The air down there

I once told friends how a fishing pal and I spent an August morning catching bluegills in 60 feet of water on a lake in Waukesha County, Wis. The fact is we were floating in a boat above water 60 feet deep, but there was no way the bluegills were down there on the bottom. What happened was that bluegills suspended about 15 feet down were catching our baits as they fell. We only thought we were pulling them out of the depths.

How do I now know this is true? Because there was no way those bluegills could have lived that deep under the surface. And why? Because there was no air down there – or more properly, not enough oxygen.

In a previous post I wrote about how lakes thermally stratify in summer – a layer of lighter, warm water on top, and a layer of denser, cold water below. The difference in density is enough so that the two layers don’t mix very much. And as a result the lower, cold layer becomes very deficient in oxygen, more and more so as summer wears on. That upper layer (called the epilimnion) gets recharged with oxygen daily and the wind kicks up waves, or just through normal diffusion. The lower layer (the hypolimnion), of course has no access to the surface, so no way of getting recharges, and meanwhile its oxygen gets steadily used up.

Algae, dead plant matter, dead fish, and all manner of organic material sink to the bottom throughout the summer. There, it is broken down by aerobic bacteria – the kind that need oxygen to live. Gradually, the amount of oxygen dissolved in the water decreases, until there is too little to support fish or much of any kind of life. The depletion may become so extreme that the job of breaking down dead matter falls to anaerobic bacteria, which exist without oxygen. So that zone toward the bottom of the lake becomes almost a dead zone – a hostile place.

This is one reason why summer anglers are cautioned not to “fish too deep.” If you drop a bait beyond the upper, oxygen-charged layer, into the lower, oxygen-poor zone, you might as well be fishing on the moon. There’s a thin zone between the two layers (sort of like the thin layer of peanut butter in a sandwich) where the temperature changes rapidly, from warm to cool. This is called the thermocline, and fish will stay at or above it – they have no choice. The bluegills my pal and I were catching that August day in all likelihood were sitting at the thermocline.

Of course, lakes do not remain stratified. At this time of year, as the days and nights cool, so does that surface water. The epilimnion gets steadily cooler and thinner, until finally the whole lake temperature equalizes, at which point the wind can stir things up, adding life-giving oxygen throughout. This is called the fall turnover. You can actually see this happen on your lake as the water all of a sudden becomes cloudy, as the dirtier water from below mixes with the cleaner water from above. You might even notice a hint of sulfurous smell – the result of hydrogen sulfide (“marsh gas”) produced by those anaerobic bacteria in the depths as they fed on dead material through the late summer.

The turnover on our lakes here in northern Wisconsin is happening about now. It’s a rite of autumn, as inevitable as the oaks and maples turning color.