Earlier this winter I went onto the pond ice – not to skate, but to look beneath the surface. Although marine ecologists once thought of the plankton in frozen lakes as dormant during winter, recent studies show that the plant-like, microscopic phytoplankton (which move with the lake’s currents) and animal-like zooplankton remain active beneath the icy surface.
In data collected from more than 100 lakes, Professor Stephanie Hampton of Washington State University found that while the base of the food web beneath the ice is reduced compared to summer, it is certainly not dormant. Hampton’s research suggests that the phytoplankton that support lake food webs is reduced by 80% compared to summer, while one level up the food web the amount of zooplankton in winter lakes decreases by 75%.
Still, there is enough activity in this web to support a variety of winter lake creatures, something the optimistic New England ice fishing community has long understood. But the fish are certainly not waiting in vain for a sudden burst of light through the ice and man-made meals like manna from heaven. They rely on a natural food web supported by phytoplankton, which uses the sun’s energy to make food. This phytoplankton is consumed by zooplankton, which in turn are large enough to provide tasty morsels for fish.
Just as the fish don’t just swim around for the sake of the winter anglers, the zooplankton aren’t in it for the sake of the fish. While fish and other organisms evolved as predators, there is no Darwinian imperative to become prey. Zooplankton evolved not to be eaten, and behavioral adaptations keep many types of danger away.
One of those adaptations, according to Ariana Chiapella, a research associate at the University of Vermont’s Rubenstein Ecosystem Science Laboratory, is comparable to “Earth’s largest daily migration.” During the summer, many zooplankton species spend their days out of sight on the bottom of the lake, although most of their food is near the bright surface. At night, protected from fishy nemesis by the veil of darkness, these zooplankton emerge to feast. The phantom mosquito, one of these nocturnal migrants, makes a living by hunting smaller zooplankton at night and settling in the mud during the day.
Most of the research documenting these nocturnal migrations was conducted in the summer. Chiapella and collaborators in Jason Stockwell’s UVM research group wondered whether these and other daily migrations also occur under winter ice when the opposite is true, rather than surface ponds being warmest as they are in summer. And so, one March morning, they set out for Shelburne Pond with ice drills and plankton nets.
They sampled 24/7 from just below the ice to more than 14 feet below the surface. To obtain their samples, the researchers lowered a pipe through ice holes and pumped five gallons of pond water into buckets from each of several depths. After initially sampling small amounts for microscopic phytoplankton, they poured the remaining water through fine-meshed nets to collect the larger zooplankton.
In their first midday rehearsals, there was no sign of phantom mosquitoes at any depth. However, what was common in the midday samples was Daphnia mendotae, a water flea species commonly eaten by phantom mosquitoes. Nightly rehearsals told a different story. The phantom gnats emerged from the depths at dusk and were found in the deepest samples. By midnight they were plentiful at all depths from 14 feet deep to just below the ice surface. And their prey, the water fleas? They went deeper at midnight and approached the ice after sunrise when the phantom gnats had descended.
I asked Stockwell why the phantom gnats don’t just go after the water fleas and stay up day and night. He reminded me that fish rely on daylight to find food, including phantom mosquitoes, so the mosquitoes dive because, as Stockwell said, “it’s better to be hungry than dead”.
These studies confirm that the base of the lake’s food web is alive, healthy, and migratory year-round, but perhaps on a calorie-restricted winter diet. My diet tends to follow an opposite pattern!
Declan McCabe teaches biology at Saint Michael’s College and produces bi-weekly Nature Snippets podcasts. The Outside Story is commissioned and published by Northern Woodlands magazine and sponsored by the Wellborn Ecology Fund of the New Hampshire Charitable Foundation.