jungle orchids—attached to large clumps of sediment caught by the
net of mucilage.
Eventually,
‘overgrowth’ occurs as the periphyton colony matures and grows
‘top-heavy’ with all this networking. The upper story of the
community simply sloughs off—usually triggered by turbulence such
as when the dams release water. This is similar to a forest fire in
the Boreal forest, which creates space and light for new colonization
and growth. The dislodged periphyton then ride the turbulent flow,
temporarily becoming plankton, and those that survive the crashing
waters provide “seed” to colonize substrates downstream.
Amid
the many dead frustules, I observed many living diatoms along with
green and blue-green algae, rotifers, Protista and bacteria feeding
on the detritus. I even saw one amoeba, actively feeding on the
nutrient-rich interface of a micro-foam organic bubble.
The
Regulated River
The
dams on the Otonabee River dramatically affect its aquatic biota by
altering hydrology, sediment transport, nutrient cycling, temperature
regimes, and the movement of organisms. One of the main impacts of a
dam is the change in flow dynamics: intensity, velocity, direction.
By their very nature, dams create more lake-like environments
(lentic) from a more river-like one (lotic). This forces aquatic
communities to adapt from free-flowing, erosional habitats to
depositional environments.
Having
a less mobile substrate along with increased temperatures and higher
nutrients in river sections behind dams will lead to increased
biomass of certain diatoms and the proliferation of filamentous green
algae (e.g. Ceratophylum,
Spirogyra,
Zygnema,
and Mougeotia—all
currently blooming in the Otonabee River along with the diatom
forests. All are responsible for taste and odor of the water.
Taste
and Odour of the Diatom Forest
In
addition to providing hydroelectric power, the Otonabee River is a
source of drinking water for the City of Peterborough. Particularly
in the warm summer months, Peterborough residents notice that their
tap water carries a complex taste and odor with hints of mostly
earthy mustiness. This is caused by several volatile organic
compounds (VOCs) created and released by benthic algae 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 diatom forest provides potentially significant
sources of biogenic taste and odor VOCs in a river. When diatoms
slough off the top-heavy 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.
Diatom
species implicated in these odors include Fragilaria,
Synedra,
Melosira,
and Stephanodiscus
among others—all identified in the scum I looked at.
Diatoms
use volatile taste and odor compounds to modify cell function (e.g.
as antioxidants, pheromones, and autoregulation) and food web
interactions (grazer deterrents, inhibitors, toxins against
predators, attractants) and in response to stress.
Although
the production and release of volatile taste and odor compounds is
natural to many river algae (in the process of cell metabolism,
growth and eventual death and decay), environmental stresses can
escalate and intensify the release of T&O compounds. Organic
enrichment and nutrient enrichment of the river from fertilizers and
urban runoff create stress by overly increasing the biomass of river
algae, leading to blooms and bloom crashes with higher incidence of
T&O.
References
Diatoms
of North America. 2021.
Munteanu,
N. & R. Maly. 1981. “The effect of current on the distribution
of diatoms settling on submerged glass slides.
Hydrobiologia
78: 273-282.
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.
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