Spring turnover

Spring turnover? No, it’s not a seasonal recipe. It’s the term ecologists use to describe what’s happening in our lakes and ponds this time of year.

You’ve probably noticed the ice has receded on most of our water bodies as the temperatures have gradually warmed up. We’ve also had more than our expected share of March’s windy days, it seems.

These two factors — melting ice and wind — combine to change the aquatic environment in spring. During the winter, the ice layer prevents much sunlight from penetrating lake surfaces.

Sealed in by the ice, the water beneath became still, and more or less uniform in temperature. With less light, aquatic plants reduced or ceased their photosynthesis, and little oxygen was produced. Most animals cut back on activity; some hibernated in the mud at the bottom of the lake.

Now, the sunlight penetrating the surface is beginning to warm the water. The temperature gradient in the water (the differences from top to bottom) is starting to even out, which causes the water to circulate as it changes. Aided by the wind, the water in the lake begins to stir itself. This is spring turnover.

What else is happening? Aquatic plants — from tiny, one-celled organisms such as algae, to pondweed and duckweed — are starting to grow again with the greater light and to resume photosynthesis, producing more dissolved oxygen. The circulation of the water is distributing oxygen and nutrients throughout the lake. Animals from turtles to tadpoles to insect larvae to fish of all kinds are coming to life and feeding on the plants and each other.

This is also the critical time of year for vernal pools, which are essential nurseries for salamanders, frogs and toads. These are small, self-contained pools of water found in woodlands and clearings that are filled by winter snowmelt, spring rains, or by groundwater seeps. They generally last for only a brief period of time in spring and early summer.

Because vernal pools are temporary, fish do not use them. Several kinds of frogs and salamanders do use them, however, for breeding and their larval stages, precisely because they are free from predatory fish. Marbled salamanders, which hatched from eggs in the late fall, are developing in vernal pools, while spotted salamanders are just now laying their eggs in them.

My thanks to Audubon naturalist and educator Ted Gilman for his advice in preparing this column.

Fred Baumgarten is a naturalist and writer. He can be reached at fredb58@sbcglobal.net. His blog is at thatbirdblog.blogspot.com.