Chap.10 - Water Density and
Layering in Lakes
Lake Stratification
z Epilimnion
– upper part of lake. Even
temperature due to wind mixing
z Hypolimnion – bottom layer of lake
water. – No wind mixing
z Thermocline or metalimnion –
temperature decreases with depth.
Located between upper and lower
zones.
Stratification
Water Density
z Water
is heaviest at 4C (40F) . It is
lighter above and below this
temperature.
z Salinity – increases density and
decreases freezing point.
z Suspended Particles – change water
density. Make it heavier.
Lake Stability
z Lake
Stability – It takes a lot of energy
to mix water of different densities.
z S= amount of work energy required to
entirely mix a lake to a uniform
temperature without adding or
subtracting heat.
Lake Classification
z Mictic
and mixis – mean mixing and
are terms used in lake classification to
describe how the lake mixes.
z Some lakes mix differently every year.
So they are hard to classify.
Amixis
z
z
z
Amicic lakes – never circulate. Covered with
ice. Antarctica and Greenland.
Some of these lakes have unusual
stratification profiles due to saline
stratification or temperature fluctuations due
to thermal springs.
Amictic lakes can also be found at high
altitudes. Peruvian Andes and on Mount
Kenya in Africa.
Lake Classification
z Holomixis
- Wind circulation mixes
entire lake.
z Oligomictic
z Monomictic
z Dimictic
z Polymictic
z Meromictic
Lake Nicaragua
Lake Maggiore Italy
Lake Classification
Lake Geneva Switzerland
z
z
Oligomictic lakes – generally equatorial
lakes. There is warm water at all depths.
Very little seasonal change. Stratified and
usually stable because water is very warm on
the surface and relatively cooler at lower
depths. Circulation is unusual, irregular and
of short duration.
Cold oligomictic lakes- large sub-alpine,
European lakes (Lake Geneva Switzerland)
and Lake Tahoe in U.S.
Lake Tahoe CA
Lake Classification
z Monomictic
Lakes - One regular
mixing period sometime during the year.
Usually exist in climates where seasonal
changes are strong
Elephant Butte NM
Lake Oneida
Lake Oneida
z
z
z
z
z
z
z
z
Cold Monomixis
Frozen during winter months. Water shielded
from wind.
Water stagnant, no mixing in winter (amictic).
0 C just below ice to 4C at bottom (inverse
stratification).
Ice melts in spring –
Stratification stability is low and water mixes.
No summer stratification.
New Quebec Crater Lake
Ex. Lake Oneida NY
Lake Hazen Ellesmere Island
z
z
z
z
z
z
z
z
z
z
Warm monomixis
Lake Atitlan
Lack ice cover
Circulate in winter – stratify in summer
Usually occurs south of 40 degrees latitude
Some Oklahoma lakes are warm monomictic
if they are deep enough and somewhat
sheltered from the wind.
Great Lakes are warm monomictic
Lake Victoria, lies across the equator.
Expect it to be holomictic ( in circulation all
the time).
However, it stratifies in the rainy season. Lake Superior
Cool rain water flows to bottom setting up
stratification from bottom up.
z
Dimictic Lakes
z
Two mixing periods
Spring and fall overturns
This is the idealized, typical temperate zone lake.
Stratification is strong during warm summer months
Falling temperatures and wind break down
stratification in the fall.
Lake Tom Wallace
Lake circulates when water reaches 4 C.
Ice forms on lake in winter, no circulation
Water becomes stratified , 4C on bottom, may be
colder on top.
Spring – ice melts. Lake temperature much the same
top to bottom
Wind mixes lake. Lake Itasca
Mixing continues until summer stratification sets in.
z
z
z
z
z
z
z
z
z
z
z
z
z
z
z
z
z
Polymictic Lakes
Lake Nicaragua
Many mixing periods or continuous
circulation.
Influenced more by daily changes than
seasonal changes.
Some Andean Lakes stratify during the day
and destratify at night.
Can happen with desert lake also.
Lake Nicaragua (very shallow) stratifies at
night but is mixed each afternoon by wind.
Ponds exposed to wind in Oklahoma can be
polymictic.
Remember! – Lake
categories can change with
unusual seasonal changes.
Meromixis
z Meromictic
Big Soda Lake NV
lakes circulate sometimes
but not completely.
z Dense stratum of bottom water remains
stagnant and usually anaerobic.
Terms for regions in a
meromictic lake.
z
z
z
Monimolimnion – permanently stagnant
bottom layer. Greater concentration of
dissolved substances then overlying waters.
Mixolimnion – more dilute, mixed by wind.
Shows seasonal changes.
Chemocline – Between the mixolimnion and
the monimolimnion. Chemical gradient.
Salinity increases rapidly with depth.
Meromixis
Lake Tanganyika
z Biogenic
Meromixis – caused by an
accumulation of substances derived
from bacterial decay, diffusion from the
sediment and carbonate precipitation
from photosynthesis.
Fayetteville Green Lake
Biogenic Meromixis
z
z
z
No accumulation of NaCl(salt).
Substances found are what you usually see
in a eutrophic lake at the end of summer.
Anoxic water, high Hydrogen sulfide. Devoid
of most life except for anaerobic bacteria.
Biogenic meromictic lakes form when lake
shape (steep sides, deep), surrounding
topography (lake sheltered by woods and
hills), and weather conditions that hinder
overturn combine with accumulation of
biologically derived substances that increase
water density on lake bottom.
Round Lake NY
Meromixis
z Ectogenic
meromixis - Water density
differences brought about from outside
sources.
Soap Lake WA
Ectogenic meromixis
Judd Lake
z
z
z
z
Can enter lake in 2 forms
1) dilute upper layer that lies above an
existing saline layer.
2) Outside source of saline water finds
its way to bottom of a freshwater lake.
Topographic, morphological and
meteorological factors not so important
with this kind of meromixis.
Soap Lake WA
Ectogenic meromixis
z This
situation is being created in some
parts of the U.S. where salt is used to
de-ice roads. This creates saltwater
runoff in the spring that flows to the
bottom of freshwater lakes.
Accumulations over time create a dense
layer of salt water at the bottom of the
lake.
Judd Lake
Ectogenic meromixis
z Hot
Lake WA. Lake water is rich in
magnesium sulfate (epson salts).
Freshwater from snow melt enters the
lake each year and forms a layer of
fresh water on top of the magnesium
sulfate rich water.
Soap Lake WA
Meromixis
z Crenogenic
meromixis – subsurface
flows of saline water into a lake plus
freshwater inflow from the surface when
in equilibrium produce this type of
meromixis.
z Lake Kivu Africa is an example.
Unusual Temperature Profiles
z Dichothermy
– coldest stratum of water
found at an intermediate depth.
z Bottom water warmed by bacterial
metabolism of geothermal activity.
Unusual Temperature Profiles
z Mesothermy
– Temperature increases
with depth then decreases before the
lake bottom is reached. Results in a
warm stratum sandwiched between
cooler strata of water. Seen in
meromictic lakes due to temperature
changes caused by salinity differences.
Unusual Temperature Profiles
z Poikothermy
– complex temperature
curves that occur when a meromictic
lake is beginning to warm in the spring.
Unusual Temperature Profiles
The Thermal Bar
z
z
z
z
z
Occurs In large cold lakes where temperature
falls below 4C during winter. circulation
Shallow water near shore warms before deep
water farther out in the lake does.
Deeper water continues to circulate while the
warmer inshore water begins to stratify.
The two regions are separated by a narrow
zone of down-welling water.
This water is at 4C and the most dense water
in the lake. It forms a barrier between
circulating off-shore water and warmer, much
less dense in-shore water.
Thermal Bar
The Thermal Bar
z
z
z
Nutrients flowing in from the water shed
cannot penetrate thermal bar so
phytoplankton blooms will occur out into the
water until they reach it.
The 2 lake regions are very different in
chemical, optical and phytoplankton species.
Thermal bar disappears when off-shore area
temperatures warm and stratify during the
summer.
The End