Coral Reefs and Their Ecosystems

Coral Feeding Mechanisms

  • Corals use two feeding methods:
    • Feeding on zooplankton.
    • Utilizing a mutualistic relationship with zooxanthellae (algae).
  • They prefer nutrient-poor water to avoid overgrowth by algae.
  • In nutrient-poor water, there is limited plankton; corals rely on zooxanthellae for energy.

Relationship Between Corals and Zooxanthellae

  • Zooxanthellae photosynthesize and provide nutrition for coral polyps.
    • Convert carbon dioxide and other compounds into sugars, fats, and oxygen.
  • Coral polyps take in sugars and fats, which are essential for their survival.

Coral Bleaching and Thermal Stress

  • Corals are stressed at water temperatures above 30-35 °C (approximately 86-90 °F).
  • Elevated temperatures can lead to the expulsion of zooxanthellae, causing coral bleaching, where corals appear white due to a lack of algae.
  • Corals can survive temporary bleached states through other feeding methods but require zooxanthellae for long-term survival.

Factors Contributing to Coral Bleaching

  • El Niño Events: Major El Niño events cause widespread coral bleaching due to increased water temperatures.
  • Climate Change: Chronic temperature increases lead to repeated and prolonged bleaching events. The changes in rates and fluctuations of these temperatures are critical for coral health.
  • A slight increase of just 1 °C above the normal temperature can trigger bleaching events.

Impact of Zooxanthellae on Coral Calcification

  • Corals produce calcium carbonate beneath their polyps, building the reef structure.
  • Healthy polyps have a stable pH environment (around 8.2), while bleached corals may see a drop in pH (down to 7.6), reducing their ability to calcify effectively.
  • Acidity impacts the formation of healthy calcium carbonate layers necessary for coral structure.

Coral Reproduction Strategies

  • Asexual Reproduction: Polyps bud off from parent corals, leading to limited genetic diversity.
  • Sexual Reproduction: Includes external fertilization, where gametes are released into the water (broadcast spawners), and internal fertilization (brooding), which leads to higher survival rates but less genetic dispersal.

Types of Coral Reefs

  1. Fringing Reefs: Located close to shorelines; they are vulnerable to human impact and usually have the highest tolerance to stressors.
  2. Barrier Reefs: Situated further from the shore, featuring a lagoon in between. These reefs can experience greater fluctuations and are prone to instability.
  3. Atolls: Formed from sinking islands. Atolls are characterized by a lagoon housing coral systems away from land, leading to stability and ideal growth conditions.

Community Interactions and Ecological Dynamics

  • Coral reefs exhibit mutualistic relationships (e.g., corals and zooxanthellae) and competition for resources like space and light.
  • Coral competition can include:
    • Growth Patterns: Corals may grow taller to access light.
    • Aggressive Interactions: They can digest neighboring corals using mesenterial filaments.

Environmental Stressors Affecting Corals

  • Corals are sensitive to changes in salinity and pollution.
  • Sedimentation smothers coral reefs and reduces photosynthesis efficiency.
  • Human-induced factors like dredging and nutrient runoff lead to increased algal growth, impacting coral health negatively.

Summary of Coral Ecosystem Functioning

  • Balancing the growth (calcification) and breakdown (bioerosion) is essential for reef health and resilience.
  • Maintaining clean, nutrient-poor water with low sedimentation is crucial for coral survival.