Winter has descended on New England and most vernal pool enthusiasts can only dream of next year’s Big Night. But I want to dispel the idea that vernal pools cease all their activity in the cold. Or even when it is dry. Or too hot. Vernal pools are active at some level all year.
One of three things is happening in pools now depending on the water. If the pools are dry then the resting eggs of invertebrate inhabitants are safe and secure. They will be stimulated by some set of cues in the spring. The order of hatching is not at all random and careful observation will reveal that the sequence is the same every year. The resting eggs are in suspended animation and can survive for years or even decades until the appropriate time.
The other thing that happens is that the ponds remain cold and wet. I think this is one of the best times. Again, with careful observation you can see that the zooplankton community is thriving. In earlier posts I wrote about Daphnia ephemeralis, a species that is a cold stenotherm. That means they survive only over a narrow range of cold temperatures. Freezing marks one extreme and the other is about 50°F. Much warmer and the animals are stressed by the heat! The species is reported from New England but I haven’t been out myself to collect it but anyone with the curiosity might find them happily swimming around even now.
Cold and wet ponds are also interesting because they have a seasonal succession of species. There are copepods and cladocerans in the cold but they are not the same ones that you will find once it gets warmer. These cold-water forms are often larger than their warm water relatives.
The other option is that the pond could freeze. Even then all activity has not stopped. There are any number of invertebrates that live in the unfrozen muck. I know this because I have been out in the late winter-early spring and collected large Daphnia covered in algae and muck swimming about in a pond that I know was frozen but a few days before.
So I write this as encouragement to all you vernal pool enthusiasts. We may not have the seed catalogs to comfort us as gardeners do, but our little habitats are safe, secure and even active for the winter.
