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Winter 2019
phytoplankton (microscopic plants) or rooted aquatic plants. The warm epilimnion supports nearly all of a lake’s phytoplankton and/or aquatic plants, meaning that it is in this zone where most of the oxygen is produced.
Below the thermocline, light becomes limiting and aquatic plants cannot continue to grow, so oxygen is not produced. Further, organic materials fall to the hypolimnion and decompose using more oxygen further reducing dissolved oxygen levels.
Because the cooler hypolimnion does not support plant growth and is isolated from the warm surface layers, it can become quite low or even devoid of oxygen by mid summer – and can become unsuitable for fish.
In lakes with very low nutrient levels, the water is clearer and light can penetrate much deeper, causing adequate oxygen to occur at much deeper depths. Very clear lakes can contain well-oxygenated water at depths of 40-feet or more during summer.
Thus, the depth where the thermocline exists will differ widely among lakes depending on their productivity, even for lakes that are only a few miles apart but differ widely in nutrient content, topography and hydrology.
Because water quality conditions and habitat are typically suitable throughout the epi- and
metalimnion, not all bass will be found around the thermocline, because bass will go where the habitat and food provides the greatest ecological advantage. However, many prey species and predators will aggregate around the thermocline particularly during the daytime.
This zone represents the coolest water in the lake that contains adequate dissolved oxygen, but it also harbors more zooplankton (micro-crustaceans) during daylight hours and thus provides high food abundance for prey fish.
Presumably, water bodies with less available structure/cover available to hold bass in areas, would see more transient fish, likely to follow bait and concentrate in the thermocline, particularly during daylight hours (e.g., spotted bass).
By late summer – much of the nutrients in the epilimnion have been greatly diminished, intensity of algae production decreases and water becomes clearer (depending upon of course the lake). We do not get the summer rains and inputs that say, mid-west or southern reservoirs would get.
As the season begins to wane, sun intensity and duration decrease, air temperatures begin to cool, and the lake begins to lose heat to the environment.
Temperature gradient and density differences between layers begins to lessen until a point is
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