organisms influence (and are influenced by) the chemistry of the surrounding
environment. For example, phytoplankton extract nutrients from the water
and zooplankton feed on phytoplankton.
Nutrients are redistributed from the upper water to the lake bottom
as the dead plankton gradually sink to lower depths and decompose. The
redistribution is partially offset by the active vertical migration
of the plankton.
to DO, essential nutrients such as
forms of phosphorus and nitrogen (dissolved phosphate, nitrate, and
ammonium) typically increase in the spring from snowmelt runoff and
from the mixing of accumulated nutrients from the bottom
during spring turnover.
Concentrations typically decrease in the epilimnion during
summer stratification as nutrients are taken up by algae and eventually
transported to the hypolimnion when the algae die and settle out. During
this period, any "new" input of nutrients into the upper water
may trigger a "bloom" of algae. Such inputs may be from upstream
tributaries after rainstorms, from die-offs of aquatic plants, from
pulses of urban stormwater, direct runoff of lawn fertilizer, or from
leaky lakeshore septic systems. In the absence of rain or snowmelt,
an injection of nutrients may occur simply from high winds that mix
a portion of the nutrient-enriched upper waters of the hypolimnion into
the epilimnion. In less productive systems, such as those in Northeastern
Minnesota, significant amounts of available nitrogen may be deposited
during rainfall or snowfall
events (wet deposition)
and during the less obvious deposition of aerosols and dust particles
(dry deposition). For instance, Lake
Superior has been enriched by as much as 300 µg/L during this
century, presumably due to air pollution. Nitrogen and phosphorus in
dry fallout and wet precipitation may also come from dust, fine soil
particles, and fertilizer from agricultural fields.