Dealing with the cold

By all standards this winter has been downright balmy. In fact, I wouldn't be surprised if the last few nights were among the coldest we've had all year - and we're only a week away from spring.
Oh, and they're calling for a foot of snow tomorrow.

Inspired by the cold, and the fact that to me hibernation sounds like a perfectly reasonable way to spend the winter, I wanted to look at how the critters that live in our ponds survive the cold.

Koi and Goldfish

Koi and goldfish under a layer of ice.
Photo credit: Sven Jakubowski

For many of us water gardeners koi and goldfish are an important addition to our ponds. They add to both the aesthetic and entertainment value, and since they can life for many years, we can get quite attached to them. This is why it's good that these fish are able to survive even the colder winters.
Key to their survival are the unique properties of water.
Most substances get gradually denser (and thus heavier) as they cool down and turn from liquid to solid, but water is an exception to this rule. It reaches it's highest density at 40℉. If it cools down further it will expand again. This is why ice floats and why even in the middle of winter there can be a pocket of 40℉ water in the bottom of the pond, assuming the pond is deep enough.
Fish can exist comfortable in that bottom layer for several months, since they're coldblooded and their metabolism slows down to the point where they require little to no food.

Fish death can still occur during the winter if the surface of the water remains covered in ice for too long. If there is no exchange of gases (oxygen, CO²) between the water and the air the fish will eventually suffocate.
This is where pond de-icers come in; they create an opening in the ice so the oxygen level in the water can be maintained.

Frogs

American Bullfrog.   Photo credit: CC BY-SA 2.5, 

https://commons.wikimedia.org/w/index.php?curid=1079191

Aquatic frogs, such as the leopard frog and bull frog, also wait out the winter in that pocket of warmer water in the bottom of the pond.
They don't bury completely in the mud, because they need to be able to extract oxygen from the water. Instead they simple rest on the bottom.
Like fish they can suffocate if the oxygen level in the water drops.
Toads and terrestrial frogs hibernate on land, either burrowed into the soil or in places such as leaf litter piles. If it gets cold enough, these frogs may actually freeze, This won't kill them, however, because their vital organs are protected by a natural anti-freeze.

By the way, frogs aren't just for show, they play an important role in your pond. They are a sign that your pond is healthy as they are sensitive to water quality issues. Also, as tadpoles they help keep your pond clean by eating algae and scavenging.

Dragonflies

Dragonflies are a welcome addition to any pond as they hunt mosquitoes and other flying insects. These areal predators prefer to survey their domain from a high vantage point, so adding tall plants such a lotus and cattail can help attract them to your pond.
They lay their eggs in the water and the larval dragonflies, called nymphs, are fully aquatic and prey on mosquito larvae and even small fish and tadpoles.

Depending on the species, dragonflies can live through several winters, but they do so underwater as eggs or nymphs. Adult dragonflies cannot survive the winter, In fact, most adult dragonflies only live for a few weeks in which they mate and lay eggs.


Garter snakes

Garter snake basking on a warm winter day,
Photo credit: Suzanne Boom

I realize garter snakes aren't thought of as one of the inhabitants of your average garden pond, but they are a common visitor of the display ponds here at Lilypons .

Garter snakes survive the winter by brumating. Brumation is the reptile equivalent of hibernation. It's a slightly different mechanism that accomplishes the same thing - reducing the body's energy requirements to a bare minimum in order to survive the winter. One of the main differences is that brumating reptiles don't actually sleep and they don't live off fat reserves. Their metabolism, heart rate and breathing slow down due to the colder temperatures.
In garter snakes brumation serves a secondary purpose - it triggers the start of the mating season.
To ensure that mates will be available as soon as the weather warms up, garter snakes overwinter in large groups of a few dozen to several hundred individuals.
Their dens can be found in tree stumps, logs or rock piles, and even under man-made structures.
Once the mating season is over, the snakes remain mostly solitary until the late fall.

And when I say as soon as the weather warms up, I do mean as soon as. the photo above was taken on February 24th, when it was 75℉ here in Maryland. Garter snakes are the first snakes to become active in the spring and the last to start brumation in the fall.

I'm going to leave it at that for today. For those in the path of nor'easter Stella; stay safe and warm!

References:
http://extension.illinois.edu/gardenerscorner/issue_06/winter_04_10.cfm
https://www.scientificamerican.com/article/how-do-frogs-survive-wint/
https://en.wikipedia.org/wiki/Insect_winter_ecology
http://www.psu.edu/dept/nkbiology/naturetrail/speciespages/gartersnake.htm
http://dnr2.maryland.gov/wildlife/Pages/habitat/wadragonflies.aspx