Encyclopedia of Environmental Science and Engineering, Volume I and II

612 LIMNOLOGY

grocery item, the cost is determined by multiplying the unit

cost and the number of items purchased. The total grocery

bill is the sum of the costs of all items. Likewise, for each

source of phosphorus input to a lake, the phosphorus load-

ing is determined by multiplying the flow rate for the source

(lake inlet, groundwater, etc.) and its phosphorus concentra-

tion over annual and seasonal periods. The total “phosphorus

bill” (total phosphorus loading from all sources) is the sum

of the loadings from each source.

Phosphorus loadings change in response to season,

storm events, upstream point sources, and land use changes.

For example, converting an acre of forest into residential

or commercial land typically increases the phosphorus

loading to a lake in that watershed fivefold to twentyfold.

This is because there will be increases in both water flow

(runoff from the newly created impervious surfaces) and

phosphorus concentration (deposition of phosphorus on

impervious surfaces). An evaluation of phosphorus load-

ings provides a basis for predicting lake responses to

changes in land use.

STRATIFICATION

Many swimmers notice that when they dive into a lake during

the summer, the deeper waters of the lake are much colder than

the surface waters. This is due to stratification, an interesting

temperature-related characteristic of most temperate climate

lakes.

TABLE 1

Phosphorus Loading Concept (U.S. EPA, 1990)

Grocery Bill Phosphorus Loading

Item Source

Quantity Flow

Unit Cost Concentration

Cost of Item Loading from Source

Total Cost of All Items Total Loading from All Sources

EPILIMNION or mixed layer-warm (light) water

HYPOLIMNION

cool (heavy) water

Dissolved

Oxygen

Temperature

Profile

Low High

Low High

METALIMNION

THERMOCLINE

FIGURE 3 Thermal stratification.

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