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Vernal Pools

Amanda Shearin leads a public program about vernal pools (Curt Chipman photo)

By Ed Robinson

The snows of winter, and the rains of mud season, are vital parts of the process for recharging unique bodies of water known as vernal pools (from the Latin word “vernus” meaning “of Spring.”).

A vernal pool is a small, seasonal pothole or wetland that forms in a shallow depression over bedrock or clay-containing soil. A key distinction from more traditional ponds lies in their variable water depth. Vernal pools lose a considerable amount of water to evaporation in drier parts of the year and often dry up completely by June or July. The cyclical fluctuation of water levels has a significant impact on the distinctive plants and animals that inhabit these pools.

Wood frog eggs (left) and Yellow spotted salamander eggs (right) (Curt Chipman photo)

A vernal pool, because of its periodic drying, does not support breeding populations of fish. This allows other species to flourish, where they have adapted to the conditions of the pool. It is common to find large egg masses from yellow spotted salamanders and wood frogs. Juveniles from these eggs are in a race for life – they are born with gills but must mature and develop their lungs before the pools dry up. Tiny crustaceans such as Daphnia and fairy shrimp live their entire life cycle within a few weeks, leaving behind egg cases that lay dormant through dry and freezing periods before hatching in next year’s waters. A pool may also have specialized plant species that grow annually, thanks to seeds left behind that can handle extended dry spells.

Walking through a woodland, it can be easy to overlook vernal pools since they are often visually underwhelming. But time your visit in March through May, and you will be amazed at the life emanating from these special habitats. In low light conditions, spring peepers provide a chorus of breeding songs. Birds and mammals use the pools as a source of food and water. If you are lucky enough to visit when spotted or blue-spotted salamanders are visible during their one to two day breeding season, you will be amazed to see dozens of these creatures together, as they spend the rest of their year well hidden in the ground or under rocks and decaying logs.

The vernal pool at the Hackett and Minot Trails (Curt Chipman photo)

Yellow backed salamander (Ed Robinson photo)

Vernal pools have long suffered from man’s ignorance and neglect, and many were destroyed in the rush of land development. Fortunately in recent years, numerous groups have taken action to educate the public about these tiny ecosystems. Not only do vernal pools offer essential habitat for many species, they also play a role in erosion control and water quality. In 2007, Maine launched a statewide effort at the community level to survey and map significant vernal pools to encourage better development planning and improved conservation efforts.

In Harpswell you can find vernal pools at the Hackett and Minot Trails, Long Reach Preserve, at the headquarters of the Harpswell Heritage Land Trust on Route 123 (in this case a roadside ditch is acting as a man-made vernal pool) and many other locations.

September 2017