by Nina Munteanu
Looking for ancient treasure, I drove north from Peterborough
to Petroglyph Park in the Great Lakes-St. Lawrence Lowlands Forest
Region, a sought-after destination for its impressive ancient
petroglyphs (rock carvings). Holes in the rock were considered
entrances to the spirit world, situated directly beneath the surface
(spirits prefer to live near water).
When I reached the
park, I discovered that the glyphs were off-limits because of COVID.
Disappointed, I looked to salvage my trip by hiking the 2 km loop
trail to McGinnis Lake. The walk from the west day use parking lot
took me through dense pine forest. Giant pines thrust high above me
like columns of a sacred cathedral. Their deep green canopies swayed
and creaked in the breeze as they strained toward the heavens in a
low baritone hush. I passed pink granite outcrops in the soft
limestone and found myself on a rocky promontory that overlooked the
over 12 m deep lake. The 4.4 ha lake’s water was a deep blue-green
jade colour rimmed by shallows of lighter green that graded to a
cream colour. Rocks, logs and large shore debris hung precariously
over the steep sides of the lake below me, covered in creamy marl.
On the opposite side
of the jade-coloured lake—accessed by the east day use parking lot,
the marl-covered shallows extended further out, creating a stunning
visual colourscape that shifted from deep blue-green to yellow and
cream.
The information sign
on the promontory describes McGinnis Lake as a rare meromictic lake.
What a treasure! I’d
studied meromictic lakes at university as a limnology student; I’d
never actually seen one before. Until now. Meromictic lakes
aren’t just rare; they are fascinating in the study of lake
formation, type and function.
Many lakes in the
northern temperate area of Canada are dimictic: they mix completely
(holomixis) twice a year, once in the spring and once in the fall. In
shallow lakes, warmed by the sun and mixed by the wind, wind-driven
currents keep the water mixed all year round. In deeper lakes, the
currents can’t compete with the active summer warming of the upper
water mass and density differences develop between upper and lower
waters. The lake stratifies into an upper epilimnion and lower
hypolimnion, separated by a metalimnion or thermocline barrier. In
the fall, with cooling, the density barrier breaks down and the
wind-driven currents penetrate into the lower layers to thoroughly
mix the lake to the bottom sediments (holomixis) in what’s called
vernal turnover.
Unlike dimictic
lakes, meromictic lakes experience incomplete vertical mixing of only
the upper water mass during the circulation period (called
meromixis). The upper water mass (mixolimnion) mixes twice yearly
like a dimictic lake; however, below this upper mixing layer lies a
salinity barrier known as a chemocline (where dissolved oxygen
decreases markedly with depth) and beneath it lies the anoxic water
mass known as the monimolimnion, which experiences a fairly constant
temperature and higher salinity. The higher dissolved salt at the
bottom—and greater associated water density—prevents wind-driven
mixing of this bottom quiescent layer and accumulates hydrogen
sulfide and methane.
Lake
morphology—particularly the relationship of depth and surface
area—contributes largely to whether a lake is meromictic and
capable of preserving undisturbed laminated sediments. A meromictic
lake may develop if it contains a deep hole in a shallow basin or is
sheltered from the prevailing wind by tall vegetation or other
barriers—like McGinnis Lake, which rests in a steep-sided limestone
basin, sheltered from the winds by a dense pine forest. McGinnis Lake
may have formed through karst erosion; it may also simply occupy a
deep glacial trough. Because of the barrier and lack of mixing, any
exchange of dissolved materials from the lower quiescent layer into
the mixing layer occurs very slowly through eddy diffusion across the
chemocline. This makes Lake McGinnis’s monimolimnion a nutrient
sink and why it is, like most meromictic lakes, unproductive
(oligotrophic).
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This drawing shows how the lake is shallow at its edges and grows deeper in two places.
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In summer, when
McGinnis Lake is stratified, the top 6 m layer of McGinnis Lake
reaches 20-22˚C and its middle 6-12 m layer is typically 7-12˚C.
However, below the chemocline, the anoxic monimolimnion (below 12 m),
stays a constant 5-6˚C year-round, and is a pinky-brown colour. Few
organisms live in the oxygen-depleted monimolinion. An exception are
the cyanobacteria (Cyanophyceae), autotrophic bacteria that can
survive on hydrogen sulfide at the lake bottom and are responsible
for lime depositing in lakes.
Brilliant Jade
Colour
The intense jade colour of marl-based McGinnis Lake is
partially explained by the presence of calcium carbonate (CaCO3) in
the lake from marl—calcium carbonate and clay. The dominant
carbonate mineral in most marls is calcite, along with other
carbonate minerals such as aragonite, dolomite and siderite. Marl
formation and settling is encouraged by bacteria, phytoplankton, and
periphyton (attached algae) as temperatures increase in summer. The
calcium carbonate—which is present in the limestone bedrock
surrounding the lake—acts like a flocculent to clear the lake of
the coloured, dissolved substances; as the brown hue is removed, blue
and green light can penetrate into the deepest parts of the lake. The
most brilliant jades can be seen when the microscopic algae thrive
and when the suspended marl increases in volume in mid to late
summer.
Presence of marl is
also why the water-sunken trees and debris and the entire shoreline
are covered in a milky cream-coloured floc—likely a combination of
marl deposit and periphyton (attached encrusting and filamentous
algae) that help deposit the marl. Examples may include stalked
diatoms (Gomphonema) and blue-green alga Oedogonium. The periphyton
secrete glycocalyx (fibrous meshwork of carbohydrates) and other
mucilage secretions that coat the sediment particles and adsorb
organics and other nutrients for their use. This is why the lake’s
shallow shores are a dramatic cream-yellow colour and grade to a
brilliant green then deep blue-green of deeper overlying waters. Marl
are tiny white coloured crystals and as the water warms in the day,
so does the volume of crystals in the water. As the summer
progresses, the clear deep blue of McGinnis Lake may transform into
lighter milkier turquoise with suspended calcium carbonate crystals.
Undisturbed
Sediments & Varves
Because a meromictic lake’s bottom
water layer doesn’t mix and is permanently anoxic (without oxygen),
no burrowing benthic organisms are present to destroy the sediment
layers (varves) laid down over time—mostly organics that don’t
decay. Because of this, these varves provide an undisturbed history
of biological succession and climate change of the last 10,000
years.
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Here's a cutaway image of varves - layers of sediment
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Undisturbed annual
sediment laminations can provide accurate chronology, just like tree
rings, over thousands of years, dating back to the late Pleistocene
and Holocene 10,000 to 12,000 years ago. This is because sediment
accumulation—just like tree growth—often follows a seasonal
pattern. Annual accumulations of sediment may consist of a simple
two-component couplet (summer vs. winter sedimentation). In summer
increased photosynthesis causes settling of CaCO3; while in winter,
when the lake is ice-covered, fine organic material and clay settles
to the bottom.
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This is a layer of diatoms in a varve
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Varve couplets
(summer-winter layers of a year) typically consist of a dark layer of
organic sludge with algal filaments, iron sulfides, and clay that
grades upward into a lacy network of diatom frustules and organic
matter; this would be overlain by a light layer of diatom frustules
and calcite that turns into pure calcite at the top. In summer,
calcium carbonate and diatoms (algae with silica shells) accumulate
on the bottom; in winter more fine organic matter and clay settle.
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These shapes are tiny diatoms!
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On my way home, I
considered my fortune: I’d found a real ancient treasure after all,
something
I hadn’t expected to see.
References:
Anderson, Roger Y., Walter E. Dean, J. Platt
Bradbury, and David Love. 1985. “Meromictic Lakes and Varved Lake
Sediments in North America. USGS Bulletin 1607.
Burkholder, JoAnn M.
1996. “Interactions of Benthic Algae with Their substrata. B. The
Edaphic Habit: Epipsammic and Epipelic Algae among Sands and Other
Sediments. Algal Ecology: Freshwater Benthic Ecosystems, R.
Jan Stevenson et al., editors. Academic Press. 753pp.
Cheek, Michael Ross.
1979. “Paleo-indicators of Meromixis.” M.Sc. Thesis, Brock
University, St. Catherines, Ontario. 129pp.
Stewart, K.M.,
G.E. Likens. 2009. “Meromictic Lakes.” In: Encyclopedia of
Inland Waters, G. E. Likens, editor-in-chief. Academic Press.
2250pp.
