Current Topics in Streams and Benthos

- 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

    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
d. Review of predicted patterns
        2. Modifications
           a. Streams enter the continuum at different points.
           b. Example
    B. Nutrient spiraling
        1. downstream fluxes of nutrients
        2. rate at which nutrients are recycled
        3. ratio of flux to recycling – gives average downstream distance traveled by a nutrient atom during a cycle – spiraling length

        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

Nutrient Spiraling Concept

    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

Serial Discontinuity Concept

    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).

    G. Geomorphic-Trophic Hypothesis

        1. LTER in Alaska, Hershey et al. 1999
        2.Topography and landscape position alter not only the biogeochemistry (already known),
                    but also the invasibility of aquatic habitats, and this can determine later ecological interactions
        3. Fish communities of lakes connected by streams are determined by basin steepness (streams impassible to certain fish),
            lake depth (freeze to bottom), lake size
        4. The fish communities are fairly predictable (~88% accuracy) based on these abiotic characteristics
        5. The fish regulate the types of benthos found in those lakes
        6. Arctic is a relatively simple system - how will this hold up in other systems?

    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

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