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Lecture 09
Limnology - study of inland waters
Ecology of Freshwater Ecosystems:
Rivers, streams, lakes and wetlands
Three basic types of freshwater ecosystems:
• Lotic: rapidly-moving water, for example
streams and rivers.
• Lentic: slow-moving water, including pools,
ponds, and lakes.
• Wetlands: areas where the soil is
saturated or inundated for at least part of
the time.
Loctic: Rivers and Streams
• Rivers and streams can be divided along
three dimensions:
– Length: Pools + runs
– Width : Wetted / active channels
– Vertical: Water surface, column
• Riparian zone is a transition area between
the aquatic and upland terrestrial
• Rivers and Streams
• River basin – area of
land drained by river
drainage network
– Separated by
Dimensions of Stream Structure
Flow Rate
• Variation a Function of:
– vegetation/nature of surrounding landscape
• Forests ‘damp out’ variation in flow
– Seasonal variation in ppt.
• Rainy/dry season climate
• Seasonal snow melt
• Impact on flooding
• Flood pulse concept: health of river
system depends on maintaining natural
variations in flow
River Continuum Concept
• Organic matter from vegetation adjacent to stream in headwaters
provides nutrient base coarse particulate organic matter
• CPOM  FPOM: provides nutrient base for food web in low
reaches of river
• See:
for more
• 4 Invert Feeding groups:
• Feeding on bacteria in organic matter of various sizes:
1. Shredders: breakdown CPOM: consume bacteria
2. Filtering and gathering collector: feed on bacteria found in FPOM
3. Grazers: feed on algae on surfaces
4. Gougers: burrow into submerged logs/wood debris
• Low spot – captures and retains water
• Formation involves geologic processes + dam building by
• Most FW resides in lakes.
– 20% in Great Lakes of North America
Many Types of Lakes - + 11 types
• Glacial lakes
• Tectonic Lakes
• Landslide lakes
• Volcanic lakes
• Shoreline lakes
• Littoral zone: Shallows –
light reaches bottom
• Limnetic zone: open
waters –
– Habitat of zooplankton
and nekton (freeswimming organisms)
• Benthic: primary place
of decomposition
• Light
– Lake color depends on light absorption and
biological activity
– Light is increasingly attenuated with water depth
• Temperature
– Lakes become thermally stratified as they warm.
– Temperatures vary seasonally with depth
• Water Movement
– Wind-driven and temperature mixing of the water
column is ecologically important.
• Oxygen can be limiting
– Eutrophic vs. oligotrophic lakes
Seasonal Temperature
Nutrients and Lakes:
• Oligotrophic:
Low nutrient availability
Low surface area to volume ratio
Low biological production
well oxygenated
May have high species diversity
Generally older lakes
• Eutrophic:
High biological production
High nutrient availability – particularly N and K
High surface area to volume ratio
may be depleted of oxygen – benthos anaerobic 
methane production
• Dystrophic:
– Acidic, high in organic matter
From Schoenherr A.A. 1992. A Natural History of California. University of California Press..
Lakes - Human Influences
• Human populations
have had profound,
usually negative effect.
– Municipal and
agricultural run-off 
– Exotic species - Zebra
Mussels  alter
• Major dam builders
– Humans
– Beaver
• Damming interrupts both nutrient spiraling and the
river continuum
– Downstream flow is greatly reduced but a constant
inflow is maintained
– The resulting lake develops a heavy bloom of
phytoplankton (or floating plants) due to high nutrient
levels of decaying material on the newly flooded land
– Disrupts seasonal fluctuations in flow
• Lentic-adapted fish (many introduced exotics) replace loticadapted fish
• Pulsed releases of water (hydroelectric dams) can wipe out
or dislodge downstream organisms
• Generally water released from hypolimnon is cold and low in
• Accumulated sediment  wet meadow
• Nutrients from outside lake  eutrophication
• Freshwater wetlands (25.6) = terrestrial
• 6% of Earth’s surface – declining
• Importance – various reasons
• Various types/degree of wetness
– permanently flooded to periodically saturated soil
• Hydrophytic plants
– Obligate wetland plants require saturated soils
– Facultative wetland plants can grow in either
saturated or upland soil
– Occasional wetland plants are usually found out
of wetland environments but can tolerate
25.6 Freshwater Wetlands Are a
Diverse Group of Ecosystems
• Wetlands occur in three topographic
– Basin wetlands develop in shallow basins,
from upland depressions to filled-in lakes and
ponds–water flow is vertical
– Riverine wetlands develop along shallow and
periodically flooded banks of rivers–water flow
is unidirectional
– Fringe wetlands occur along the coasts of
large lakes–water flow is in two directions
Reduced Oxygen Levels and Wetlands as
Biological Filters
• decreased breakdown of organic matter
• Wetlands = nutrient sinks
– Nutrient sink = environment that traps
• Nutrient source = net release of nutrients – results
as oxygen is introduced
• Biological filter: remove potentially
damaging chemicals from waterways
Hydrologic Regulators
• Act as hydrologic buffers
– Water amounts entering may vary
• Excess water absorbed and retained
– Water amount leaving remains constant
• Impact:
– Water may percolate into aquifers
– Prevent down stream flooding
• Prevent damage
• Reduce leaching of nutrients
Ecological Issues: The
Continuing Decline of Wetlands
• The loss of wetlands has reached a point
where both environmental and
socioeconomic values are in jeopardy
– Waterfowl habitat
– Groundwater supply and quality
– Floodwater storage
– Sediment trapping
Wetland Protection
Endangered Species Act
Clean Water Act of 1972 – section 404
State Laws
– California Coastal Commission
– Other: