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 the bioavailable
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
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