13 Nov 2025 6 min read

Fallen Angels

As if autumn leaves weren't amazing enough

Autumn leaves accumulating in a stream. Photo by David Lukas

The ethereal realm of fluttering leaves completely transforms in the fall as these windblown bits of magic plummet to earth and become part of something much larger.

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Everywhere I walk at this time of year, the ground is thickly carpeted in leaves. It's fascinating to watch them slowly break down and decompose, and their contribution to compost is well known to any gardener. But, out of sight, and less well known, fallen leaves also play an incredible role in stream ecology.

fallen leaves — We pay attention to autumn leaves on the ground, while missing the fact that they also fall in streams and lakes. Photo by David Lukas

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In today's newsletter, I'll refer to "streams," but understand that we're talking about everything from ponds and lakes to creeks and rivers because this story applies to all bodies of water.

When leaves accumulate in water, it might look messy, but it's a sign of health. In fact, a stream with no fallen leaves is as good as dead because fallen leaves are the fundamental source of energy for the entire food chain that exists in and around the stream.

leaves in stream — Leaves in the water look messy but they're a vital part of stream ecology. Photo by David Lukas

Why is this? While streams have some built-in sources of food, the bulk of the organic matter that powers aquatic food chains comes from external sources, namely the leaves (and branches) that fall into the water from streamside plants.

river with fall colors — These trees are more than just a pretty picture. Photo by David Lukas

Leaves are comprised of complex carbohydrates, minerals, and a variety of chemical compounds. And as soon as a leaf falls in the water, these components begin to break down, with a long lineup of organisms waiting to process and hand off these elements in an incredibly complicated choreography.

leaves in water — A massive pulse of leaves falling in the autumn sets up a food chain that feeds a stream for an entire year. Photo by David Lukas

However, the bonanza of food stored in a leaf isn't available until the leaf is first transformed. Within 10 days of falling in the water, a leaf is colonized by aquatic fungi whose hyphae penetrate the leaf, softening the tissue and adding nitrogen and protein that will make the leaf palatable to detritus eaters.

leaf in water — As soon as a leaf falls in the water it is colonized by fungi that begin the process of breaking the leaf down. Photo by David Lukas

As the fungal breakdown proceeds, bacteria begin invading the leaf, attacking surfaces prepared by fungi and hitching rides on fungal hyphae as they penetrate the interior of the leaf.

This combination of fungi, bacteria, and increasingly fragmented leaf particles soon attracts a group of aquatic invertebrates with strong mouthparts that are known as shredders. While the larvae of aquatic insects like caddisflies and stoneflies cannot eat whole leaves, they have evolved to nibble on leaf fragments produced by fungi and bacteria.

stonefly larvae — You rarely see stoneflies until their larvae emerge from water and transform into flying adults. Photo by David Lukas

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Biofilms I wrote about biofilms in an earlier newsletter, and they play a fascinating role in stream ecology. Not only do fungi and bacteria begin to break down leaves, but they also produce extracellular polysaccharides (a kind of sugar) that cover river rocks and other surfaces with an amorphous slime. These polysaccharides are so abundant that they account for ten times more biomass than all the living cells in a stream combined, and they dominate the flow of nutrients in a stream.

aquatic invertebrates — Aquatic invertebrates are grouped into different guilds based on their feeding strategies. Image from Jane Marks

In turn, shredders are messy eaters that leave behind (and poop out!) many smaller fragments that are then eaten by another group of aquatic invertebrates known as collectors. Collectors include the larvae of superabundant insects like mayflies and midges, and you can already begin to see how fallen leaves build food chains that feed every organism from bacteria to bears and bald eagles.

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Tangled Webs There is a vital relationship between a stream and its surrounding environment. For example, half of the foods eaten by fish like salmon and trout come from flying and ground-based insects that fall into the water, while emerging aquatic insects account for 25-100% of the food eaten by streamside birds, bats, lizards, and spiders.

Hundreds of papers have been written on this topic, and these aquatic food webs are one of the best-known aspects of stream ecology. However, this is just the tip of the iceberg because I want to mention a few other aspects of this story.

While fallen leaves provide energy to aquatic food chains, not all leaves provide the same benefits. Each plant growing along a stream provides different kinds of nutrients over different time spans. Some leaves break down quickly, while other leaves break down slowly, releasing peaks of nutrients over the course of the entire year. Not only are there key differences between species, but individual plants of the same species can have even sharper differences. What this means in practical terms is that the variety of plants growing along a stream is what matters most.

trees along river — When a streamside forest is cut down or reduced to a narrow strip, it has a massive impact on the stream ecosystem. Photo by Uwe Kienle from Pixabay

Another critical point is that leaves have a ratio of carbon and nitrogen. If the soils and plant communities along a stream are rich in nutrients, plants produce nitrogen-rich leaves, which in turn stimulate aquatic decomposers. And conversely, when the atmosphere is full of excess carbon due to human activity, leaves end up with more carbon than nitrogen, which greatly reduces the activity of aquatic decomposers.

trees around water — A glimpse of a small healthy riparian ecosystem in the fall. Photo by David Lukas

In other words, this is a big story, and it's one way that humans are having a bigger impact on stream ecology than you might realize.

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