Stream Ecology (NR 280)

Chapter 1 – Introduction to Fluvial SystemsBasic Concepts

Stream Order – Strahler NumberArthur Strahler (1957) after Robert Horton (1945)

Source: http://www.krisweb.com/stream/stream_order_kris.htm

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• 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