Under A Frozen Shell

Under the frozen surface of the garden pond, life winds down as plants and creatures adapt to survive winter’s harsh bite

- Nathan Hill

IT IS DEEPEST winter, and the garden pond is buried beneath bleached ground. Sparse stems of robust plants jut through the pure white cover, as though reaching for warmer heights, their remaining leaves brittle and laced with frost. Settled snow hides the pool’s periphery, smoothing the edges as shoreline and surface are blended into uniformity.

The pond dozes through a seasonally imposed, cryogenic sleep. Aside the tracks of passing cats and birds pressed into the crisp covering, there is empty desolation. Yet still, life remains; it has just sunken. Below the surface, biological clocks tick and skeleton workforces continue to labour.

The upper levels are abandoned. As ponds cool, the life they contain relocates progressively deeper, an exodus from the surface ice to follow. Governing this are changes to the physical properties of water, brought about by the low temperatures. For aquatic life, these changes are essential for survival, as without them, the pond would freeze solid.

The main parameter altering the pond’s very dynamic is water density. Liquid water is most dense at 4°C. At temperatures below that, it thins again, until it solidifies as ice. This results in thermal stratification, where dense water sinks, and cooler water forms layers on top. This is partly why ice starts at the surface, instead of throughout.

Vital protection

For aquatic residents, the denser, warmer layer represents a safe oasis at the very limit of their temperature tolerance. This safety can be easily compromised. Depth is critical: if the pond is too shallow, then stratification will not occur. Small bodies of water, less than 35in (90cm) deep, can be hostile to even hardy life. Fish may need to be rehoused indoors while the worst of the season passes.