by Steven Awve – Most people work a 9-to-5 job. But for some of the smallest creatures in our lakes, the workday begins at sundown. Their shift is still 9-to-5, but the AM’s and PM’s are switched.
Each night, under cover of darkness, zooplankton—tiny aquatic animals—rise from deep waters to the surface to feed. By morning, they retreat to the depths to avoid being eaten themselves. This phenomenon, called diel vertical migration, happens in nearly every lake on Earth. In the ocean, it comprises one of the largest daily movements of biomass on the planet.
It is also no small feat. In a post on this blog from 2022 some quick math concluded that these daily migrations are the equivalent of running multiple marathons just to get dinner.
But how do we know this migration is happening when zooplankton are nearly invisible to the naked eye?
I got the chance (or depending on how much you like to sleep, maybe the misfortune) to help sample three lakes for zooplankton. To capture this daily migration, we had to sample at night and again during the day. At 9 PM, our team left Trout Lake Station and headed to the UNDERC property (University of Notre Dame Environmental Research Center), home to three famously studied lakes that are part of the long running Cascade Research Program.
Working under dim red headlamps to avoid disturbing the plankton behavior, we paddled to the center of each lake. One team traveled out in a two seater row boat and took a snapshot of the lake’s layers by collecting water quality data every ten centimeters throughout the water column using a sensor that measured temperature, dissolved oxygen, and chlorophyll. Meanwhile, my team collected zooplankton from different depths using a device called a Schindler trap—a cube-shaped sampler with ends that seal off once you’ve lowered it to your desired depth. The Shindler trap captures only water (and zooplankton) from that specific layer of the lake.
We filtered out the water, and placed the remaining contents into jars. These jars were preserved, labeled, and stored to be rigorously counted and measured under the microscope. All this information combined will allow us to better understand how the layers of algae and predators interact to drive zooplankton movements from day to night.
But we weren’t the only ones on the lake. As we wrapped up the last few samples a loud crash snapped us to attention—a beaver slapping its tail on the water. We couldn’t see it, but we could hear it moving closer, the sound of water slaps echoing across the still lake. An eerie—but awesome—reminder that we aren’t alone on the lake after dark.
By the time we finished sampling all three lakes and preserved our jars with ethanol, it was nearly 3 AM. Then came the 9 AM wake-up call to go back out and repeat the process, collecting the same data during the day.
Why does this matter?
Recently published research shows that if we only sample during the day, we may be drastically underestimating how many zooplankton are actually in the lake. That’s because many species spend daylight hours deep in the water column to avoid predators, only rising to the surface under the cover of darkness. In fact, nighttime surface samples can contain 5 to 500 times more zooplankton than daytime surface samples. If we don’t look at night, we miss a huge part of the story.
Plankton are often considered the base of the aquatic food web. This nuanced understanding of plankton behavior is essential to correctly understanding and modeling other aspects of lake ecology like predator-prey dynamics, lake food webs, and how energy flows through aquatic ecosystems. And every sleepless night spent studying them brings us one step closer to a clearer picture of the lakes we love.
Featured photo: acrylic painting by Christina Weatherford