Freshwater Ecosystems - Bowie Aquatic Science



Freshwater Ecosystems - Bowie Aquatic Science
The study of fresh bodies of water
Lentic: standing water (lakes and ponds)
Lotic: flowing water (streams and rivers)
How might they be different?
Glacial erosion and deposition - movement
of glaciers eroded land (Great Lakes)
Deposition and erosion of sediment in a
meandering river creates an oxbow lake
Geologic Activity- tectonic movement; rift
valleys and volcanic craters fill with water
Manmade: dams, log jams, strip mining (all
lakes in TX except one, which one???)
High specific heat of water make aquatic
environment temperatures more stable than
Epilimnion: surface water, small temperature change
Metalimnion: middle mass of water with a rapid
temperature decline (1°C/ meter)
Thermocline: temperature gradient found in
Hypolimnion: deep cold layer cutoff from air; most
Summer- large thermocline. Why?
Winter- Surface water loses heat to
atmosphere therefore thermocline
Overturn: surface water temp. decreases
and causes water to mix and stir up
nutrients and dissolved oxygen because of
density changes (Fall and Spring)
Enters the water by
◦ 1. absorption from the atmosphere
◦ 2. photosynthesis
The amount of oxygen and other gases
water can hold depends upon pressure and
As temperature increases- solubility of
dissolved oxygen decreases
As pressure increase- solubility of DO
Water loses oxygen through
◦ increased temperature
◦ increased respiration of aquatic life
◦ aerobic decomposition
During the summer, oxygen may become
stratified in lakes and ponds
The quantity of oxygen decreases with depth
because of decomposition in the bottom
During Spring and Fall overturn- water
circulation (churning) replenishes oxygen in
the bottom
In winter- DO decreases slightly with depth;
DO is more stable because cold water can
hold more DO than warm water
The depth to which light penetrates is limited by
turbidity of the water and the absorption of light
Trophogenic zone- layer through which light can
penetrate and where photosynthesis occurs (photic
Tropholytic zone- layer through which light cannot
pass through and where decomposition occurs
Compensation level-where photosynthesis ends and
decomposition begins
1. Littoral zone- (horizontal) shallow water
zone where light penetrates the bottom
emergents- plants whose roots are
underwater and stems and leaves are above
water. Live in the littoral zone.
submergents-completely below water
2. Limnetic Zone -Open water zone where
photosynthesis occurs
Plankton- organism that cannot swim
against a current
◦ Phytoplankton- photosynthetic plankton; algae
◦ Zooplankton- animal-like plankton; rotifer
Nekton-free swimming organisms
Is carried out in the limnetic zone by
phytoplankton (ALGAE) and in the littoral
zone by macrophytes- large aquatic plants
(emergents and submergents)
6CO2 + 6H2O + light  C6H12O6 + 6O2
(photosynthetic equation)
1)Title your paper Lentic Ecosystem- mid-summer
2) Draw a cross section of a lake on your paper (this should
take up a considerable amount of room on your paper).
3) Use your Freshwater Ecosystem Notes and laptop to label
following on the cross section of a lake:
Trophogenic Zone, Tropholytic
Zone, Compensation Level,
Littoral Zone, Limnetic Zone
Epilimnion, Metalimnion,
Hypolimnion, Thermocline,
Benthic Zone
4) Use your freshwater Ecosystem Notes and laptop and draw in
examples in their proper places of the following the cross
section of a lake:
sun, sun rays, emergents,
Label where decomposition and
submergents, phytoplankton, fish.
photosynthesis are occurring.
Rainwater runs off - dissolves and carries
nutrients and sediment into lakes.
Water carries with it silt, clay, organic
matter, and nutrients in solution to enrich
the aquatic ecosystem
Human activities including road building,
logging, mining, construction, and
agriculture add to the amount of silt and
organic matter
Eutrophication- nutrient enrichment of an
aquatic ecosystem
Oligotrophication- nutrient reduction in an
aquatic ecosystem
Deeper, steep sides
Poorly developed littoral zone
Clear blue water
High in DO
Poor in nutrients: phosphorous, nitrogen,
and calcium
Few organisms but a high diversity
Very little organic matter
Rich in organic matter and nutrients
DO depletion in hypolimnion during summer
due to decomposition
Eventually will turn into a bog or marsh
Lots of organisms, low diversity
For a healthy ecosystem you want a balance
between eutrophic and oligotrophic.
There needs to be enough nutrients for
organisms to grow and reproduce.

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