Subtidal Benthic, Part II.

V. Kelp Beds and Forests
    A. What are they?
        1. macroalgae (brown algae)
        2. kelp beds -- if no surface canopy
        3. kelp forests -- when there is a floating surface canopy; can reach 20-30 meters

    B. Structure and distribution
        1. kelp structure
            a. holdfast -- acts like roots
            b. stipe -- like a stem or trunk
            c. pneumatocyst -- float
            d. blades or fronds -- like leaves
        2. canopy and understory, like a forest
        3. light
        4. hard surface for the holdfasts
        5. temperature and nutrient requirements
            a. take up nutrients directly from seawater
            b. rely on constant water movement to bring nutrients
            c. limited by high temperature (generally comes with low nutrients)
    C. Life cycle
        1. grow from tissues at the holdfast
        2. grow rapidly – up to 50 cm/day
        3. life cycle has an alternation of generations, between an asexual sporophyte (macroscopic kelp)
                and the sexual gametophyte (microscopic -- produces the male and female gametes) --
                patterns of recruitment may be important

    D. Human Uses

    E. Ecology
        1. Net annual productivity of kelp beds (~800-2,000 g C/m2) can be several times the production of phytoplankton
        2. Mortality
            a. waves
            b. nutrient depletion
            c. high water temperatures
            d. Not much of the kelp is actually grazed by herbivores (only ~10%); the rest decays and becomes DOM
                (1) urchins
                    (a) controlled by sea otter predation, disease and storms

                    (b) may limit recovery of kelp from storms and from El Nino events
                    (c) killer whale to sea otter to sea urchin – top down effects on kelps
                (2) fish; equivocal evidence
            3. Zonation (light, wave action)
    F. Factors affecting population dynamics
        1. abiotic – wave action, mechanical damage, light, nutrients, temperature
        2. biotic – grazing, recruitment of sporophytes, keystone predators

VI. Seagrass communities
    A. Composition and distribution
        1. only about 50 species -- not algae; are flowering plants
        2. more in tropics than in temperate zone
        3. very productive -- most common on soft substrate
        4. seagrasses are often colonized by various epiphytic plants and animals
        5. high diversity of animals in seagrass beds
    B. Environmental conditions
        1. abiotic conditions
            a. water and sediment movement (tend to accumulate sediment)
                (1) temperate sea grass beds (Zostera) - accumulate externally derived sediment
                (2) tropical sea grass beds accumulate sediment/organic matter from the bed itself
            b. light and turbidity (occur to depths of 50 or 60 meters)
            c. temperature --
                (1) ice scouring
                (2) high temperatures (>30 degrees C) can be fatal to temperate species
            d. desiccation
            e. anaerobic conditions - organic sediments (bacterial respiration); promotes nutrient recycling
            f. tolerant of salinity changes
        2. biotic conditions
            a. grazing -- few organisms graze the seagrass itself

            b. disease -- 'wasting disease'
            c. competition for space and light
    C. Productivity (tropical rainforest is 1,000-3,500 g C/m2 /yr, as is cultivated land)
        1. temperate – 500-1,000 g C/m2 /yr
        2. tropical - >4,000 g C/m2 /yr
    D. Structure and Biological Interactions
        1. may be more biomass of epiphytes and epizooics than of seagrass!
        2. epiphytes may shade the seagrass
        3. protection from predation for other organisms (clams, fish)
        4. protection from sun and desiccation for other organisms
    E. Relationships to other systems
        1. like a forest – lots of standing biomass that is relatively inedible
        2. important detrital component -- export DOM to many other systems
        3. important to sediment stabilization – depositional environment
        4. refuge for animals that move between systems
        5. nursery grounds in tropical areas
    F. Succession
        1. not seen in Zostera (temperate) -- colonizes directly
        2. In temperate Pacific, Phyllospadix colonizes hard substrates (exposed) after the green algae Ulva and some red algae invade
        3. Thalassia seems to vary with substrate -- requiring algae and/or mussels
    G. Human uses/services
        1. commercially important species -- scallops
        2. stabilize seashore

VII. Polar seas
    A. Physical conditions of the Arctic and Antarctic
        1. Common conditions
            a. ice and snow
            b. cold year-round
            c. drastic changes in photoperiod over the year
        2. Arctic ocean – beyond 70 degrees N
            a. cold sea with perennial ice layer (pack ice -- hard and thick -- 3-4 m)
            b. land around the arctic ocean has lots of shallow shelves
            c. arctic basin is deep and isolated from the Atlantic and Pacific with shallow connections at the
                    Bering Strait (70 m deep) and the Fram Strait (400 m)
            d. some large river input – brings sediments and freshwater
        3. Antarctic ocean -- 50-60 and 65-70 degrees S
            a. Antarctic is an ice-covered land mass
            b. continent surrounded by a circumpolar ring of water
            c. continental shelf is narrow
            d. connected to both oceans
            e. pack ice is usually seasonal; 1-2 m thick
            f. no rivers, no sediment input

    B. Abiotic factors
        1. Wave action is not as important as in other benthic communities
        2. Ice is more important
            a. scours bottom and freezes and removes organisms -- disturbed zone
            b. anchor ice forms on the bottom to about 30 m

    C. Biological comparisons of the Arctic and Antarctic
        1. Thorson’s rule (cold water benthic organisms should exhibit a predominance of nonpelagic development)
                does not hold in the Antarctic (but pelagic larvae develop slowly)
        2. Antarctic
            a. greater benthic species diversity and endemism than the arctic
            b. high biomass (10-100X that in the arctic)
            c. mostly epifauna
            d. fish less important
                (1) but many are endemic;
                (2) have ‘antifreeze’ and specialized physiological adaptations to cold
            e. many birds at sea ice edge – similar around entire Antarctic
        3. Arctic fauna
            a. many forms of Atlantic origin
            b. mostly infauna (lots of sediment)
            c. no specialized endemic fish
            d. few birds at sea ice; birds differ longitudinally
        4. Dominant species
            a. Antarctic - euphausid shrimp is Euphausia superba
                (1) reaches 6 cm!
                (2) eaten by whales, seals, penguins, fish
            b. No such dominant species in the Arctic Ocean

    D. Ecological communities of the polar seas
        1. sea ice communities
            a. no apparent freshwater parallel
            b. well-developed microbial food web
            c. over 200 species live in association with sea ice
            d. abundant at thin edges with more light
            e. algae are grazed by amphipods and krill

        2. Antarctic soft-sediment community
            a. Less common
            b. Three zones:
                (1) canopy (anemone)
                (2) understory (crustacean deposit feeders and predators)
                (3) subsurface (polychaetes)

        3. Antarctic hard-bottom community
            a. 3 zones
                (1) 0-15 m depth – ice scouring allows only mobile organisms to survive; nothing there during ice periods
                (2) 15-30 m depth – anchor ice has less influence; cnidarians (mostly anemones) dominate – sessile animals
                (3) >30 m – no anchor ice; sponges dominate, but there is a diverse fauna; very stable; seems to be regulated by predation

 Return to Marine Ecology Lecture homepage

 Return to K.L. Schulz's homepage