Big Indiana Bass

So it's been hot as hell - well, almost - for well over a week now. Water temperatures are starting to sizzle, with surface temps at or above 90 on many of our local reservoirs. The bite seems to have really slowed down in many places, and I'm seeing and hearing of guys talking about fishing deep, but that might not be the right thing to be doing. Let's talk about lake biology for minute here in Part I.

Lakes are classified by their level of primary productivity (amount of carbon fixed into organic compounds), typically corresponding to the amount of algae present in the water.  Very productive lakes are called eutrophic and usually have warm, turbid water. That happens to be what a good majority of Indiana's are classified as. Such waters tend to have high algal levels. While some algae is essential to the aquatic food web, too much is aesthetically displeasing to lake users and can disrupt the ecological balance.

Temperature and oxygen

In the summer, lakes typically develop a thermal stratification.  In eutrophic lakes, the upper portion (epiliminion) is warmer and more well-oxygenated, while the lower portion (hypolimnion) is cooler and has less oxygen available.  The zone of transition is called the thermocline, and is defined by a temperature decrease of at least 1 degree Celsius for each meter of depth.  In eutrophic lakes, the oxygen and temperature profiles are expected to correspond to one another.

The amount of oxygen dissolved in water can be expressed either as a relative percentage or as milligrams of oxygen per liter of water (mg/L).  Saturation values of greater than 100% are possible in some cases, such as from high photosynthetic rates or mechanical agitation of the water.  The solubility of oxygen decreases as the water temperature rises, and at a water temperature of 25 Celsius, 8.4 mg/L is considered to be 100% saturation.  Super-saturation as it is frequently referred to commonly indicates high levels of algal photosynthesis.  For fish, increasing water temperatures result in increased metabolism and need for oxygen.  In cases where dissolved oxygen is low in the deeper water, fish are limited by to the warmer, shallower depths.

Light

Light decreases rapidly with depth in stained waters. The 1% level is that which photosynthesis equals respiration.  Deeper than this, the amount of light is insufficient to stimulate adequate photosynthesis to produce enough oxygen to replace that which is lost by respiration of organisms, resulting in a net loss of oxygen. In many cases you can have an algal community that is self-shading.  Excessive amounts of algae at shallow depths block light to such a degree that photosynthesis and therefore oxygen dwindle rapidly with increasing depth.

Water chemistry and plankton

Plankton in a lake forms the foundation of the food pyramid.  Usually the algal community is dominated by green algae, with diatoms also being abundant.  Both green algae and diatoms are food sources for zooplankton.  Blue-green algae ("toxic" algae) are undesirable because they are not as palatable for zooplankton and can also form nuisance blooms.  Zooplankton is the major food source for young fish.

Coming next in Part II - How to use this information to your bass catching advantage.