the most fundamental set of properties of lakes relates to the interactions
of light, temperature and wind mixing. The absorption and attenuation
of light by the water column are major factors controlling temperature
and potential photosynthesis.
provides the food that supports much of the food web. It also provides
much of the dissolved oxygen in the water.
Solar radiation is the major
source of heat
to the water column and is a major factor determining
wind patterns in the lake basin and water movements.
at the lake surface varies seasonally and with cloud cover and decreases
with depth down the water column. The deeper into the water column that
light can penetrate, the deeper photosynthesis can occur. Photosynthetic
organisms include algae suspended in the water
attached to surfaces (periphyton), and vascular aquatic plants
at which light decreases with depth depends upon the amount of light-absorbing
dissolved substances (mostly organic carbon compounds washed in from
decomposing vegetation in the watershed) and the amount of absorption
and scattering caused by suspended materials (soil particles from the
watershed, algae and detritus).
of the surface light absorbed or scattered in a 1 meter long vertical
column of water, is called the vertical extinction
coefficient. This parameter is symbolized
with low k-values, light penetrates deeper than in those with high k-values.
Figure 2 shows the light attenuation
from two lakes with attenuation coefficients of 0.2/m and 0.9/m.
depth at which algae and macrophytes can grow is determined by light
levels. Limnologists estimate this depth to be the point at which the
amount of light available is reduced to 0.5%1% of the amount of
light available at the lake surface. This is called the euphotic zone. A general rule of thumb is that this depth is about 2 to 3 times
the limit of visibility as estimated using a Secchi
disk. Light may be measured in a variety of ways for a number of
different characteristics. The reader is referred to the reference texts
for more information. Since photosynthesis depends fundamentally on
light, significant changes in light penetration in a lake will produce
a variety of direct and indirect biological and chemical effects. Significant
changes in lake transparency are most often the result of human activities,
usually in association with landuse activities
in the watershed.
Also see "What
for an excellent primer on light.
1. Estimated ranges of water transparency values for various lakes.
"Clear" water refers to the lack of bog-staining color.
WOW lakes in bold face.
Vertical extinction coefficient, k
defined previously. Secchi depths in meters.
||Euphotic Zone (m)
|Crater Lake (OR)
||Clear, sky blue ultra-oligotrophic lake
|Lake Tahoe (CA/NV)
||As above but decreasing clarity since1960s due to watershed overdevelopment
most oligotrophic of the Laurentian Great Lakes
(Green water near Duluth)
||Western arm near Duluth
and St. Louis River and harbor inputs
|St. Louis River (Duluth-Superior
||Brown (bog) stained from river plus high suspended
||210 (19701990) >10 (19931995)
||Eutrophic (clarity improving
recently due to zebra mussels)
|Lake Baikal, Siberia
(Pine County, MN)
||Mesotrophic, water is fairly
stained or colored
(Itasca County, MN)
|Lake Minnetonka (Hennepin