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.