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PAUL NELSON FISHING: Summer is finally here on area lakes

Summer finally reached the Bemidji area this past week, with some of the hottest temperatures of the season.

Summers are short in the Bemidji area, with July usually the hottest month and August the second-hottest month in an average year.

Surface-water temperatures in the local lakes have passed the 70 degree mark, with most now in the low 70s. Summer patterns start to take over the lakes once water temperatures exceed 70.

The major insect hatches are coming to an end, with the largest species of mayflies hatching this past week. Minor insect hatches continue into the fall, with dragonflies, midges and other aquatic insects hatching periodically during the entire open water season.

Algae blooms begin to increase once water temperatures exceed 70 degrees. Lakes without zebra mussels will begin to develop a green tint to the water, while lakes infested with zebra mussels will remain much clearer most of the summer.

Each zebra mussel filters through about one liter of lake water per day, sifting most of phytoplankton out of the water.

Phytoplankton (algae) is the base of the food pyramid in the lakes and zooplankton (microscopic critters) are the next step up the food chain, eating the phytoplankton. Zooplankton is then eaten by minnows, panfish and other smaller fish, which in turn get eaten by larger fish all the way up the food chain.

If the lake have extra fertility and a surplus of phytoplankton, the zebra mussels may actually help boost the fish populations by cleaning up much of the extra algae (see Lake Erie).

If the lake is already clear and doesn't have much excess fertility and a small surplus of phytoplankton, the effects on the lake and anglers are going to be more negative (see Cass Lake).

There are other changes that happen in the lakes once water temperatures exceed the 70 degree mark. One of the things that directly affects people is "swimmer's itch," which starts to show up in some lakes as the water temperatures increase.

The facts about swimmer's itch are not pleasant. The little parasites accumulate in the shallows where people swim during the heat of summer (there are fewer parasites away from shore in the middle of the lake).

The typical host of the parasites that cause swimmer's itch are ducks and geese, but that doesn't stop them from trying to infect humans.

When people go swimming and come out of the water, the little parasites are in the water that clings to the swimmer's skin. As the skin dries, the microscopic parasites dive down a hair follicle and get under the swimmer's skin.

If swimmers immediately dry off with a towel or rinse off with fresh water before their skin has the chance to dry, most of the parasites will be wiped off or washed away.

People are not suitable hosts for the swimmer's itch parasites, so they die under swimmer's skin and the infected sites itch until the the dead parasites are absorbed.

Another thing that starts to happen in deep lakes once the surface water temperatures exceed 70 degrees is the water begins to stratify by temperature and set up a thermocline (the narrow band of water between the cold and warm water).

Once a thermocline sets up in a lake, the water below the thermocline and the water above the thermocline stop mixing together.

There is a finite amount of oxygen in the water below the thermocline, with springs the only thing that can add oxygen.

The water above the thermocline continuously adds oxygen from vegetation, rain water, inlets and the waves act like an aerator in the water.

Any fish respiration or decomposition of plant and animal matter that occurs below the thermocline depletes the limited supply of oxygen.

Some lakes with large amounts of deep water have enough oxygen below the thermocline to support fish all year long.

These lakes are often called "two-story lakes" because they are capable of supporting warm water species like walleyes, pike and bass above the thermocline and cold water species like trout below the thermocline.

Paul Nelson
Paul Nelson writes a weekly fishing column for the Bemidji Pioneer. He runs the Bemidji Area Lakes Guide Service.
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