Kelp Forests

Kelp Forests Overview

  • Kelp Beds and Forests
    • Dominated by brown seaweeds, particularly from the group Laminariales.
    • Size can vary significantly; growth rates can be quite high.
    • Common species: Macrocystis and Laminaria which form beds or forests.

Kelp Requirements

  • Environmental Conditions for Growth:
    • Cold water: optimal temperature generally less than 20˚C.
    • Hard substrates required for anchoring.
    • Continuous supply of nutrients must be available in the water.
    • Sufficient light: primarily found at depths less than 30 m; may exist in deeper tropics where water is clear and nutrient-rich.

Distribution of Kelp Beds

  • Kelp forests are distributed globally, with dominant genera being:
    • Laminaria
    • Macrocystis
  • Distribution patterns indicate significant geographic diversity, affected by factors like temperature and nutrient availability.

Growth Dynamics of Kelp

  • Growth Patterns:
    • Kelp blades grow towards light; Macrocystis features gas-filled floats aiding in buoyancy.
    • Macrocystis can grow up to 50 cm/day, with a frond lifespan of about 6 months. Total lifespan can range from 1 to 7 years.
    • Laminaria grows from the base up; older tissue is subject to erosion.
    • Kelp is a vital source of organic detritus, supporting ecosystems.

Kelp Community Interactions

  • Biodiversity and Species Interaction:
    • Kelp forests provide food, shelter, and habitat for numerous organisms including:
    • Large fish
    • Smaller invertebrates and algae attached to kelp blades, holdfasts, or sea floor
    • Kelp communities have high species diversity, including herbivores, carnivores, and scavengers.

Grazing Impact on Kelp

  • Grazers:
    • Only a small fraction (10%) of kelp production is consumed.
    • Main grazers include sea urchins, abalone, limpets, certain fish, and gribbles.
    • Historical example: the Steller's sea cow was a significant grazer but is now extinct.

Urchin Population Dynamics

  • Damage from Urchins:
    • Urchins are major grazers that can substantially damage kelp holdfasts and young sporophytes.
    • Natural predators such as sea otters, lobsters, and cod help regulate urchin populations.
    • A decline in kelp beds off Nova Scotia has been linked to increased urchin populations due to overfishing of their predators.

Sea Otters as Keystone Predators

  • Role in Kelp Ecosystems:
    • Sea otters exert top-down control on urchin populations, maintaining kelp health.
    • Historically abundant on the Pacific coast but nearly driven to extinction in the 19th century.
    • They have made a successful comeback since being protected in 1911, especially in California.

Factors Affecting Kelp Community Structure

  • Control Dynamics:
    • Kelp communities may be affected by a combination of top-down (predation) and bottom-up (nutrient supply) controls.
    • Important influences include:
    • Storm events
    • Behavior of herbivores
    • Presence or absence of sea otters.

Effects of Storms on Kelp Forests

  • Storm Impact Variation:
    • In North America's West Coast, storms can uproot and damage kelp forests.
    • Upper canopy species are especially susceptible.
    • El Niño events can affect kelp health by deepening the thermocline, reducing nutrient levels, and resulting in more frequent storms.

Urchin Behavioral Changes Post-Storms

  • Changes in Urchin Mobility:
    • Urchins typically are sedentary in dense kelp forests, fending off drift kelp.
    • Post-storm, as kelp disappears, urchins become more mobile, grazing on newly recruited seaweeds, potentially leading to 'urchin barrens'.

Stability and Community Succession

  • Stable States vs. Urchin Impact:
    • Kelp abundance supports sedentary feeding behavior in urchins.
    • After a storm, with loss of drift algae, urchin mobility increases and can inhibit kelp recovery.
    • Increased roving behavior results from higher urchin densities, establishing a cycle promoting barren grounds.

Importance of Urchin Predators in Kelp Forests

  • Case Studies:
    • Research from the Aleutian Islands revealed:
    • Amchitka Island: dense otter population correlates to low urchin density and healthy kelp forests.
    • Shemya Island: absence of otters leads to high urchin density and no kelp forests.
    • Historically, predators like Pycnopodia helianthoides contribute to kelp patchiness.

Trophic Cascade Effects

  • Interactions and Biomass Relationships:
    • Study data illustrate the relationship between sea otter abundance and urchin biomass, affecting overall kelp density.

Southern California Kelp Food Web

  • Predator-Prey Dynamics:
    • Various predators including starfish and octopus influence urchin and kelp populations within the ecosystem, highlighting the complex food web interactions in the kelp forest.

Historical Human Interaction with Kelp Forests

  • Kelp Highway Theory:
    • The colonization patterns of the Americas may have been influenced by human fishing communities following kelp-rich coastlines from Northeast Asia to the Pacific Coast during the last ice age.
    • Kelp provided critical resources such as stable food sources and a transport buffer for boats, revealing its importance in human history.