Coral Reefs

Coral Reefs

  • Introduction

    • Study of coral reefs in Biol 3711, Winter 2025.

  • Tropical Waters

    • Characterized by relatively nutrient-poor ocean waters.
    • Clear, blue water due to low plankton levels.
    • Coral reefs function as oases in a desert.

  • Coral Reefs Overview

    • Most diverse marine habitats with significant biodiversity.
    • Framework builders include:
    • Corals: Precipitate calcium carbonate.
    • Coralline Algae: Contributes to structural complexity.
    • Sponges: Play a role in framework stability.
    • Living surface growth occurs upwards and seawards, contributing to high diversity.

  • Reef-Building Corals

    • Comprised of scleractinian corals (or hermatypic corals).
    • Mostly colonial; consist of thousands of interconnected polyps.
    • Predatory with nematocysts or suspension feeders with symbiotic zooxanthellae (tropical only).
    • Types include:
    • Massive forms: Characterized by slow growth.
    • Branching forms: Show rapid growth.

  • Zooxanthellae

    • Dinoflagellate symbionts residing in coral endodermal tissue.
    • Located concentrated in tentacles of corals.
    • At least 7 independent clades, with 4 found in corals.
    • Acquired from the environment, with distinct clades adapted for various light conditions.

  • Mutualism Benefits

    • Corals provide protection and nutrients (e.g., nitrogen in the form of NH4) to zooxanthellae.
    • Zooxanthellae translocate up to 90% of coral's carbon requirements through carbohydrates.
    • Enhance calcification in corals.

  • Distribution of Coral Reefs

    • Factors limiting reefs include:
    • Temperature (optimum at 26-27˚C; restricted to between 25˚N and 25˚S).
    • Salinity, light, sedimentation, and exposure.
    • Absent from freshwater areas and regions with high sediment loads.

  • Development of Reefs

    • Balance between coral growth and bioerosion.
    • Coral growth is extremely slow (1 cm per 1000 years in Jamaica).
    • Bioerosion by organisms such as boring invertebrates (sponges) and parrotfish.
    • Fine calcareous sediments produced contribute to cementation from invertebrates.

  • Types of Reefs

    • Atolls: Circular or horseshoe-shaped islands forming around a lagoon, typically from subsiding volcanoes.
    • Coastal Reefs: Structures near coastlines but separated from it, such as the Great Barrier Reef.
    • Fringing Reefs: Thin layers of coral on hard substrate, like found in the Red Sea.

  • Zonation in Reefs

    • Dominance of different coral species varies by depth due to factors like:
    • Wave and current strength.
    • Light availability.
    • Suspended sediments.
    • Space competition leads to overgrowth.

  • Reef Zones

    • Reef Front: Seaward side with rich coral growth; energy dispersed here.
    • Reef Crest: Highest point, faces most sunlight and wave energy; may have algal ridges.
    • Reef Flat: Often contains seagrass beds and exhibits lower wave action.

  • Coral Form and Zonation

    • Reef Front: Dominated by massive forms such as brain corals.
    • Reef Crest: Usually features branching forms due to increased stress.
    • Reef Flat: Hosting small, delicate coral species in calm water.

  • Primary Production

    • Major contributors include zooxanthellae, seaweeds, phytoplankton, and seagrasses, with combined biomass exceeding that of reef animals.
    • Symbiotic relationships allow a direct, efficient transfer of nutrients to consumers.
    • Nutrient enrichment can lead to eutrophication, favoring seaweeds over corals.

  • Seaweeds and Herbivores

    • Seaweeds thrive in lagoons and on reef surfaces.
    • Herbivores play a critical role in reducing algal cover, aiding coral recruitment.
    • Key species include parrotfishes and surgeonfishes, characterized by specialized mouthparts.
    • Urchins, particularly Diadema, also contribute to this dynamic.

  • Corallivores

    • Organisms like fish, molluscs, and echinoderms that feed on coral.
    • The balance of corallivores can influence reef health:
    • High abundance in Marine Protected Areas (MPAs) leads to healthier reefs.
    • Crown-of-thorns sea star population increases can be detrimental to coral reefs.

  • Top Predators in Reefs

    • Larger predatory fish that exert top-down control on reef communities.
    • Presence of top predators in protected reefs enhances coral recruitment.

  • Threats to Coral Reefs

    • Natural threats, such as hurricanes and El Niño events, can cause significant disruption.

  • Human Activities Impacting Coral Reefs

    • Destructive fishing practices lead to overfishing of herbivores, promoting algal overgrowth.
    • Harmful fishing methods include:
    • Use of poisonous chemicals.
    • Explosives and bottom trawling.
    • Coastal development generates nutrient runoff and increases sedimentation, changing water flow patterns.
    • Coral mining for use in construction and jewelry remains a concern.

  • Climate Change Effects

    • Rising CO2 levels from fossil fuel use lead to ocean warming, stressing corals and increasing susceptibility to bleaching and disease.
    • Ocean acidification negatively impacts calcium carbonate deposition.

  • Coral Bleaching

    • Result of symbiotic zooxanthellae expulsion primarily due to thermal stress.
    • Corals may recover if stress levels are not extreme; bleaching resistance varies among species.

  • Coral Disease

    • Major cause of reef mortality and degradation; includes:
    • Black band disease (dark band migration across coral tissue).
    • White pox (appearance of white lesions).
    • Caribbean yellow band disease.
    • Causes include environmental stressors and dysbiosis.