I. Zones and distributions
    A. Physical factors
        1. Changes in water levels – seasonal; some unpredictable
        2. Changes in temperature – changes oxygen capacity
            a. Small streams; unshaded streams
            b. Stratification rare except in pools.
        3. Oxygen
        4. Chemistry – determined by catchment:
            “In every respect, the valley rules the stream” (Hynes 1975)

        5. Light – if stream has canopy or is turbid, low light may limit primary production in the stream itself
        6. Flow
            a. Advantages
                1. respiration
                2. filter feeding
                3. transportation (if organisms can control it)
                4. chemical communication – water flow increases chemical movements – prey can detect upstream predators

            b. Disadvantages –
                1. can dislodge organisms
                2. shearing action of flowing water transports and deposits material, continually changing the physical environment
    B. Riparian zone – normally above water line; may be inundated during floods
        1. Allochthonous inputs – inputs to the system from outside – DOM, leaves, etc.
        2. Water and nutrient inputs

            Chemical transformation -- e.g.,  NH₄⁺ to NO₃^-
             adsorption of nutrients

    C. Shore zone – often bare; colonization difficult – water level often fluctuates
    D. Water column
        1. Potamoplankton – river plankton; usually algae
        2. ‘tychoplankton’ – don’t belong there but are washed in
        3. drift – mostly aquatic insects – organisms being carried downstream; may include zooplankton in large rivers
        4. fish
    E. Benthos - – attached or free-living on bottom
        1. Aufwuchs – fungi, algae, bacteria, protozoans and some organisms feeding on them
            a. Epipelic
            b. Epilithic
            c. Epiphytic
        2. rooted plants
        3. animals: aquatic insects, mollusks, fish
    F. Hyporheic – “below current”

II. Adaptations
    A. Algae
        1. firmly attached to hard substrates
        2. motile
        3. body form
            a. flattened – trying to remain in boundary layer where there is little current
            b. trailing filaments – increase exposure to nutrients
    B. Higher plants (angiosperms, liverworts, mosses)
        1. attached to rocks
        2. rooted in substrate – tough yet flexible stems
    C. Potamoplankton
        1. River size – there are more potamoplankton as go downstream with increased size of stream and often get decreased velocity areas
                (pools)
        2. No special adaptations
        3. Seasonal changes due to export from nearby quieter waters

III. Benthic Invertebrates – most adaptations, wide phylogenetic diversity
    1. Mollusca (Gastropoda, Bivalvia);
    2. Turbellaria (flatworms)
    3. Crustacea (crayfish, amphipods, isopods),
    4. Oligochaetes, Hirudinea (leeches)
    5. Acari (water mites), Porifera (sponges)
    6. Cnidaria (hydra)
    7. Nematoda (roundworms)
    8. Major orders of stream insects
        a. Plecoptera - stone flies; mostly in temperate regions; rare in tropics; cool, clean streams of low orders;
                sensitive to low oxygen; tolerant of low pH; adults are poor fliers
        b. Trichoptera - caddis flies; worldwide distribution; both free-living case-building species
        c. Ephemeroptera – mayflies; world-wide distribution; gills for respiration; sensitive to low pH; adult lifespan short and do not feed 
            as adults.
        d. Odonata - dragonflies and damselflies; occur worldwide predators; stalk their prey; can eat vertebrates as well
        e. Diptera (true flies)- midges(Chironomidae - nonbiting midges); black flies (Simuliidae)
        f. Coleoptera (beetles) aquatic beetles tend to live in water both as larvae and as adults
 

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