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Shallow-water subtidal benthic associations Chapter 5

Shallow-water subtidal benthic associations Chapter 5 . General characteristics. To depths of ~ 200m Always underwater (subtidal) Generally soft sands & muds; some rocky subtidal habitats Dominated by infauna Macrofauna (> 0.5 mm) Meiofauna (0.062 – 0.5 mm )

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Shallow-water subtidal benthic associations Chapter 5

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  1. Shallow-water subtidal benthic associationsChapter 5

  2. General characteristics • To depths of ~ 200m • Always underwater (subtidal) • Generally soft sands & muds; some rocky subtidal habitats • Dominated by infauna • Macrofauna (> 0.5 mm) • Meiofauna (0.062 – 0.5 mm) • Microfauna (< 0.062 mm) – bacteria, protists

  3. Variable environmental conditions • turbulent bottom currents, waves, upwelling • little thermal stratification  nutrients rarely limiting  high productivity • bottom sediments can be stirred up & resuspended: particle sorting by wave action • heavy wave action: coarse particles left (sand) • low wave action: fine particles (silts, clays)

  4. Variable environmental conditions • salinity changes are minimal, except at sites near a major terrestrial discharge • seasonal temperature changes • low light penetration (turbidity, plankton) • abundant food: phytoplankton & macroalgae • more variable in many conditions than offshore / deep sea • less variable than intertidal

  5. Subtidal food webs • deposit feeders & suspension feeders dominate • some predators: worms, molluscs, crustaceans, echinoderms • some carnivores: bottom fish

  6. Factors influencing subtidal benthic distributions • distributions are generally patchy • organisms clustered by functional types: • sediment destabilizers / bioturbators • deposit feeders • feeding & defecation result in destabilization • e.g. bivalves (burrowing), some polychaetes • sediment stabilizers • tube builders (e.g. worms, amphipods) • seagrasses • Zonation driven by frequency of disturbance

  7. Trophic group amensalism • Exclusion of one group by the activities of another(Rhodes & Young, 1970) • e.g. deposit feeders exclude suspension feeders • e.g. tube-builders exclude other benthic organisms • Deposit feeders in muddier bottoms • Suspension feeders in sandier bottoms • Still debating the frequency with which these two feeding types co-occur • Some organisms can switch between both feeding types

  8. Subtidal predators Cause of disturbances: surface, burrowing, digging, infaunal predators

  9. Rocky subtidal communities • less common than sedimentary substrates • generally covered with plants & sessile organisms • space is at a premium!! • no burrowing: life in 2-D • biological factors & physical disturbance can structure communities • sea urchins as major disturbers: when absent, algal cover increases • high disturbance alternate steady states:community structure can alternate between different states in short periods of time

  10. Rocky subtidal communities How do the following things change with depth in rocky subtidal habitats? : • Wave energy • Sedimentation • Light • Predators • Plants • Temperature

  11. Kelp beds and kelp forests • Found in cold, temperate regions • On hard subtidal substrates (for holdfast)

  12. Kelp abundances are influenced by: • low nitrogen (e.g. El Niño years on U.S. Pacific Coast) • warm water temps. (same as above) • grazing (sea urchins & barrens) (-) • sea otter feeding on sea urchins (+)

  13. Seagrasses • flowering plants, submerged • creeping rhizomes, roots, blades, inflorescences • stabilize bottom sediments, dampen wave action • high productivity

  14. Seagrasses • diverse communities live in seagrass beds: they create structure & physical refuge • epiphytic & epizoic organisms • burrowers • mobile organisms • not a lot of organisms eat seagrasses (predators: birds, urchins, sea turtles, manatees) • provide food in parts that break off & are transported out of area • *** major exporters of organic material

  15. Polar seas • Ice scour in winter destroys benthic fauna to the depth of ice penetration • Some organisms (fish) have adapted to cold with “antifreeze” in blood • Antarctic zooplankton dominated by a euphausiid (krill) • euphausiids are major prey for whales, seals, penguins, fish

  16. Polar seas • Arctic: covered in pack ice year-round; only 2 outlets for water • Antarctic: less ice year-round than Arctic • circumpolar current around continent • upwelling south of the Antarctic Convergence; nutrient-rich water upwelled from below

  17. Arctic, before ice scour Arctic, after ice scour Arctic vs. Antarctic • Arctic • more permanent ice • fewer species, lower biomass • mostly sedimentary habitats • more frequent disturbances • Antarctic • seasonal ice • greater species diversity • high degree of endemism • high biomass (1-2 “OOM”s) • both sedimentary and hard-bottomed habitats

  18. Sea ice communities

  19. Euphausiids (krill) dominate the Southern Ocean

  20. Impacts of reduced sea ice • Many marine mammals and birds are obligate associates of sea ice • Reproduction: nurture young on sea ice • Molting: birds and seals • Loss of food source for polar organisms, especially euphausiids in Antarctic • Potential impacts on entire food web • Global warming is reducing sea ice cover

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