Stream Ecology (NR 280)
Chapter 1 – Introduction to Fluvial Systems
Basic Concepts
Stream Order – Strahler Number
Arthur Strahler (1957) after Robert Horton (1945)
Source: http://www.krisweb.com/stream/stream_order_kris.
River continuum concept
• Vannote et al. (1980)
Canadian Journal of
Fisheries and Aquatic
Sciences
• Rivers as gradients:
An array of physical, chemical
and biological characteristics
change continually and
gradually with distance
downstream
Figure from the original paper
RCC Forcing
Factors
• Slope decline
• Discharge increase
Note: Hypothesized decrease
in velocity is not true
How might these
landscape factors
affect “master”
environmental
variables like light
and temperature?
Longitudinal Trend in Temperature
(and Light)
• Tends to decrease with elevation drop and
canopy spreading
• Daily range in temperature is most extreme in
middle reaches- canopy is open and depth is
shallow
Light and Temperature Strongly
Control Primary production
• Primary Production (P): the
product of photosynthesis
• Respiration (R): the product
of energy generation
• What happens to biomass if
P<R? P>R? P=R?
• In the Vannote et al. where
might P:R be greatest?
Lowest?
Is primary production the primary
source of “fuel” for stream ecosystems?
• Detritus – biomass that was
once alive and is now dead.
Potential food.
• Autochthonous production –
produced within the stream (i.e.
primary production)
• Allochthonous production –
produced in the terrestrial
environment and transported to
streams
• Note: All three of these terms
were borrowed from Geology!
Energy sources for the stream foodweb
• Detritus is continually
carried downstream
and processed along
the way
• CPOM becomes FPOM
• FPOM becomes finer
and more recalcitrant
• The stream is a detritus
processing factory
Logical consequences of the RCC
• What benthic macroinvertebrates
eat determines how they are
ordered along the longitudinal
profile (functional groups)
– Shredders: low-order streams
– Grazers: middle order
– Filter Feeders: middle order??
– Collectors: high-order
– Predators: evenly distributed
• Fish respond to physical habitat and
to food resources.
Linkages between physical habitat,
food resources, and biotic diversity
Challenges to the RCC
• RCC assumes a forested
watershed
– Bulk of stream studies in such
systems
• Many streams run through
prairie, agricultural areas, deserts,
tundra or alpine regions
– Tree leaf input minimal
– No canopy
– Here autotrophs may peak at low
order, P>R throughout
Challenges to the RCC
• The amount of streamside
vegetation is assumed to remain
constant
• In fact riparian vegetation is
discontinuous
– Streams pass through areas of logging,
agriculture, villages…
– Canopy opens and closes; CPOM input varies
• CPOM input is season dependent
and so is stream shading and solar
input
Challenges to the RCC
• Dams disrupt the continuum
– Reservoirs alter temperature,
sediment, and flow regimes,
altering productivity and
diversity.
• The Serial Discontinuity Concept
(Ward and Stanford 1983)
– Each impoundment sets back the
river continuum to a new starting
point
Challenges to the RCC
• Assumption that autotrophs are
light limited may not always be
true
– If nutrients are limiting, productivity
will not increase in middle reaches
no matter how much the canopy
opens
• Assumption that food resources
or physical factors control animal
populations can be wrong
– They may be controlled by predators
Challenges to the RCC
• Assumption that all (or most) FPOM is
produced upstream and carried
downstream could be wrong
– Doesn’t take into account inputs from
backwaters, marshes and the floodplain
• The Flood-pulse Concept (Junk et al.
1989) presents an alternative (tropics)
– The most important hydrological feature of
large rivers is the annual flood pulse, which
extends the river out onto its floodplain
– Biotic communities have adapted to this
pulse and use it to exploit the floodplain
environment
Which concept is right?
• In environmental sciences a
theory that is uniformly
“right” for all places is an
anomaly. (Doesn’t exist?)
• Elements of each of these
theories has merit in some
systems.
• The RCC spawned decades of
productive research on
streams and rivers that has
contributed substantially to
our knowledge of stream
ecosystems.
Opportunities for future research
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Lateral organic matter and nutrient exchange
Hyporheic processes
The effects of nutrient spiraling
Processing of DOM and DOC
Impacts of high flow events
Minimum flow thresholds (“river needs”)
Climate change impacts
Research on large rivers in general
Research on urban or highly developed rivers