Summer:
The lake is stratified with a layer of
warmer water (the epilimnion) on top of the
colder and denser water of the depths (the hypolimnion). A layer of steep
temperature change with depth, the thermocline, separates the warmer and colder
water. Nutrient circulations are flow through the plankton, the shore community, the
benthos or bottom community.
The friction of the wind causes the upper
layer of water to circulate, so that nutrients
are transported between the plankton and shore communities. Particles and
organisms settle through the deeper water, carrying nutrients to the bottom, where
they may be circulated between water and sediment, but where there is net
movement into the sediment.
Autumn:
The surface water cools until temperature
is the same at all depths, and the whole
lake is circulated by the wind. Winter: The water is stagnant and near 4°C., except
close to the surface ice cover.
Spring:
The surface water warms until temperature
is the same at all depths, and the lake is
again circulated. Further warming of the surface water leads again to summer
stratification.
These movements are important limiting
factors of lake productivity.
When phosphate concentration in the water
is increased by adding phosphate
fertilizer, the concentration in the water and plankton thereafter declines (usually
within some weeks) back to the level before fertilization. A single fertilization thus
produces only a temporary increase in productivity. Some of the fertilizer phosphate
may leave the lake in the stream that flows from it.
The lake retains in its basin, however,
most of the phosphorus that enters it,
whether it enters as fertilizer, or as the normal, continuing input in streamwater, or
from another source. In a typical lake only a fraction (a third, say) of the phosphorus
that enters the lake leaves it in the outflowing stream. The remaining phosphorus
does not simply accumulate, but after being taken up by lake organisms and used
as a basis of their productivity is lost to the lake by the net movement into
permanent deposit in the sediments.
Because of the steady loss of phosphorus
(and other nutrients) into the sediments,
as well as stream outflow, the lake is dependent on the flow of nutrients into it from
its watershed to maintain its productivity.
The uptake of phosphorus by shore plants
indicates the importance of these plants
for nutrient movement and productivity. In many lakes much, and in some shallow
ponds most, of the primary productivity is by shore plants and not by the open-water
plankton. The sinking of plankton and particles removes phosphorus and other
nutrients from the surface waters in stratified lakes and limits their productivity.
Stratification affects nutrient movement in a number of ways that relate to annual
cycles of productivity. Those cycles vary widely in different lakes.
During the winter the water is both cold
(near the temperature of maximum density,
4°C, at most depths) and fairly stable (particularly if the surface water is below 4°C
or frozen). Productivity is low, and nutrients tend to accumulate in the deeper water.
In the spring, when the waters are at the same temperature (which may be near
4°C) at all depths, there is no density contrast; and the waters of the lake are turned
over by the wind. Nutrients are brought to the surface at a time when light intensity is
increasing and surface temperatures are 4°C or warmer, and the result is a spring
bloom or peak of plankton productivity. In summer the lake may be stratified, with
the wind mixing only the warmer, less dense, surface waters while the deeper
waters remain cold, generally near 4°C. Through the summer the nutrient content of
the surface waters, and the productivity supported by them, decrease. If, however,
the lake is a productive one, the large amount of dead organic matter (including
some from shore plants and from outside the lake as well as from plankton) that
sinks into the deeper waters may exhaust the oxygen there as bacteria use the
oxygen to decompose organic matter. The nutrients released by decomposition
partly remain in the water and partly accumulate in the mud; but the concentration of
nutrients in the water will be higher if the lake is productive and if oxygen is
exhausted in the deeper waters. In the fall the surface waters cool; and when water
temperature and density are the same at all depths, the lake waters again turn over.
Nutrients are then brought to the surface where they may support a secondary, fall
peak of productivity. Productivity decreases into the winter but does not end while
sunlight reaches the surface water. Even in midwinter and even under ice cover that
admits light, some plankton algae carry on photosynthesis.
