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 Generalized Phosphorus Budget

phosporus cycle diagram

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.

oxygen profile

You can play with this and other data visualization tools, by going to the data section of this website ( ).


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