Under the Water, December’s Peak Leaf Season

By December, foliage season is long over for us humans, but it’s peak season under the water. Last month, as the last bus of tourists departed for home, fallen leaves accumulated in our streams and rivers, starting a process that’s critical for the nourishment of everything from caddisflies on up the food chain to eagles and even people. In fact, most of a New England stream’s food supply originates in the form of fallen leaves.

The bright yellow and red piles that accumulate on river rocks and fallen branches are not nearly ready for consumption by discerning invertebrates. The witch’s brew of natural chemical compounds that discourages insects from eating green leaves on trees, can be just as repellent to creatures that scavenge freshly fallen leaves under water.  First, cold water must leach out those chemicals. Imagine the process as soaking and re-soaking a teabag. During this period, the leaves are also colonized by microscopic organisms. For a hungry invertebrate, the cleansed layered leaves, covered in fungi, bacteria, and algae, make a sandwich of which Dagwood Bumstead could be proud.

If all leaves were created equal, then the river food supply would consist of a brief crop polished off by whatever ravenous insect species got there first. But of course not all tree and shrub species lose their foliage at exactly the same time, and their leaves decompose at different rates. Some are tough, leathery, and laden with toxins. Certain laurel species can release so much cyanide that entomologists have used them to kill study specimens. Other species, such as linden, are comparatively soft and become an early underwater food source. Oaks and beech trees hold onto their tough leaves late into the season and when they finally fall, they are slow to reach an edible condition.


Variability in the timing of leaf fall, and in the rates at which leaves become palatable, means that edible leaves are available for most of winter. This is also the season when much aquatic invertebrate growth occurs.  A caddisfly species called Pycnopsyche gentilis acts like a chronicler of the leaf supply.  In fall and early winter, its larvae cut disks from leaves, and use silk to make and line pencil-diameter cases in which they live. In early spring, as leaves become scarce, and the larvae keep growing, they add to their cases using large sand grains.  This is why one can sometimes find cases with leaves at the back end and sand grains at the front.

Although fish aren’t typically leaf eaters, they nonetheless rely on the underwater foliage season. Cranefly larvae and stoneflies, fattened up on leaves, are protein-packed snacks for hungry trout, salmon, and bass.  Without invertebrates, fish would go hungry.  Without some knowledge of invertebrates, few anglers would know what to put on a hook to lure a spectacular brook trout.

Cleanest streams host most invertebrate species and simply counting the number of species can be the basis for an interesting study. For example, each fall, I visit forested and urban streams with Saint Michael's College biology students.  This year we found more than 20 species in Browns River, downstream from Mount Mansfield State Forest.  Centennial Brook drains South Burlington neighborhoods and parking lots, and hosts fewer than half that many invertebrate species. Browns River macroinvertebrates include clean-water loving giant stoneflies, and an assortment of mayflies. I have yet to meet a Centennial Brook stonefly because the community is dominated by midges, craneflies, and a particularly tough caddisfly that makes its living by filtering particles from the water.

A view into the world of submerged invertebrates is just a bunch of leaves away.  You can easily lift an intact chunk of leafy habitat from a streambed for study.  Equipment can be as simple as a basin of stream water. Drop a handful of streambed leaves into the basin and observations can begin. Rinse and remove the leaves one at a time to reveal the small creatures living among them.

Declan McCabe teaches biology at Saint Michael’s College. His work with student researchers on insect communities in the Champlain Basin is funded by Vermont EPSCoR’s Grant EPS-1101317 from the National Science Foundation.

This article was written for Northern Woodlands Magazine's Outside Story and first published on December 14th 2015

The photograph of a stonefly was downloaded from Wikimedia Commons under a CC BY-SA 3.0 Creative Commons license.  Download the Article

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Zebra Mussels

Invasive species have earned their bad reputations. English sparrows compete with native birds from Newfoundland to South America. Australian brown tree snakes are well on their way to exterminating every last bird from the forests of Guam. And I don’t think anyone can fully predict how Columbia’s rivers will change in response to drug lord Pablo Escobar’s escaped hippopotamus population.

While our climate protects us from rampaging hippos, the Northeast has plenty of exotic species in its waterways, including some that cause serious damage. Zebra mussels are possibly the most familiar of these. They were first discovered in Lake Champlain in 1993 by a precocious 14 year old, Matthew Toomey, who recognized one based on an identification card he’d received at school. Since then, the mussels have spread throughout the lake and their effects have been well chronicled. They kill native mussels; coat surfaces with razor-sharp shells; foul anchor chains; block water intake pipes; and steal plankton and other food from native fish.

With all of the negative press regarding the species, you might find it jarring to read anything positive about zebra mussels, particularly anything written by a biologist. Discussing positive effects of invaders is practically taboo. We don’t speak ill of the dead; we never praise invasive species. I’m certainly not advocating zebra mussel propagation, but like them or not, they are here to stay. These mussels are an important part of European ecosystems, and it’s interesting to consider what native organisms benefit from their presence.

Zebra mussels are voracious filter feeders. A single mussel can suck a liter of water through its body daily. All of this filtration removes plankton and particles from lake waters, but these particles don’t just disappear. The phrase immortalized in Minna Unchi’s children’s book Everyone Poops applies. Along with excrement, unpalatable particles rejected by zebra mussels are mixed with mucus and dropped on the lake floor. Mussel excrement and mucus might not sound appetizing, but it’s a smorgasbord for lake floor invertebrates and fish.

In addition to covering rocky surfaces, zebra mussels often carpet lake floor sand and silt. Formerly soft sediments that provided foraging grounds for sturgeon and other fish can become a tangled mess of living and dead mussels several inches thick. Not surprisingly, fish such as log perch, bullhead, and sculpins have difficulty finding their insect prey amongst the clutter of shells layered over their sandy habitats. When given the choice, juvenile sturgeon avoid zebra mussels and spend their time on sandy or rocky areas.

What’s bad for these predators may be good for their prey. To figure out just how good or bad zebra mussels could be for Lake Champlain invertebrates, we ran experiments under 30 feet of water in sandy areas of Appletree Bay. When my colleagues Ellen Marsden, Mark Beekey, and I fenced off lake floor patches with and without zebra mussels, twice as many invertebrates colonized areas with zebra mussels. More species also moved in. After a month, the number of species in experiments with added mussels doubled and included some species more typical of rocky lake floors. Nooks and crannies between zebra mussel shells seem to act like very small, natural shark cages that protect tiny insects from hungry fish. And when we placed insects and fish in aquariums, far more invertebrates survived with zebra mussels than without.

On balance, I would rather have a lake without zebra mussels than with them. But unless ways are found to eliminate them, it will remain important to understand how they affect native species. In Lake Champlain, the zebra mussel population grew rapidly and has since fallen below peak numbers, as often happens with this species in a new location. This month, for the first time in recent years, we pulled up a lake floor sample in Burlington Bay that entirely lacked zebra mussels. Perhaps we are reaching a new equilibrium?

Declan McCabe teaches biology at Saint Michael’s College. His work with student researchers on insect communities in the Champlain Basin is funded by Vermont EPSCoR’s Grant EPS-1101317 from the National Science Foundation.

This article was written for Northern Woodlands Magazine's Outside Story and first published on October 5 2015

The photograph of zebra mussels was downloaded from Wikimedia Commons under a CC BY-SA 3.0 Creative Commons license.

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