Lake Ecology
Nature of Lakes
• Lakes are enclosed basins which can trap
standing water
• Water retention time of lakes (the time an
average water molecule stays in the lake)
varies from a few days to hundreds of
years
• Water retention time depends on the size
of the lake and the rate of inflow/outflow
Lake Basins
• The lake basin is the “bowl” or depression that contains
the water
• Lake basins are formed by numerous processes, the
principal being:
–
–
–
–
–
Glacial activity
Crustal movement
Rivers
Solution processes
Human activity
• These processes often occur in restricted areas giving
rise to “lake districts”, areas in which there are a lot of
lakes (eg. The Adirondacks, Minnesota, African rift
valley)
Lakes formed by Glacial Processes
• Glacial activity has
resulted in the greatest
number of lakes and
some of the largest lakes
in area
• The lakes of Minnesota
(“Land of 10,000 Lakes”)
and the Adirondacks in
New York are attributable
to glacial activity
• The Great Lakes are also
glacial in origin
Lakes formed by Glacial Processes
• Glacial lakes are
found in areas of
steep terrain where
scour has been the
mechanism
Lakes formed by Glacial Processes
• They are also found
in flat terrain where
damming by moraines
or ice blocks left
behind in glacial drift
is the mechanism
Origin of Lakes – Crustal
Movement
• Tectonic Activity
(crustal instability
and movement)
– Graben = faulttrough = rift lake
– Formed between
two faults
Lakes formed
by Crustal
Movement
• The deepest and
oldest lakes in
the world are
those formed by
crustal
movement
• The deepest and
oldest lake in the
world is Lake
Baikal in Siberia
Lakes formed by Crustal Movement
• Earthquake Lakes
– Reelfoot Lake, TNKY
– Major earthquake (8
on Richter scale)
– Caused surface to
uplift in some areas
and subside in
others
– Mississippi R was
diverted into a
subsidence region
for several days
forming Reelfoot
Lake
Lakes formed by Crustal Movement
• Landslide Lakes
– Mountain Lake, VA
• One of two natural lakes
in Virginia
• Formed when landslide
dammed a mountain
valley
• The lake is estimated to
be about 6,000 years
old and geologists
believe it must have
been formed by rock
slides and damming
Lakes formed by Crustal Movement
• Crater/caldera Lakes
– Lake occupies a
caldera or collapsed
volcanic crater/cone
– If cone blows out the
side like Mt. St.
Helens, no basin left
– Ex. Crater Lake, OR
Rivers Formed Lakes
• Alluvial rivers leave
behind bends that
become oxbow lakes
• Oxbow lakes are
localized to areas in
alluvial floodplains, like
the lower Mississippi
valley
Solution Lakes
• Lake basins can be formed
when subsurface mineral
deposits (like halite or
limestone) dissolve leaving a
void which collapses resulting
in a basin
• The lakes of central Florida
form a solution basin lake
district
Origin of Lakes – Solution Lakes
• Salt collapse basins
– Underground
seepage dissolves
salt lenses, ground
collapses and basin
fills
– Montezuma Well,
AZ
Lakes formed by Human Activity
• These may be intentional, as
in the case of reservoirs
created for recreation, flood
control, irrigation, navigation,
hydropower
• Or they may be incidental, as
in the case of flooded peat digs
or rock quarries
Light in Lakes
• Sun is virtually the only source of energy
in natural aquatic habitat: photosynthesis
and heat
• Solar constant
– Rate at which radiation arrives at edge of
Earth’s atmosphere
– ≈ 2 cal/cm2/min
– More than half of this is lost coming through
the atmosphere
Solar Radiation Reaching Lake
Surface
• Absorption by
different
chemicals in
atmosphere
• Water and ozone
(O3) are
especially
important
• Ozone is the
most important in
the UV range
Solar Radiation
Entering Lakes
• Solar radiation enters
lakes and is absorbed at
a constant rate
• Absorption rate varies
with wavelength
• There is more light
available near the surface
and this decreases
exponentially with depth
• This light energy affects
– The temperature of the
water in a lake
– The growth of primary
producers in a lake
Lake Stratification and Mixing
• Due to the changes in density
with temperature, lakes
generally stratify in summer
with warmer, lighter water
overlaying colder, heavier
water
• This creates a stable layering
of water which can last well
into the fall
• As temperatures drop in the
fall, the surface water cools
and gradually reaches the
temperature of the bottom
water
• When this occurs, we have
“turnover” in which water mixes
throughout all lake depths
Dimictic Lakes – Annual Cycle
Seasonal heating and cooling
Wind creating turbulence
Polymictic Lakes
• Shallow temperate zone lakes can also be
polymictic including the GMU Pond
• Note the daily stratification and mixing pattern
Lake Layers during Stratification
• The upper layer
of the lake is
called the
epilimnion
• And the lower
layer is the
hypolimnion
Stratification Affects
Lake Chemistry
• During stratification, the hypoliminion is cut off from the
oxygen in the air
• If the lake is productive, there will be organic matter
from the epilimnion settling into the hypolimnion
• This organic matter will be broken down by microbial
respiration resulting in a decrease in dissolved oxygen
• This may leave the hypolimnion critically deficient in
dissolved oxygen so that it cannot support many
animals like fish
Lake Chemistry - Oxygen
• Vertical
Distribution
– Varies with
lake type
– Very
productive
lakes lose
oxygen
during
stratification
Lake Chemistry - Phosphorus
• P limits
biological
production in
lakes
• P cycle in lakes
• P accumulates
in the
sediments
Zonation of Biota
• Biological zonation is strongly influenced by light availability
• The littoral zone is the portion of the lake which has
sufficient light for photosynthesis to the bottom
• The limnetic zone is the open water area in which sufficient
light for primary producers is only available in the top of the
water column
• The dark portion of the open water is sometime called the
profundal zone
Types of Lake Organisms
• Macrophytes: large leafy plants with attached
microscopic periphyton
• Plankton: Suspended small organisms controlled by
currents
• Benthos: Bottom dwellers
• Nekton: Larger, mobile organisms
• Note which zone each is found in
Typical Macrophytes
• submersed
Floating
leaved
•
•
Emergent
Typical Phytoplankton
flagellate
diatoms
cyanobacterium
desmid
Typical Zooplankton
------------------- 0.5 mm
Copepod: grazer
on phytoplankton
Rotifer:grazer on
phytoplankton
Water flea: grazer on phytoplankton
Typical Benthos
• Midge larvae
Dragonfly
nymph
•
bivalves
Typical Nekton
Bass: a piscivore (fish eater)
Bluegill: a planktivore
and benthivore
Catfish: a detritivore (scavenger)
Lake Food Web
• Nutrients like N and P
together with CO2
and light stimulate
phytoplankton
• They are fed upon by
zooplankton which in
turn provide food for
juvenile fish
Lake Food Web
• The larger fish
generally eat other
fish (piscivorous)
and provide the top
of the food web
•There is sometimes
even a second tier
of even larger fish
Overview of the Lake Food Web
Lake Trophic Status
• Oligotrophic
– Low productivity,
clear water, life
more sparse
• Somewhat
Eutrophic
– High productivity,
murkier water,
but more life
Excess Nutrients – N&P
Natural Eutrophication
• Productivity of lakes are
determined by a number
of factors:
– Geology and soils of
watershed
– Water residence time
– Lake morphometry
– Water mixing regime
• Over thousands of years
these factors gradually
change resulting in lakes
becoming more
productive
Cultural Eutrophication
• Human activities can alter
the balance of these
factors, esp. when excess
nutrients (P in freshwater)
are introduced
• Untreated sewage for
example has a TP conc of
5-15 mg/L
• Even conventionally treated
sewage has about ½ that.
• Compare that with inlake
concentrations of 0.03 mg/L
that can cause eutrophic
conditions
• So, even small amounts of
sewage can cause
problems
Cultural
Eutrophication
• Problems associated with
cultural eutrophication include
– Anoxic hypolimnion
• Part of lake removed as
habitat
• Some fish species eliminated
• Chemical release from
sediments
– Toxic and undesirable
phytoplankton
• Blooms of toxic cyanobacteria
• Phytoplankton dominated by
cyanobacteria and other
algae that are poor food for
consumers
– Fewer macrophytes
• Elimination of habitat for
invertebrates and fish
– Esthetics
Cultural
Eutrophication –
Case Studies
• Lake Washington
– Following WWII, pop’n
increases in the Seattle
area resulted in increases
in sewage discharge (sec
trted) to Lake Washington
– Secchi depth decreased
from about 4 m to 1-2 m
as algae bloomed from
sewage P
– Diversion system was
built and effluent was
diverted to Puget Sound
in mid 1960’s
– Algae subsided and water
clarity increase
– Daphnia reestablished
itself and further clarified
the lake
Cultural
Eutrophication –
Case Studies
• Norfolk Broads, England
• Shallow systems where
macrophytes dominated
• Increased runoff of
nutrients, first from
sewage and then from
farming stimulated algae
• First periphyton bloomed
and caused a shift from
bottom macrophytes to
canopy formers
• Then phytoplankton
bloomed and cut off even
the canopy macrophytes
and their periphyton
Case Study: Gunston Cove on the Potomac River
Households in the
Gunston Cove
watershed have grown
dramatically since the
mid-1970’s. Since the
study began in 1984
the number of
households has grown
by about 50%. All other
things equal, an
increase in households
should produce an
increase in nonpoint
contributions.
The point source P
load declined
dramatically in the late
1970’s and early
1980’s.
Formal study initiated
in 1983.
Gunston Cove Recovery
• Improvements in
water clarity related to
P-limitation and
decline of
phytoplankton were
correlated with an
increase in submersed
macrophyte coverage
in Gunston Cove
• Since 1 m colonization
depth was achieved
(2004), macrophyte
coverage has
increased strongly
References
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http://waterontheweb.org/under/lakeecology/index.html
http://pearl.spatial.maine.edu/default.htm
http://www.co.cayuga.ny.us/wqma/weedswatchout/biology.htm