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Chapter 3

BIOL150 Chapter Notes - Chapter 3: Limnetic Zone, Cyanobacteria, Oceanic Zone

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Rebecca Rooney

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Biology 150 Chapter 3 The Aquatic Environment
Organismal and Evolutionary Ecology Reading notes
3.1 Aquatic Ecosystems
- Nutrient availability, water depth, and water movement are the 3 key physical
factors affect the distribution and abundance or organisms
3.2 Nutrient Availability
- Nutrient levels limit growth rates in the photosynthetic organisms that provide food
for other species
- In still water, nutrient fall to the bottom and collect in the form of debris; in moving
water, nutrient can be washed away
- The 2 important events that affect nutrient availability in lakes and oceans:
(1) Ocean upwelling
Winds blow: along the coast, the prevailing winds blow north, pushing water at
the surface to the north Surface water moves: as the earth rotates, the
moving surface water is forced offshore Upwelling: as the surface water
moves away from the coast, it is replaced by nutrient-laden water moving up
from the ocean bottom
(2) Lake turnover
Winter stratification: Dense 4 degrees Celsius water at the bottom is nutrient
rich but lack of oxygen while colder water near surface becomes oxygenated. A
gradient in temperaturethermocline (heat slope), thermal stratification occurs.
Spring turnover: Surface water warms to 4 degrees Celsius and sinks carrying
oxygen down and driving nutrients up.
Summer stratification: Dense 4 degrees Celsius water at the bottom becomes
nutrient rich while warmer water near surface becomes oxygenated
Fall turnover: Surface water cools to 4 degrees Celsius and sinks, carrying oxygen
down and driving nutrients up
3.3 Water movement/flow
- Water movement has an effect on productivity and is a physical force that
organisms have to contend with
3.4 Water Depth
- Water absorbs and scatters light, so the amount and types of wavelengths
available to organisms change dramatically as depth increases
- Productivity: the total amount of carbon fixed by photosynthesis per unit area
per year
- Only certain wavelengths of light are available under water, and green algae
photosynthesis is driven most efficiently by wavelengths of about 425 nm and
680 nm
- Intensity of light declines with water depth, light penetrates farther in freshwater
than seawater

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Biology 150 Chapter 3 The Aquatic Environment
Organismal and Evolutionary Ecology Reading notes
3.5 Freshwater Environment: Lakes and Ponds
- Lakes and ponds are distinguished from each other by size: ponds are small,
whereas lakes are large enough that the water in them can be mixed by wind and
wave action
- They occur in high latitudes
- Water Depth
(1) Littoral (“seashore”) zone: consists of the shallow waters along the shore
(2) Limnetic (“lake”) zone: offshore and comprises water that receives enough
light to support photosynthesis
(3) Benthic (“depth”) zone: made up of the substrate
(4) Photic zone: regions of the littoral, limnetic, and benthic zones that receive
(5) Aphotic zone: portions of a lake or pond that do not receive sunlight
- Water Flow and Nutrient Availability
Littoral and limnetic zones are typically much warmer and better oxygenated
than the benthic zone, because they receive more solar radiation and are in
contact with oxygen. Benthic zone is more nutrient rich because of the sinking
dead decomposing bodies.
- Organisms
Plankton (cyanobacteria, algae and other microscopic organisms) live in the
photic zone, as do the fish and small crustaceans that eat them. Rooted plants
are common in the shallow parts of the photic zone. Animals that consume dead
organic matter are abundant in the benthic zone.
3.6 Freshwater Environment: Wetlands
- Wetlands are shallow-water habitats where the soil is saturated with water for at
least part of the year
- One thing to be distinguished from terrestrial habitats is the presence of
“indicator” plants that grow only in saturated soils
- Water depth
Different from lakes and ponds for 2 reasons: shallow and have emergent
vegetation (plants that grow above the surface of the water). All or most of the
water in wetlands receives sunlight.
- Water Flow and Nutrient Availability
Types of wetland
Water Flow
Slow but
Slow but
Develop in depressions where
water flow is low or non-existent
Oxygen poor or even anoxic;
nutrient poor
If the water is stagnant, oxygen is sued up during the decomposition of dead
organic matter faster than it enters via diffusion from the atmosphere. Once the
oxygen I the water is deleted, decomposition slows. Organic acids and other
acids build up, lowering the pH of the water. At low pH, nitrogen becomes
unavailable to plants.
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