activity peaks during the spring and summer when photosynthetic activity
is driven by high solar radiation.
Furthermore, during the summer most lakes in temperate
climates are stratified.
The combination of thermal
stratification and biological activity causes characteristic patterns
in water chemistry. Figure 9 shows the typical seasonal changes in dissolved
oxygen (DO) and temperature. The top scale in each graph is oxygen
levels in mg O2/L. The bottom scale is temperature in °C.
In the spring and fall, both oligotrophic
lakes tend to have uniform, well-mixed conditions throughout the water
column. During summer stratification,
the conditions in each layer diverge.
concentration in the epilimnion
remains high throughout the summer because of photosynthesis
from the atmosphere. However, conditions in the hypolimnion
vary with trophic
status. In eutrophic (more productive) lakes, hypolimnetic DO declines
during the summer because it is cut-off from all sources of oxygen,
while organisms continue to respire and consume oxygen. The bottom layer
of the lake and even the entire hypolimnion may eventually become anoxic,
that is, totally devoid of oxygen.
In oligotrophic lakes, low algal biomass
allows deeper light penetration and less decomposition.
Algae are able to grow relatively deeper in the water
column and less oxygen is consumed by decomposition. The DO concentrations
may therefore increase with depth below the thermocline
where colder water is "carrying" higher DO leftover from spring
mixing (recall that oxygen is more soluble in colder water). In extremely
deep, unproductive lakes such as Crater Lake, OR, Lake Tahoe, CA/NV,
and Lake Superior, DO may persist at high concentrations, near 100%
saturation, throughout the water column all year. These differences
between eutrophic and oligotrophic lakes tend to disappear with fall
In the winter,
oligotrophic lakes generally have uniform conditions. Ice-covered eutrophic
lakes, however, may develop a winter stratification of dissolved oxygen. If
there is little or no snow cover to block sunlight, phytoplankton
and some macrophytes
may continue to photosynthesize, resulting in a small increase in DO just
below the ice. But as microorganisms continue to decompose material in the
lower water column and in the sediments, they consume oxygen, and the DO is
depleted. No oxygen input from the air occurs because of the ice cover, and,
if snow covers the ice, it becomes too dark for photosynthesis. This condition
can cause high fish mortality during the winter, known as "winter kill." Low
DO in the water overlying the sediments can exacerbate water quality deterioration,
because when the DO level drops below 1 mg O2/L chemical processes
at the sediment-water interface frequently cause release of phosphorus
from the sediments into the water. When a lake mixes in the spring, this new
phosphorus and ammonium that has built up in the bottom water fuels increased