When covid shut down pools, a lot of swimmers took to lakes, rivers, and the ocean. For sure, there was a robust open water swimming community pre-covid, including folks who complete all sorts of very long and very cold swims. For the rest of us, we had to figure out our tolerance for cold water (and our tolerance for spending money on more gear to better endure those cold temperatures). During the 1.5 years that the pool where I typically swim was unavailable to me, I had the pleasure of swimming in many ponds, most commonly Walden Pond in Concord, MA. I was quickly sold on swimming outside at dawn, spotting bald eagles and great blue heron, hearing loons, and enjoying fall foliage. I’m told there are a lot of fish to see as well, but I have a funny habit of closing my eyes underwater. After two open water swimming seasons, I have determined that water colder than 55 F is pretty much too cold for me. I swam twice in 53 F water, and neither time went very well despite a gear upgrade for the second try. (My gear-buying tolerance did not match that of my friends with real, non-PhD student jobs, but the pull of swimming is strong.) As the open water swimming season approaches once again, the calibration of our bodies to water temperature has me thinking about phenology. The University of Wisconsin Extension defines phenology as, “a branch of science that studies the relationships between periodic biological events—usually the life cycles of plants and animals—and environmental changes.” The date when Walden Pond is warm enough for me to swim is not particularly important, but the date when specific plant species leaf out or flower has ecosystem consequences. Animal migration and pollinator emergence are also important events, and changes in phenology does not affect all species in the same way. Changes in phenology are one response to climate change. Walden Pond was three degrees cooler on the second Sunday in May in 2022 than it was in 2021, but the National Phenology Network reports the spring leaf out to be days to weeks earlier in the Northeast this year than the past 30-year average. They also reported in 2018 that across the U.S. spring was generally starting earlier than 20th century averages. Changes in phenology can disrupt ecosystem processes, such as pollination, animal migration, and when light reaches a forest understory. Phenology is also important for more human-centered events such as when to plant, fertilize, and harvest; optimal timing for managing invasive species; and allergy and mosquito season. Long-term phenological datasets, such as those kept by Aldo Leopold and Henry David Thoreau, provide the data needed to examine links between changes in phenology and climate change. Starting with Thoreau, years of records in Concord, MA, have allowed researchers to document many climate change-induced changes, including the loss of about a quarter of the town’s wildflower species. I appreciate the opportunity to swim in a beautiful place that has contributed to our understanding of natural history and climate change. Luckily, this spring, I can swim in an indoor pool as I wait for Walden Pond to warm to a tolerable temperature. (All Walden Pond images by Ian Waitz.)
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