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Pond scum is kind of a rude name, isn’t it? It feels kind of appropriate when you’re wading through murky waters, and you might not be able to see it, but you sure can feel it—whether or not you want to.

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They signed up to give us a monthly contribution that goes directly into the production of these episodes. So, we’d like to start off today by saying thank you to them, and if you’d like to join them you can go to and sign up today. Pond scum is kind of a rude name, isn’t it?

I mean, sure, it feels kind of appropriate when you’re wading through murky waters, and you feel that disconcerting combination of fuzzy and slick textures sliding against the bottom of your feet. You might not be able to see it, but you can sure feel it— whether or not you want to. And it feels like anything could be there, anything at all.

But even still. Scum? That seems a little judgmental.

Technically, the word “scum” was originally used in the 14th century to describe the frothy layers on top of liquids. It wasn’t until the 16th century that it began to be used more as an insult against people. So sure, “scum” was probably meant more to describe the way pond scum obscures our water than as a moral criticism of those green mats.

But there is actually something kind of… scummy about pond scum. It’s not something they necessarily intend to be. In fact, it’s a contradiction of sorts, a process that takes them from an organism that provides oxygen to its fellow pond neighbors, to an organism that suffocates them.

There are different types of green that accumulate in our ponds, including clouds of cyanobacteria that disperse when you wave a stick through them. But pond scum is different. If you drag a stick through pond scum, it’ll come out of the water looking like a wet mat woven together with green threads.

It looks like that, because that is what pond scum is. Those mats are made out of filamentous algae, a range of species who bring a mixture of greens and yellows and browns to their joint endeavor. Up close, with a microscope, you can see the individual characteristics that make up those algae, almost as if you’re examining the silk threads that have been stitched together to form a glorious tapestry.

And by “glorious tapestry,” we of course mean “pond scum.” And yes, that feels like a contradiction. But look at these Spirogyra and their distinct spiral shape that their chloroplasts wind through them. They’re like a spectacular off-color candy cane.

When you feel pond scum on you, this is what you’re feeling: beautiful microscopic strands built from chains of cells that divide and divide to elongate the thread binding their lives together. Filamentous algae usually begin growing at the bottom of ponds, in shallow areas where there is plenty of light to drive their photosynthesis. As the algae divides and extends its chains, the filaments begin to tangle and weave together, forming a dense mat.

Because the algae are photosynthesizing, they are also releasing oxygen into the water— oxygen that gets trapped as bubbles within the growing mat. Those bubbles lift the mat of filamentous algae up so that it’s no longer trapped at the bottom of the pond. And that creates a sort of floating city, with threads of algae that house many residents of the microcosmos.

In the background here, you can see the blurry form of a rotifer snapping up any snack that passes close enough for it to grasp. And here we can see a strange hypotrich popping out from underneath a filament as it feeds, maybe using the algae like a log to hide under before it ambushes its unsuspecting prey. And if you look hard enough, you might find an aeolosoma, our favorite polka-dotted worm, sandwiched between strands of algae, maybe casually wiggling around on its way to a cannibalistic gathering. (Yes, that’s a thing.

We have a whole episode about it, if you want to know more.) All-in-all, it turns out that pond scum is both made of beautiful algae threads, and houses all sorts of fascinating creatures. You might be tempted to call it a little microbial utopia. But James, our master of microscopes, often gets emails from people who want to know how to get rid of the scum growing in their artificial ponds. “But they’re so wonderful,” you might say, now that you know that underlying all that muck is a wondrous world of worms and rotifers and mysterious creatures housed in a living home that sustains them.

And yet every year, ponds around the world become the site of a battle between the filamentous algae that come to dominate them, and… well, pretty much everything else in the pond. This is a different situation from, say, the cyanobacteria we mentioned earlier, or certain algal blooms, that can kill their fellow pond residents by producing toxins. Filamentous algae wouldn’t do something nefarious like poison their neighbors.

They just suffocate them instead. The organisms living inside ponds rely on oxygen that gets dissolved in the water. Some of that oxygen comes from outside the pond, diffusing and circulating into the water.

But of course, filamentous algae and their fellow photosynthetic pond residents contribute to the oxygen content as well. Which is quite kind of the filamentous algae. The problem is that they don’t just make oxygen.

They also consume it. And if the weather conditions aren’t quite right, the algae will consume more oxygen than they make. This depletion can be exacerbated by nutrients like phosphorus and nitrogen, which might enter the pond from fertilizer or runoff from around the pond.

These nutrients cause the algae to grow and grow and grow until their numbers skew the oxygen balance in their favor and against everyone else’s. And even if the algae die off all at once— like through the use of a chemical algaecide— that doesn’t solve the oxygen problem. You’ve now provided the bacteria in the pond plenty of dead algae to decompose, which means they consume a lot of oxygen instead.

That doesn’t mean that you can’t use chemicals to kill algae. However, it requires careful consideration of the pond conditions and the algae involved to make sure that it makes sense to turn to those algaecides. There are other methods for short-term control of these algae populations.

Like physically removing them with a rake, or using water-soluble dyes that limit sunshine to the algae when they’re still at the bottom of ponds. But these are short-term solutions. The fact is that long-term approaches means preventing excess nutrients from entering the ponds in the first place, whether by thinking through our fertilizer choices or adding buffers to our ponds that prevent runoff from entering or any number of other approaches that help maintain a healthy pond.

Because maybe in the long-term, dealing with filamentous algae is rooted in acknowledging the way they suit the word “scum” in all senses of the word. They are grimy and gross, but also something that is simply a part of the water. And in between the negative connotations of the word and its more neutral origins is something wondrous that requires balance on our end to maintain.

Thank you for coming on this journey with us as we explore the unseen world that surrounds us. Those names you’re seeing on the screen right now are some of our Patreon patrons, and they are the reason that this show can exist. If you’d like to join them in supporting Journey To The Microcosmos, you can go to to get weekly wallpapers and monthly uncut videos If you’d like to see more from our Master of Microscopes, James Weiss, you can check out Jam & Germs on Instagram.

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