/Ponds, Beaver Ponds, Arctic and Antarctic Ponds, and Walden Pond
UncategorizedSeptember 26, 2020

Ponds, Beaver Ponds, Arctic and Antarctic Ponds, and Walden Pond


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By Lambert Strether of Corrente

When I read this sad story, of which more below, I had a forehead-striking moment that having posted on estuaries, rivers, streams, lakes, and swamps, I had never posted on ponds. So herewith. Getting the obligatory Monty Python reference out of the way:

The peasant Dennis undermines Arthur’s legitimacy not only with reason (“no basis for a system of government”) but by challenging the rhetoric of the Arthurian Legend itself, substituting the plebian pond (“strange women lying in ponds”) for the wellborn Lake (“The Lady of the Lake”).

And indeed there’s something subversive about ponds. Although their etymology comes from pound, enclosure, they seem ill-defined at the edges. They seem ephemeral. Though sometimes they are not. Beaver ponds and alligator holes enrich ecologies. Ponds from melting permafrost or glaciers signal (and accelerate) climactic change. Ponds resist classification: The Wikipedia entry for Walden Pond begins: “Walden Pond is a lake…” but then goes on to refer to it as a pond (lower case “p”) throughout. Ponds are strange things!

Regardless, I went looking for a classification system for ponds, along with a definition. Wikipedia defines a pond as “an area filled with water, either natural or artificial, that is smaller than a lake” (and we see again Dennis’s subtle diminution of Arthur). Which strikes me as pretty vague, and Wikipedia goes on to admit that “the technical distinction between a pond and a lake has not been universally standardized…. Accordingly, some organizations and researchers have settled on technical definitions of pond and lake that rely on size alone.” (Of course, they can’t agree on the size. Wikipedia only presents technical definitions based on size; later I’ll present what looks to me like a better one.) We can forget about artificial ponds; I’ll leave ornamental ponds, swimming pools, Crystal Lagoons®, hog lagoons, and water hazards on golf courses for another time.

Small size, I think, leads people to think of “this pond” as “my pond.” I’ve never seen so many photos of any body of water as I saw when collecting material for this post. Many are beautiful, since most ponds are still water[1], their unruffled surfaces reflect sky, trees, clouds:

And this one:

“The pond” is my pond:

Of course, not all ponds are placid, or that small:

Pace Wikipedia, there are at least two classification systems for ponds that are not size-driven. The first comes from The Nature Conservancy, “A Lake and Pond Classification System for the Northeast and Mid-Atlantic States” (PDF). This was a 2014 effort to design a classification system that would cover all the lakes and ponds fo the Northeast. Here is the key slide:

Initially, I was tempted to classify this classification system as a failure, simply because there were a lot of different agencies and stakeholders round the table. I’ve been to meetings like that, and I saw the complexity (33 pond types? Really) as a resolution of institutional conflict. (And I’m also extremely suspicious of classification systems that have a bucket called “Miscellaneous,” because the lazy or ignorant will throw too much in that bucket. So, “Unclassifiable”? Really?) But on further consideration, I’m not sure I’m correct; nature is complex, and the Northeast is big. Plus the projects. maps are beautiful.

Nevertheless, this classification system is more congenial to me, because it reminds me of the ponds on the plains of my youth in the Midwest. From United States Department of The Interior Fish And Wildlife Service, Bureau Of Sport Fisheries And Wildlife, “Classification of Natural Ponds And Lakes in the Glaciated Prairie Region” (1971):

Here is the method used to design the classification, from pp 7-8:

Seven major classes of wetland in natural basin are recognized on the basis of ecological differentiation. Each class is distinguished by the vegetational zone occurring in the central or deeper part and occupying 5 per cent or more of the total wetland area being classified.

The Nature Conservancy taxonomy considers temperature, trophic state, alkalinity, and depth. Unlike the Fish and Wildlife taxonomy, it does not consider vegetation. That seems odd. If one wishes the consider the pond as an ecological being, surely the vegetation it contains and that surrounds it should be part of the equation? In any case, choosing a pond classification system is above my paygrade; but at least we know that the problem is more interesting than Wikipedia makes out.

Ponds may also be classified by life-cycle stages (and their lives tends to end). From the Missouri Botanical Garden, which also provides a brutally simple definition of the term:

A geological event, such as a glacier or sink hole, can create a pond. Ponds are nothing more than shallow holes where water collects[2]. Yet, if left alone, ponds will fill in with dirt and debris until they become land.

More formally, from artificial pond-marker Kasko, “Pond & Lake Life Cycle“:

Ponds or lakes are divided into 3 categories; they are either Oligotrophic, Mesotrophic, or Eutrophic stages of their life (listed youngest to oldest).

Oligotrophic bodies of water are considered new or young ponds or lakes in the overall scheme of things. Oligotrophic ponds and lakes have a low concentration of nutrients, such as nitrogen and phosphorus. They typically have steep sloping shorelines and are deep and clear. The bottom of the pond or lake is typically sand, gravel, or rock.

When Thoreau describes Walden Pond — supposing it to be a pond — he is describing an oligotrophic pond:

The scenery of Walden is on a humble scale, and, though very beautiful, does not approach to grandeur, nor can it much concern one who has not long frequented it or lived by its shore; yet this pond is so remarkable for its depth and purity as to merit a particular description. It is a clear and deep green well… The surrounding hills rise abruptly from the water to the height of forty to eighty feet… Walden is blue at one time and green at another, even from the same point of view. Lying between the earth and the heavens, it partakes of the color of both. Viewed from a hill-top it reflects the color of the sky; but near at hand it is of a yellowish tint next the shore where you can see the sand, then a light green, which gradually deepens to a uniform dark green in the body of the pond. In some lights, viewed even from a hill-top, it is of a vivid green next the shore.

Back to Kasko:

Mesotrophic bodies of water are considered middle aged, geologically. Mesotrophic lakes fall in the middle, between oligotrophic and eutrophic lakes. They have more nutrients and, therefore, more plant and algae growth than oligotrophic lakes and pond, but less than eutrophic. As a pond or lake ages from oligotrophic to mesotrophic, the sides of the pond begin to slope less and the bottom of the pond begins to fill in with organic material. The substrate that was once rock, sand, or gravel, now consists of mud on top of the rocks.

Eutrophic bodies of water are considered old or dying ponds or lakes. Eutrophic lakes and ponds are extremely well nourished with nitrogen and phosphorus, leading to an abundance of aquatic plant growth. As the pond or lake continues to age, the sides continue to flatten out and what were once steep sides is now gently sloping. The bottom of the pond is now filled with organic sediment and mud. The overall depth of the pond or lake is continually decreasing and the clarity continues to decrease. As the pond or lake fills in and the weeds grow larger, the total open water area shrinks as well. If left alone, the pond or lake will eventually fill in completely, and become a swamp or wetland at best.

Of course, these stages are not inevitable; they are affected by the pond’s ecology, in particular megafauna. From the Guardian, “Blasts from the past: how ice age ponds are coming back to life“:

“If you leave a pond it will naturally, in most cases, silt up and turn into a bog or a woodland,” says Dave Hutton, ice age ponds project officer at Herefordshire Wildlife Trust. “Without those natural processes, like aurochs and large mammals traipsing around and keeping them open, ponds and their wildlife tend to disappear. We’re acting like beavers and other large herbivores and keeping them open.”

And aurochs bring me to the poor elephants whose fate induced me to write this post. From Smithsonian, “Toxic Algae Caused Mysterious Widespread Deaths of 330 Elephants in Botswana“:

For months, what killed the more than 300 elephants between late April and June was a mystery, with many wondering if poachers were somehow involved or if something sinister might be at play. Now, officials say the pachyderms were laid low by toxic blue-green algae that had polluted their drinking water, reports BBC News.

Botswana is home to the world’s largest population of elephants—roughly 130,000 and rising—making the country a premier destination for wildlife tourism, report Mqondisi Dube and Max Bearak for the Washington Post.

The blooms of blue-green algae, which is actually not a true algae but a type of cyanobacteria, took hold in seasonal pools of water used by elephants, says Cyril Taolo, Botswana’s acting director of the Department of Wildlife and National Parks. The deaths came to a halt once these ephemeral ponds dried up, reports Sello Motseta of the Associated Press.

In other words, the ponds had reached the Eutrophic stage, and the blue-green algae produced by eutrophication killed the elephants.

From aurochs and elephants, who come to ponds to drink, we turn to megafauna that build ponds[3]. First, beavers. On the bright side, Smoky the Beaver prevents forest fires (or at least ameliorates their effects). From Emily Fairfax, Beavers and Wildfire:

TL;DR: water from beaver ponds is spread around the landscape in little channels the beavers dig. The pond water slowly seeps into the soil, keeping it wet and plants green. When wildfires come through, the beaver wetlands are too wet to burn. Can’t start a campfire with soggy sticks. Beavers = Firefighters

Fairfax presents these aerial photographs:

The caption:

Top panel photograph from California Manter Fire Burned Area Emergency Response (BAER ) Team. Bottom panels from Google Earth satellite imagery of Buzzard Complex fire in Oregon.

That’s the bright side. On the dark, or at least a different side, beavers are accelerating permafrost melt. From WBUR, “The Unusual Connection Between Beavers, Permafrost And Climate Change“:

Over the last 20 to 50 years, satellite imagery has shown beavers moving from the boreal forest to build ponds in the Arctic tundra…. The influx of beavers building ponds is starting to thaw the permafrost — land that’s been frozen for at least two years but often hundreds or thousands of years — under the ground…. “Evolution has taught beavers to be almost perfect hydrologic engineers,” [University of Alaska Fairbanks researcher Ken Tape] says. “They know where to put their dams. They are very efficient, and they’re incredibly industrious.”…. Permafrost makes the beavers moving up into the tundra a global concern. When beavers flood the tundra to make ponds, the water transfers heat to the ground and starts thawing the permafrost, releasing the greenhouse gases stored inside, he says…. “[Beavers are] really creating these focal points or oases on the landscape for boreal species to gain a foothold in the Arctic,” Tape says.

I wonder if the climate models include the effects of beavers building ponds, and what the effects are. (Since they tend to complexify the ecology, I would speculate they are good.)

Our second pond-building megafauna is the alligator (amazingly enough). From Wired — this article really is fun, I now stan for gators — “The Creature Feature: 10 Fun Facts About the American Alligator“:

5. Alligators are ecosystem engineers. Alligators play an important role in their wetland ecosystems by creating small ponds known as alligator holes. Alligator holes retain water during the dry season and provide habitats for other animals.

Here is a gator hole (Anita Gould):

(It really does seem to be hard to take a picture of a pond that’s not beautiful.) In Defense of Plants describes how and why alligators build their holes, and the habitats they create in “Alligators Increase Plant Diversity“:

[Alligator] activity level changes during the dry season when water is in short supply. Gators don’t sit back and let nature take its course. They spring into action and create their own aquatic refuges.

As the surrounding landscape begins to dry, gators will excavate holes or pits in the soggy ground called gator holes. These holes hold onto water when most of the surrounding landscape isn’t. … When a gator excavates a gator hole, it creates variation in both hydrology and soil conditions.

Soils that have built up over time are lifted out of the hole and piled into mounds. Mounded soils are not only rich in nutrients, they also dry at different rates, creating a gradient in water availability. Plants that normally can’t germinate and grow in saturated soils find suitable spots to live up on the soil mounds while emergent aquatic vegetation fills in along the parameter. Plants that normally prefer to grow in deeper water can also establish within the gator hole itself. In the midst of fairly uniform marsh vegetation, a gator hole quickly becomes a hotbed of plant diversity. The differences in vegetation can be so stark compared to the surrounding landscape that some scientists can actually map gator holes using aerial scans simply by measuring the differences in infrared radiation given off by the leaves of all the different plants that establish around them.

Of course, all of that plant diversity has a huge effect on other organisms as well. Gator holes become important areas for various reptiles, amphibians, birds, and so much more. The paths that alligators take to and from their holes even act like fire breaks, changing the way fire moves through the landscape, which only increases the heterogeneity of the immediate area. Fish, though occasionally eaten, greatly benefit from the stability of water levels within a gator hole. All in all, gator holes are extremely important habitats.

Finally, melt ponds may accelerate, or at least affect, climate change in both Arctic permafrost and the Antarctic ice sheet. First, the arctic. From Phys.org, “Permafrost in the Arctic can thaw faster than presumed“:

Air temperatures are increasing in high latitudes and in high mountain areas dominated by permafrost in the ground…. [One] consequence is that ice layers in the ground start to melt, so that the ground subsides and depressions with ponds and lakes form. This landscape change [is] known as “thermokarst.” … Among scientists, the general assumption is that thermokarst processes locally lead to faster thawing of permafrost. “However, with our model, we have also considered stabilizing processes that can slow down thawing. We were amazed that under a moderate warming scenario, thermokarst processes can even limit the thawing of permafrost”, [says Jan Nitzbon from the Alfred Wegener Institute for Polar and Marine Research in Germany]. Under a stronger warming scenario, however, self-reinforcing processes dominated, which would drastically change these landscapes due to accelerated permafrost thaw.

Here’s a photograph of thermokarst:

And now the Antartic. From The Conversation, “Antarctica now has more than 65,000 ‘meltwater lakes’ as summer ice melts.” They call them lakes, but I’m gonna think of them as ponds:

Scientists already knew that lakes form on the Antarctic ice sheet….[S]cientists are particularly interested in these lakes because they may contribute to destabilising the ice shelves and ice sheet in future.

Like a sponge, the more that ice shelves become saturated with meltwater, the less they are able to absorb, meaning more water pools on their surfaces as lakes. More surface lakes mean a greater likelihood that water will drain out, fill crevasses and potentially trigger flexing and fracturing. If this were to occur, other ice shelves around Antarctica may start to disintegrate like Larsen B. Glaciers with floating ice tongues protruding into the ocean may also be vulnerable.

* * *

So concludes another perambulation through the biosphere, this one truly mind-bending for me, since I had not thought that “shallow holes where water collects” could deliver so many of what Davos Man would call ecosystem services, ka-ching. Having started with Monty Python, I will end with Pink Floyd:

Grantchester Meadows once included a fen and a marsh, hence we may think of them as a Eutrophic, filled-in pond.

NOTES

[1] Freshwater ecosystems are generally divided into two categories: lotic (running water, as rivers, streams, creeks, etc.) and lentic (still water, as ponds, lakes, marshes, bogs, etc.).

[2] “Nothing more.” Sounds like the writer has been fighting through some of the same source material I’ve been.

[3] I have not included feral hog wallows. I do not know whether they are beneficial or not. I suspect not.

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