Surface water temperatures have risen into the mid 70s on most lakes, with the water temperatures still on the rise. The peak water temperatures of the summer often happen towards the end of July or earlier in August.
There are higher spikes in surface water temperature on days with a lot of sun and calm winds. The surface temperatures only go down the first few inches of the water column, so they are not a true measure of water temperatures.
The most accurate measure of surface water temperature is done in the morning, before the sun has had a chance to warm the lakes.
The surface water mixes with the water below overnight. The amount of heat the lakes can hold overnight is a more accurate measurement, so the numbers are not skewed by the daily spikes in water temperature.
Many different things start to happen to the lakes during the heat of summer. Algae blooms increase in all lakes, although lakes infested with zebra mussels are much less likely to have heavy algae blooms.
Each zebra mussel can siphon the phytoplankton (algae) out of one liter of water every day. When there are millions of zebra mussels in a lake, that’s a lot of algae removed from the water each day.
“Swimmer’s Itch” increases in many lakes as water temperatures increase over 70 degrees. The effects on swimmers can be reduced by toweling off completely immediately after leaving the water.
Lake water farther from shore is less likely to be affected by swimmer’s itch than water right on the beaches.The little critters that cause swimmer’s itch (a parasite hosted by ducks and geese and not humans), gets blown into the windward shoreline and is usually most concentrated where waterfowl are located.
The warm water temperatures in the lakes can also have a negative effect on the day bite of light sensitive species like walleyes and crappies. The toughest time to be on the water for these species is in the middle of hot sunny days with little wind.
Algae blooms can actually help walleye fishing, because it reduces visibility and light penetration into the water. This allows walleyes to move on top of structures and feed more comfortably and for longer periods of time during the day.
Algae gives lake water a green tint that in extreme cases can become like a layer of pea soup on top of the water during extended periods of light winds.
Some algae is good, but too much is a sign of poor water quality often caused by a combination of agricultural run-off, excessive lawn fertilizers and poor septic systems.
Oxygen levels also play a part in fish location during the summer. Fish can use deep water as long as there is enough oxygen and food there to support them.
When lakes set up a thermocline, the water column in the lakes become stratified by temperature, with the coldest water on the bottom and the warmest water on the surface.
Green vegetation, springs and wave action on the surface of the lakes add oxygen to the water during the summer to the part of the water column above the thermocline.
The water below the thermocline stops mixing with the water above the thermocline. The amount of oxygen below the thermocline depends on many things, but once the oxygen levels drop below a point, the fish have to leave the deep water and move above the thermocline to survive.
Shallow lakes do not set up a thermocline during the summer because the whole water column keeps mixing all the way to the bottom during high winds and storms.
Shallow lakes and shallow bays of large lakes without a thermocline include Winnibigoshish, the shallow bays and main lake portion of Leech Lake, Big Traverse Bay of Lake of the Woods, Upper Red Lake and Mille Lacs.
This list includes some of the best walleye lakes in Minnesota and also some of the most consistent walleye action during the summer.
Paul A. Nelson runs the Bemidji Area Lakes Guide Service. Guided trips for 2019 can be booked by calling or texting 218-760-7751 or by email at firstname.lastname@example.org.