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Natural
inputs
Phosphorus (P) is relatively sparse in natural soils and exists
primarily as the phosphate molecule that tends to stick to soil
as water moves through it. Therefore, in the absence of human-caused
impacts, P concentrations in the surface and groundwater that flows
into lakes tends to be very low and so usually regulates the potential
amount of algal growth in the system. In pristine parts of the world,
there is also very little phosphorus in precipitation and in the
dry portion of atmospheric inputs referred to as dry fallout.
Human
inputs
Human activities lead to increased inputs of P in streams and sometimes
in groundwater and even in atmospheric inputs. The most obvious
sources are from municipal wastewater (sewage) treatment plants
and from industry and are called point sources that are regulated
by monitoring loads at the ends of their discharge pipes and setting
strict limits. Diffuse, or non point sources, are much more difficult
to measure and to control. Agricultural fertilizer-P is a major
source of phosphorus pollution in streams throughout the US.
The major sources
of P to most urban lakes are non point, are all controllable to
a large extent by homeowners and/or local community agencies and
typically include:
*
soil-P from erosion (construction sites, road banks, shoreline disturbance,
lawns & gardens)
* road runoff (street sweepings
of crud that accumulates between rainfalls)
* roof runoff
* lawn clippings
* excess lawn fertilizer runoff
* sewage from leaky sewer lines
or from on-site septics drainfields
Lake
internal inputs
Over long periods of time, urban lake sediments become greatly enriched
in phosphorus and then release a portion back into the water. This
internal release can occur sporadically and may exceed annual inputs
from surface waters, as is the case for Halsteds Bay in Lake Minnetonka
(see the What's New story, Thunderstorm
Watch.
In productive, moderately deep lakes like Medicine Lake that stratify
thermally in summer and become anoxic (no oxygen) in their hypolimnetic
bottom waters, large amounts of this historically deposited phosphorus
is released from the sediments into the water. It can then be mixed
into sunlit surface waters during windstorms and fuel algal blooms.
An example of such a storm is shown in the graph below for late
summer 2001 that shows short-term mixing events (when there's a
vertical brown stripe from surface to bottom). Turbulence from the
wind can also resuspend high-P sediment from shallow areas, as can
boat and jetski wakes. This latter source is worsened when the shoreline
and nearshore zone submergent and emergent vegetation (weeds) have
been removed since they stabilize the bottom sediment and act to
dissipate wave energy.
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