•Lakes have zonation structured by physical forces such as light, wind and waves.
•different zones in the lake had different types of plants and animals
•Zones in a river system are less
distinct
•But they are functionally very
important
The River Continuum Concept
Physical forces change gradually along a river
•Elevation ↓
•Slope ↓
•Temperature and nutrients ↑
•Drainage area and discharge ↑
•Width of channel and floodplain ↑
•Mean velocity ↑
•Mean depth ↑
•Turbidity ↑
•Sediments, erosional, alluvial, to depositional
•Shading ↓
•Periphyton, macrophytes ↑, then ↓
•Phytoplankton and zooplankton ↑
•Coarse detritus input highest upstream
•Fine detritus accumulates downstream
•Benthic invertebrate community changes
shredders, grazers, collectors
•Fish community changes
•Cold water to warm water species
http://www.d.umn.edu/~seawww/depth/rivers/art/figure1_4.jpg
Allochthonous input—Detritus processing
•Dead plant biomass breaks down slowly
and their nutrients can remain tied up in as
organic detritus for long periods of time
•Primary production in many ecosystems
depends more on its recycling rate ie mainly
decomposition of plant detritus, than on
loading rates
•Aquatic plants break down more rapidly
than terrestrial plants, and woody plants are
very slow to decompose because they
contain lignin, which most bacteria and fungi
can’t digest.
Leaf processing
•Wetting and breadown of cuticle
•Leaching of soluble components (DOM)
•Colonization by bacteria and fungi
•Increase in protein content
•Colonization by invertebrates
•Enhances microbial action
•Breakdown into small fragments
Invertebrate detritiivores find
leaves much more to their
liking after they have been
colonized by bacteria and
fungi
Detritus processing in a stream
Shredders enhance
microbial action
(bacteria & fungi)
•convert CPOM to FPOM
•Food for microdetritivores
Processing of FPOM by microdetritivores
Shredders-macrodetritivores
collectors-microdetritivores
Filter-feeders, deposit-feeders
Litter bag experiments have been used to study decomposition of detritus
•Nutrient content of the
detritus, especially N
greatly increases
decomposition rate,
•as does increased
temperature
•and mesh size
100 %
Weight
remaining %
Larger invertebrates get
into the litter bags if the
mesh is coarse
0.5 mm mesh
2 mm mesh
10
20
days
30
The interplay between the autochothonous and the allochtonous food chain
Allochthonous input
Autochthonous input
•Mountain and prairie
headwater streams are
unshaded
•Therefore less detritus
dominated than eastern
deciduous streams which
start at lower altitude
•Their seasonal range of
discharges also tends to
be higher than most
eastern streams.
http://www.btwatershed.org/Newsletter/2002Newsletter/Fall2002/nabs%20river%20continuum%20concept20%25.jpg
The River Discontinuum: Dams and wiers
Stream Fragmentation, A wier blocking fish movement
a hanging culvert can block fish movement
http://www.cee.mtu.edu/~dwatkins/images/aqua3pics/hatchery-weir.jpg
http://www.nzfreshwater.org/thumbnails/culvert.jpg
Dams/Reservoirs interrupt the river continuum