Mill
Creek under Cover
It's been a hard winter. The Aberfoyle Reach froze
early this year, a bit before Christmas and even with January's mini-thaw has
remained ice covered. The frozen creek has become a corridor to get out and
explore the winter landscape.
Even in the deepest winter, some of the stream is
open, particularly in fast flowing shallower reaches and in areas of significant groundwater input. Groundwater,
at a constant 8 oC throughout
the year, is significantly warmer than
the creek. While open reaches help to maintain
high dissolved oxygen concentrations throughout the stream, they also have
their drawbacks. Air temperatures below -6 oC can lead to the
formation of frazil ice, a Slushee-like mixture of ice and supercooled water that behaves a
lot like lava or flowing cement. Under the wrong conditions, frazil ice can
build up into anchor ice, blocking channels and scouring the stream bottom.
Beneath the ice, life continues. Under the gravel,
brook and brown trout eggs, deposited in redds dug last autumn, are slowly
developing into fry which will emerge with spring's freshets. Groundwater upwelling through the gravel
helps protect the eggs from freezing and
provides a refuge for adult and juvenile fish, when deep pools in main channels
may become unsuitable because of the development of frazil ice.
As both the fish and the aquatic invertebrates
(collectively referred to as bugs) are coldblooded, their metabolic rate
decreases with the cold temperature of the stream and the pace of life slows
down overwinter. Slows down, but doesn't stop. In the sediment and water
column, terrestrial organic material, such as leaf fall, is processed and used for growth by the
assorted invertebrate guilds of shredders, scrapers, grazers, collectors,
predators, omnivores, and scavengers.
These bugs then provide food for the stream's fishes.
Although their metabolic rate is reduced by the
colder temperature, adult and juvenile fish still need to eat: just ask any ice
fisherman. While adult brook and brown trout spend the winter recovering from
energy spent in their fall reproduction, spring spawning fish, such as suckers
and pike, are busy packing energy into their eggs and sperm, which they will
release in April and May. For juvenile fish, winter diet is important to
maintaining energy reserves so they can make it through to the next spring.
Mill Creek is also home to a range of mammals
Further downstream, beavers are quietly spending the winter in their warm cosy
lodges, venturing out now and then to collect the branches they gathered last
fall and stored in underwater food banks. They may share their pond and even
their lodge with muskrats which feed on under ice vegetation. Meanwhile, mink
will forage for fish, mussels, crayfish and even frogs and turtles buried in
the mud. Walking along the creek, we see the tracks of coyote, racoon and even
wild turkey, and there are deer and rabbit signs in the vegetation to either
side of the stream. Yet winter can also be harsh: a frozen opossum lies under
one of the cedars, the same stand where we found a deer skeleton a couple
springs ago.
But now the
days grow longer. All too soon, Mill Creek's winter tale will be past and a
busy spring upon us.
For more information on Mill Creek and its Friends visit http://www.friendsofmillcreek.org/