The Outside Story: The Food Web Under the Ice | Weekend magazine


Earlier this winter, I went out on the ice at the pond – not to skate, but to peek below the surface. Although lake ecologists once considered plankton in frozen lakes to be dormant during the winter, recent studies reveal that microscopic plant-like phytoplankton (which move with lake currents) and animal-like zooplankton remain active below the frozen surface.

In data collected from more than 100 lakes, Stephanie Hampton, a professor at Washington State University, found that while the food web base under the ice is reduced compared to summer, it is not certainly not dormant. Hampton’s research suggests that phytoplankton supporting lake food webs are reduced by 80% compared to summer, while higher up in the food web, zooplankton abundance drops by 75% in lakes. ‘winter.

Yet there is enough activity in this network to support a variety of winter lake life, something

The optimistic New England ice fishing community has long understood this. The fish, however, certainly don’t dawdle in vain hopes of a sudden ray of light through the ice and the meals provided by humans like manna from heaven. They depend on a natural food web supported by phytoplankton, which harnesses the sun’s energy to make food. This phytoplankton is consumed by zooplankton, which is, in turn, large enough to provide tasty morsels to fish.

Just as fish don’t swim just for the benefit of winter anglers, zooplankton aren’t there for the benefit of fish. While fish and other organisms evolved as predators, there is no Darwinian imperative to become prey. Zooplankton evolved to avoid being eaten, and behavioral adaptations keep many species out of harm’s way.

One such adaptation is akin to “the largest daily migration on Earth,” according to Ariana Chiapella, a research associate at the University of Vermont’s Rubenstein Ecosystem Science Laboratory. During the summer, many species of zooplankton spend their days out of sight on the bottom of lakes, even though most of their food is near the shiny surface. At night, protected from the enemies of fish by the veil of darkness, these zooplanktons arise to feast. The ghost midge, one of these nocturnal migrants, earns its living by hunting smaller zooplankton at night and settles in mud during the day.

Most of the research documenting these nocturnal migrations has been done during summers. Chiapella and his collaborators from Jason Stockwell’s UVM research group wondered if this migration and other daily migrations also occur under the winter ice, when instead the ponds are the warmest on the surface. , as they are in summer, the opposite is true. So one morning in March they went to Shelburne Pond with ice augers and plankton nets.

They sampled around the clock from just below the ice to over 14 feet below the surface. To get their samples, the researchers lowered a pipe through ice holes and pumped five gallons of pond water into buckets from several depths. After first sampling small volumes of microscopic phytoplankton, they poured the remaining water through fine-mesh nets to collect the larger zooplankton.

In their first midday samples, there was no sign of ghost midges at any depth. What was common in the midday samples, however, was Daphnia mendotae, a species of water flea commonly eaten by ghost midges. Nighttime samples told a different story. Ghost midges came from the depths at dusk and were found in the deepest samples. At midnight they were abundant at all depths from 14 feet down to just below the ice surface. And their prey, water fleas? They went deeper at midnight and approached the ice after sunrise when the phantom gnats set.

I asked Stockwell why the phantom gnats didn’t just follow the water fleas and watch day and night. He reminded me that fish depend on daylight to find food, including ghost gnats, so gnats dive because, as Stockwell said, “it’s better to be hungry than to die.”

These studies confirm that the base of the lake’s food web is alive and well and migrates year-round, albeit perhaps with a low-calorie winter diet. My diet tends to follow a reverse pattern!

Declan McCabe teaches biology at Saint Michael’s College. You can hear his bi-weekly “Nature Snippets” wherever you find your podcasts. Illustration by Adelaide Murphy Tyrol. The Outside Story is attributed to and edited by Northern Woodlands magazine and sponsored by the Wellborn Ecology Fund of the New Hampshire Charitable Foundation:


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