Subtidal Benthic Ecosytems, Part I

I. Background
    A. From edge of the continental shelf (<200 m) to the lowest tides – sublittoral zone
    B. Dominated by infauna
        1. Macrofauna -- greater than 0.5 mm
        2. Meiofauna -- 0.5 to 0.062 mm
        3. Microfauna -- less than 0.062 mm
II. Environmental Conditions
    A. More variability than epipelagic or deep sea
    B. Very productive
    C. Important factors
        1. Wave action
        2. Salinity
        3. Temperature - seasonal changes
        4. Light penetration reduced
        5. Food supply abundant -- nutrients; high production of attached plants
        6. few large grazing animals
    D. Community differences based on type of substrate
        1. Regions with soft bottoms
            a. Size of grains, chemical composition, percent organic matter of the sediment are important
            b. More infauna than epifauna
        2. Rocky areas - less infauna than epifauna
    E. Four major habitats
        1. open vegetated sedimentary environments
        2. hard substrates with low-encrusting plants and animals
        3. kelp beds and forests
        4. seagrass beds
III. Unvegetated sedimentary environments
    A. Sampling
        1. Grab samplers –
            a. quantitative - Petersen grab, Smith-McIntyre grab; Reineck box corer

            b. non-quantitative - bottom trawls
        2. SCUBA and Sub or ROV with video
    B. Infaunal associations –
        1. Who's there
            a. producers
                (1) allochthonous (detritus; plankton)
                (2) autochthonous -- mostly diatoms on surface
            b. Class Polychaeta (Phylum Annelida)
                (1) most abundant
                (2) burrow, build tubes
                (3) some are suspension feeders; some scavengers and predators
            c. Crustacea (Phylum Arthropoda) -- mostly on surface
            d. Echinoderms
            e. Molluscs -- burrowing bivalves; some gastropods at surface
        2. Relatively few species dominate
        3. Benthic communities named for their most dominant members
    C. Community Patterns
        1. parallel bottom communities
 

        2. spatial variation
        3. temporal variation -- daily, seasonal, annual, longer
        4. functional groups of organisms
            a. how they feed
                (1) deposit feeders
 
                (2) suspension feeders
                (3) scavengers
                (4) algal grazers
                (5) predators
            b. what they do to the sediments
                (1) stabilizers -- create structures that bind sediment or slow the flow of water

                (2) destabilizers or bioturbators -- activities cause sediment to move, be resuspended,
                        erode or change; many also oxygenate deeper sediments
            c. trophic group
    D. Community Structure
        1. Abiotic factors
            a. Temperature
            b. Oxygen
            c. Wave action
                (1) Sedimentation -- burial
                (2) Erosion – exposure
            d. examples
                (1) California subtidal

                (2) Massachusetts -- tube-dwelling amphipods; snails; polychaetes; Severe winter storms

        2. Biotic factors
            a. Competition
                (1) Direct -- contact between organisms or the direct interference
                    (a) Overgrowth - space competition; not as common in soft sediments as in hard
                    (b) Can be direct aggression
                (2) Indirect -- one organism prevents resource use by the other
                    (a) more common in soft sediments
                    (b) trophic group amensalism -- exclusion of a trophic group by another's modification of the environment

e.g., burrowing deposit feeders abundant where muddy sediments have high organic matter; exclude suspension feeders – fine particles clog filtering apparatus of suspension feeders and bury the suspension feeder larvae
                    i. clams
                    ii. sea cucumber
 
ecosystem engineers - either directly or indirectly control the availability of resources to other organisms by causing changes in the physical state of biotic or abiotic materials and by changing energy flows
facilitation


                    (c) competitive interference -- exclusion by interference with normal activities

            b. Predation
                (1) Surface predators
                    (a) fish, crabs, predatory worms
                    (b) suspension feeders may eat the larvae of conspecifics

                (2) Burrowing predators -- don't disturb sediments
                    (a) ‘weasel-like predators’ - move down tubes and attack the organisms -- nemertean worms; some gastropods

                    (b) Starfish, Pisaster brevispinus can extend its tube feet 20 cm

                (3) Digging predators -- excavate holes
                    (a) Greater effect on the infauna because they disturb the sediment
                    (b) rays
                (4) infaunal predators
                    (a) nemertine worms and predatory polychaetes
                    (b) can consume 20-50% of standing stock of prey
                (5) Often studied by using cages that exclude predators
            c. Recruitment
                (1) should theoretically be important in some cases
                    (a) little mortality
                    (b) high recruitment rate -- swamp predators
                    (c) or overall low recruitment with none of the settling species a competitive dominant
                (2) recruitment of juveniles from motile larvae; of mobile adults

                (3) lottery hypothesis

            d. Disease and parasitism

    E. Vertical Distribution and Competition

        1. Infauna is not distributed randomly in the sediments
        2. Competition for space may select for commensalisms

    F. Community Patterns, Stability and Variability
        1. Stable for several years

        2. Cyclical oscillations between 'stable states'

        3. Mosaic of patches
            Zuma beach in CA
            sand dollar; sea pansy occupies area inshore of the sand dollar

IV. Rocky Subtidal Communities
    A. Who lives there?
        1. Low-profile encrusting species or gregarious forms, kelp forests and coral reefs -- have more types of algae; more primary production in these areas
        2. Motile animals -- sea urchins, crabs and gastropods
        3. Many suspension feeders
            a. colonial encrusting species -- sponges, tunicates, bryozoans, cnidarians
            b. single individuals that settle in groups -- bivalve mollusks, sea urchins…
        4. harder to sample
            a. dredges that scrape the bottom
            b. direct observation and manipulation
    B. Abiotic factors
        1. rock type (may be selected by larvae)
        2. substrate angle is important (slope) – horizontal vs. vertical

        3. disturbance -- waves, currents; creates free space
        4. flow rate -- food, settlement, sedimentation
        5. light
    C. Biotic factors
        1. space competition
        2. grazing -- sea urchins are often important grazers
        3. mass recruitment
        4. mutualism
        5. keystone species (disproportionately important in maintaining community composition) and top down control
        6. disease -- sea urchin and sponge diseases (e.g., paramoebiasis)
    D. Examples

    E. Alternate stable states -- existence in the same area of different groups of organisms
        1. occur when dominant species is disturbed
        2. get shift to new community
        3. example (in book)
            2 islands 4 km apart off of S. Africa; lobsters; mussels; gastropods

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