Naturespeak A naturalist's view of the world

December 13, 2009

On Thin Ice

Filed under: Uncategorized — wykes @ 10:16 pm

The arrival of the icing season has put a whole new spin on life in the aquatic world. Creatures who were once at home in the transition zone where water meets air now have to choose between one or the other. Others have to accept the presence of an overhead  roof and the threat of oxygen depletion. All have to adapt, but then again that’s the way it has always been.

For the muskrat, the first layer of mid-December ice offered a new opportunity. For one Kent Lake (Kensington Metropark) ‘rat, the solid crust provided a base of operations for exploiting and harvesting a lush bed of aquatic plants (see above). Instead of venturing to and from an offshore plant bed, as he had all summer, this critter could now place himself directly over it. The submerged prize – probably some sort of tuber – was now obtainable via a short 30 second dive (see here) into an open lead of water. Eating could begin immediately after the muskrat crawled back out onto the ice. Such ice feeding sites are easy to spot, when the ‘rat is not there, due to the  fresh vegetation piled onto the ice.

I watched this fellow for the better part of ten minutes as he dove, rose, ate, and dove again. He stopped for a little grooming every now and then, but kept on task for the most part. Even though it was a bitter cold day with wind driven flurries, he looked quite happy with his lot – like a child who’d just discovered a bridge leading to a candy shop. As the ice thickens over the season he’ll have to maintain these open spots if he wants to continue. For now, however, life is good for a ‘rat on thin ice.

For another muskrat, the thicker ice layer on one of the Lake Erie Metropark lagoons forced him to remain submerged. He took advantage of the cold pack to do a little fishing. Fish become sluggish when trapped under the ice. Like otters, muskrats – at least our hardy Lake Erie ‘rats – frequently chase down and eat a few of these finny treats. The muskrat’s swimming abilities are normally more than a match for the fish but slower fish make for easier prey. Fortunately the ice was still thin enough to see through as one frisky ‘rat performed his piscatorial duties underneath the glassy ceiling (see below – a photo taken last year of the same situation).

After the muskrat left the scene, a very slow but very alive Painted Turtle could be seen crawling along the bottom. Among the most cold tolerant of reptiles, the sight of one of these reptiles under the ice is not all that unusual. It was a first for me, I have to admit. For this turtle, life under the ice will eventually require him to slow down. It’s not the cold or the lack of food that will grind him down- there’ll be plenty of dead fish to eat – but the lack of oxygen. Remarkably, Painted Turtles don’t need to surface as long as there is oxygen in the water. They can absorb the O2 through the skin lining their mouth and cloaca (yes, that’s the rear end) and, as long as they don’t do a whole lot, can survive on this.

Under solid ice conditions, however, the available oxygen supply eventually peters out. Turtles will then have to resort to anaerobic respiration- a process requiring that the creature literally begin digesting itself. Given the proper conditions a Painter can survive submerged in oxygen-depleted waters for over 5 months if necessary. A potentially fatal side effect of this process is the build up of lactic acid in the bloodstream. To get around this the turtle releases carbonate (calcium salts) into the blood stream in order to neutralize the acid. Most creatures don’t have enough extra bone in them to afford this costly procedure, but turtles can make use of their very substantial and very bony shells. By next spring, this wandering turtle will basically look the same but will weigh less. That’s a shell of a way to get by.

Many large bullfrog tadpoles share the Erie Marsh world of the Muskrat and Painted Turtle. As gill breathers they will depend upon the oxygen in the water to get them through the winter. Without any real bones with which to perform the “turtle trick” they resort to air gulping when O2 supplies are low. I watched a steady procession of tadpoles make their way to the “surface,” only to be halted by the thick  double layer of bubbly new ice. They groped about for a while until bumping into one of the many entrapped air bubbles. Then, probably after sucking in a bit of the gassy sweet stuff of life, they descended out of view. Theirs will be a long winter, but then again, they’re used to it.

Perhaps the most satisfying of thin ice scenarios involved several Canada Geese. These obnoxious birds are prime examples of the type of creatures which are forced to make a choice when they can no longer enjoy the benefit of swimming in the liquid water.

As witness to that moment in time when the ice was still forming on Washago Pond, I saw many geese pressed into making a choice. Many of them chose to stand on the land and avoid the water but when pressed these birds retreated to the water. They attempted to swim but found the water harder than usual and, after blundering about for a moment or two, soon broke through. Once in, they swam about within their mini-ponds (see below and here) until something in their pea-brained psyche told them move on. Their clumsy efforts and their ponderous weight prevented them from making any headway (see movie here).

The only way to final freedom was to resort to flying up and out until solid, albeit slippery, aqua-firma was reached. You’d think that they’d get the message. Mother Nature is telling them something.  Actually, they have several choices: they can die, leave, stand on the ice, or live under it (Admittedly the last choice will result in the first one). Real Canada Geese are supposed to migrate south to places where ice is not a factor. It’s their own fault if they stay here. The muskrat, tadpole, and turtle? Well, they have no choice.

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