In the zone

Scientists like to name and classify things, so it should come as no surprise that they have names for different zones in our lakes. Mainly they talk about four of them.

The one we think most about, that we interact most with, is the littoral zone. This is the area from the shoreline out to the point where the water is deep (or murky) enough so that there is too little light at the bottom to support rooted plants.

The Minnesota DNR pegs this zone as from shore to a depth of about 15 feet, but that depth can vary a lot with water color and clarity. The width of the littoral zone also varies. Where the bottom slopes down steeply, it may be quite narrow. If the bottom slopes gradually, the littoral zone may extend far into the water. In fact, a shallow lake may be all littoral zone.

Life can be incredibly diverse in this zone. Most fish spend most of their time there (and as a consequence so do anglers). It’s a rich environment, with relatively warm water, plenty of light, and nutrient-rich bottom sediments.

All manner of plants grow here, from emergent species like bulrushes, cattails and arrowhead, to floating-leaf plants like water lily, spatterdock and watershield, to submerged vegetation like pondweeds, wild celery and milfoils. Algae are also abundant, some species clinging to the larger plants. The plants provide cover for young fish, which in turn attract larger predators. Frogs, muskrats, turtles, insects and other creatures populate this zone.

Out beyond the littoral zone lies the limnetic zone. This is the open-water world. It begins where the littoral zone ends; its depth again depends on how deep the light can penetrate. Fish move in and out of this zone, but for the most part its inhabitants are plankton – one-celled algae of various kinds (phytoplankton) and tiny creatures (zooplankton) that eat by filtering algae out of the water. Plankton are critical to a lake’s food web, and the phytoplankton are responsible for most of the photosynthesis (thus oxygen production) that occurs in the lake.

Below the limnetic zone lies the profundal zone. This zone may not exist in shallower lakes. It’s the deep water where light penetration is greatly limited. In essence, this is where dead matter from above goes to decay. It is relatively cold, dark and oxygen-poor. The primary life in this zone consists of heterotrophs – small creatures that eat dead material.

Finally, there is the benthic zone, essentially the lake bottom sediments. Residents include bacteria and fungi that break down organic matter, releasing and recycling nutrients. Life just under the sediment surface can be quite diverse. Most benthic zone organisms are invertebrates. The eggs and larval stages of insects like mayflies and midges can be found here, along with worms and small crustaceans.

Sometimes as you look out over your lake, try to picture these zones. You’ll understand a little bit better how your lake ecosystem functions.