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
and eutrophic lakes tend to have uniform,
well-mixed conditions throughout the water column. During summer stratification,
the conditions in each layer diverge.
Figure 9. (adapted from Figure 8-1 in Wetzel, R.G. 1975. Limnology. W.B.Saunders Company)
concentration in the epilimnion remains
high throughout the summer because
and diffusion 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
column and less oxygen is consumed by decomposition. The DO concentrations
may therefore increase with depth below
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
turnover (Figure 9).
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 algal growth.