Encyclopedia of Environmental Science and Engineering, Volume I and II

610 LIMNOLOGY

relative to the volume of water, more complete wind mixing

of the lake water, and the large littoral zone along the lake

perimeter that can be colonized by plants. Shallow lakes often

have most of their plants in littoral areas and have little pelagic

habitat. On the other hand, deep lakes have fewer areas that

receive enough light for rooted aquatic plants to grow, and

therefore have a high proportion of pelagic habitat and less

littoral habitat.

HYDROLOGIC CYCLE AND WATER BUDGET

Since precipitation and surface water runoff have direct influ-

ences on lake ecosystems, understanding the hydrologic cycle

and water budget are key concepts in limnology. The hydro-

logic (water) cycle refers to the circulation of water between

the Earth’s surface and the atmosphere. This is powered by

the sun. Water falls to Earth as precipitation. About 75 percent

of the precipitation is returned to the atmosphere as vapor

through direct evaporation and transpiration from both ter-

restrial and aquatic plants during photosynthesis. The remain-

ing 25 percent of the precipitation is stored in ice caps, drains

directly off the land into lakes, streams, wetlands, rivers, and

oceans, or infiltrates the soil and underlying rock layers and

enters the groundwater system. Groundwater enters lakes

and streams through underwater seeps, springs, or surface

channels (Cooke et al. , 1986; U.S. EPA, 1990; Wetzel, 1983).

Drainage lakes are formed primarily by inflowing rivers

and streams. Therefore, their water levels vary with the sur-

face water runoff from their watersheds. On the other hand,

seepage lakes form where groundwater intersects with the

land surface. Since seepage lakes are maintained primarily

by groundwater inflow, their water levels fluctuate with sea-

sonal variations in the local water table. For both drainage

and seepage lakes, the balance between water inputs and

outputs influences the supply of plant nutrients (nitrogen

and phosphorus) to the lake and the lake’s hydraulic (water)

residence time, thereby influencing the lake’s water quality

and biological productivity (U.S. EPA, 1990).

The hydraulic (water) residence time is the amount of

time that water entering a lake will remain in it or the aver-

age amount of time required to completely renew a lake’s

water volume. The amount of water entering a lake from its

watershed controls the volume of the lake. The hydraulic

residence time is calculated by dividing the water volume of

a lake by its flow rate, and varies greatly among lakes. For

example, if a lake has a volume of 500 acre-feet and the out-

flow rate is 10 acre-feet per day, then the hydraulic residence

time would be 50 days. If the hydraulic residence time of a

lake is 100 days to several years, this means that plant nutri-

ents and pollutants remain in the water column long enough

to degrade water quality and to allow plants to accumulate

(U.S. EPA, 1990; Wetzel, 1983).

Each lake has a water balance, in which water input 


water output  the change in the amount of water stored in the

lake. If inputs are greater than outputs, lake levels rise as water

is stored in the lake. When outputs are greater than inputs, lake

levels fall. This happens during summer droughts.

A related concept is the lake water budget, which is a

measure of the sources of water entering and flowing out of a

lake over the course of a year. A lake’s water budget is affected

by the hydrologic cycle, and the quantity and timing of water

entering and leaving the lake. Types of data used in calculating

a water budget include precipitation, stream flow into and out

of the lake, and lake surface elevation (water level). Sources

of water input or inflow include the lake inlet(s), precipitation,

surface water runoff, point source discharges, and ground-

water. Sources of water output or outflow include the lake

outlet(s), evaporation, transpiration from lake plants, ground-

water seepage, and water withdrawals for domestic, agricul-

tural, and industrial purposes. The change in storage accounts

for changes in surface elevation over the year. This change is

positive if lake volume increases over the year, negative if lake

volume decreases (U.S. EPA, 1990; Wetzel, 1983).

Land use and geology of the surrounding watershed

affect the water budget. For example, lakes in areas with

permeable soils receive inflowing groundwater throughout

the year. Lakes in areas with impervious surfaces can receive

large volumes of stormwater runoff.

PHOSPHORUS BUDGET AND LOADING

Another important characteristic of lakes is the phosphorus

budget, a measure of the sources of phosphorus entering

and leaving the lake over the course of a year. Phosphorus

is a nutrient that is essential in plant growth. The amount of

phosphorus in a lake directly influences biological produc-

tivity. The phosphorus budget will indicate if the phosphorus

in the lake is coming from within the lake, from sources in

the watershed, or from both internal and external sources.

For a given lake, phosphorus inputs (inflow loading)

–phosphorus outputs (outflow loading)  net sedimentation 

change in storage. This means that phosphorus inputs to the

lake equal phosphorus losses from the lake plus or minus the

change in the total amount of phosphorus stored in the lake.

Change in phosphorus storage within the lake equals the

amount of phosphorus entering the lake minus the amount of

phosphorus leaving the lake minus the net loss of phosphorus

to the lake sediments. Sources of phosphorus inputs to a lake

are the lake inlet(s), point sources discharging directly to the

lake, precipitation, surface water runoff, leachate from mal-

functioning shoreline septic tanks, other groundwater inputs,

and migrant waterfowl wastes. Sources of phosphorus out-

puts from a lake are the lake outlet(s), groundwater seep-

age, and water withdrawals for domestic, agricultural, and

industrial purposes (Cooke et al. , 1993b; U.S. EPA, 1990;

Wetzel, 1983). Figure 2 illustrates the phosphorus budget for

Cottage Lake, near Seattle, Washington. As indicated in the

figure, most of the phosphorus in the lake comes from the

Daniels Creek inlet and from the lake sediments. Most of

the phosphorus that leaves the lake does so via the lake outlet

(KCM, 1994; Solomon et al. , 1996).

Net sedimentation refers to the amount of phosphorous

accumulated in lake bottom sediments, i.e., the difference in

the amount of phosphorus that binds to the sediments and

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