- New models and hypotheses are more often considering the stream and its watershed together and incorporating changes
    (rivers are not static systems)

Synthetic Conceptual Models

REVIEW
    A. River Continuum Concept – RCC
        1. Classic model

a. Rivers are dominated by physical factors that change chemistry and nutrient sources
b. These abiotic factors select for specific ecological communities
c. The kinds of producers, consumers, organic inputs, degree of autochthonous and allocthonous production are all
     predictable
d. Review of predicted patterns

        4. Predictions about how the spiraling length will be affected by different retention and biological processing rates and how ecosystem
               responses and stability will react

    C. Serial Discontinuity
        1. Related to RCC
        2. Ward and Stanford (1983)
        3. concern with dams and impoundments instead of pristine systems
        4. argued that regulating structures reset the river continuum (not always in the same direction)
        5. discontinuity distance
        6. parameter intensity

    D. Flood Pulse Concept
        1. Junk et al. 1989
        2. This deals with rivers that interact strongly with the floodplain by rising out of the channel bed – large unregulated rivers
        3. Incorporates a lateral dimension
        4. Nonflood periods – floodplain (like a wetland) has its own nutrient cycles
        5. Seasonal flood sends nutrients and river biota over the floodplain
        6. If pulses are predictable then organisms are adapted to take advantage of the pulse because it amplifies resource availability
        7. Flood pulse is postulated to enhance diversity and productivity by structuring the plants, nutrients, detritus, and sediments
        8. During flood is a release of nutrients – maximum aquatic production
        9. Subsequent decomposition and retreat of floodwaters
        10. Very relevant to tropics

    E. Hydraulic food-chain model
        1. Power et al. 1995
        2. Hydrologic fluctuations impose mortality on stream benthos;
                reset the system before competitive exclusion or predator-induced extinction can occur
        3. Therefore natural hydrological fluctuations may enhance the persistence of ecological communities
                by reducing the chances that their constituent populations will go extinct
        4. Attempt to link mathematically predator prey and competition models with hydrodynamic flow of streams
        5. Predicts loss of trophic levels and changes in communities when flow regime is altered

    F. Telescoping Ecosystem Model (TEM)
        1. Fisher et al. 1998
        2. Typical stream review
            a. Hyporheic zone – saturated sediments below the stream
            b. Parafluvial zone – mud or gravel overlying the saturated soils
            c. Riparian zone – bankside vegetation
        3. This model emphasizes heterogeneity in space and time
        4. Focus - what landscape factors influence the ability of rivers to retain matter
        5. Processing length – length of subsystem required for biogeochemical transformation of some substance (intentionally vague)
        6. Key difference with nutrient spiraling is that processing lengths in the hyporheic, parafluvial and riparian are thought to be quite different
            -e.g., NO₃ concentration in Sycamore Creek, Arizona
              
        7. They think of river systems as going through seasonal succession, with shorter and shorter processing lengths until an
                episodic disturbance (natural would be floods, but could be fire, dumping of pollutants) resets the system
        8. Analogy of stream system zonation and differences in processing lengths in each to cylinders of a telescope
        9. Predictions:
                a. After a disturbance, the different subsystems will alter their processing lengths, but differently.
                b. The resistance of the system – how little it changes when perturbed, is inversely proportional to the processing
                    length change and will be greatest away from the center of the telescope (greatest change in the stream)
                b. The resilience of the stream telescope (how fast it returns to the previous state) increases toward the center
                    (fastest return in the stream).

    H. Human impacts on rivers – threats to biodiversity (modified from Limburg et al. 2000)
 

Human Impact	Proximate Effects	Productivity change	Dominant species change	Species richness change
Damming	Decrease and altered patterns of flow, sediment flux and turbidity	Decreases downstream due to nutrient reduction (used up in lake behind dam)	Shifts in algae associated with changes in nutrient ratios, flow reduction, and barriers to migration	Decline in migratory species
Stream channelization	Increase in flow velocity, disruption of benthos and hyporheos	?	Species shifts to those that can withstand high flow	?
Nutrient loading	Increased production (and consequent increase in decomposition)	Increase in GPP, respiration and oxygen demand	Species shifts associated with changes in nutrient ratios and oxygen levels	Declines if there is anoxia; may have declines in richness with eutrophication
Toxic substance loading	Increased mortality of resident species; reproductive failure	May decrease	Shifts to tolerant/resistant species	Short-term decline
Exotic species	Increase competition and possibly predation	?	Can be dramatic	If successful invasion, species richness often decreases
Land use change (logging, urbanization, agriculture)	Often increased sediment and nutrients; hydrological alterations; possible toxic increases	Can be increase with increased nutrients, or decrease with increased sediment turbidity	Shifts -- Depends on type of abiotic change	Often decreases
Overharvesting of species	Depletion of target species; alters predator/prey dynamics	?	Shifts to nontarget species; food chain effects	?
Climate change	Changes in temperature, precipitation, evaporation, and atmospheric CO2	Depends on location	?	?

Return to Limnology Lecture homepage

 Return to K.L. Schulz's homepage