Salts such as sodium, potassium, magnesium and calcium
___1___ constantly
introduced into a lake by rivers and rain. Lakes also
contain much dissolved salts and minerals from
surrounding rock. While salts generally ___2___
at low levels in fresh or open lakes, they tend to
concentrate in closed lakes, in which pure water escapes
only through the process of evaporation. Lake water
also contains dissolved gases such as oxygen, carbon
dioxide, and methane. From a biological standpoint,
oxygen is the most important of the gases. Since oxygen
is ___3___ by the
photosynthesis of water plants, oxygen levels tend to be
highest near the surface of a deep lake, where plants
and plankton are abundant, and ___4___
at the bottom, where dead plant and animal matter
accumulates.
A lake's water temperatures follow one of two common
patterns, depending on the lake's geographical location.
In tropical regions, lakes maintain a relatively
constant year-round temperature. More complex are the
temperature dynamics of lakes in regions where summer
and winter temperature ___5___
greatly. In summer, the Sun warms a thin layer of light
surface water that does not mix much with the colder
heavier water beneath it. In winter, this layer of
surface water cools. It then sinks and is replaced by
water from below. |