Coral Reefs & Sea-Level Changes

1. Coral Reefs

Coral reefs are large underwater structures composed of the calcium carbonate skeletons of colonial marine invertebrates called corals. They are the "rainforests of the sea," occupying <0.1% of the ocean's surface but providing a home for 25% of all marine species.

Formation and Conditions for Growth

Reef-building corals live in a symbiotic relationship with microscopic algae called zooxanthellae that provide them with food (via photosynthesis) and their vibrant colors.

• Optimal Temperature: Shallow, warm tropical waters between 20°C and 21°C (rarely found beyond 30° N/S).
• Depth: Typically live within 50m to 60m depth because zooxanthellae require abundant sunlight for photosynthesis.
• Salinity: Prefer moderate salinity (27-30 ppt). Sensitive to influxes of completely fresh water from large rivers.
• Sediment-free water: High sedimentation clogs their mouth-like tentacles and blocks sunlight.

Types of Coral Reefs (Darwin's Theory of Subsidence)

1. Fringing Reefs: Grow directly attached to the shore of a continent or volcanic island, forming a border (e.g., reefs off the Andaman Islands).
2. Barrier Reefs: Separated from the mainland or island shore by a deep, wide lagoon. The Great Barrier Reef in Australia (largest in the world) is a classic example.
3. Atolls: Roughly circular oceanic reef rings enclosing a central lagoon, usually formed when the central volcanic island completely subsides beneath the surface (e.g., Lakshadweep islands, Maldives).

2. Coral Bleaching

When corals are subjected to severe environmental stress, they expel their symbiotic zooxanthellae algae. The coral loses its main nutrient source and its color, revealing the stark white calcium carbonate skeleton—this is Coral Bleaching. If the stress is not relieved, the coral starves and dies.

• Major Causes:
  • Global Warming/El Niño: Unusually elevated oceanic temperatures (even +1°C for 4 weeks).
  • Ocean Acidification: Increased atmospheric CO2 dissolves in the ocean, forming carbonic acid, which lowers oceanic pH and dissolves the corals' calcium skeletons.
  • Pollution & Runoff: Agricultural fertilizers (causing algal blooms that block sunlight), oil spills, and sunscreen chemicals.

3. Sea-Level Changes

Global (Eustatic) sea levels have constantly fluctuated through geological time, but are currently rising at an accelerated rate.

• Causes of Current Rise:
  1. Thermal Expansion: As ocean water absorbs the excess heat trapped by GHGs, it expands in volume. (Accounts for ~50% of the recent rise).
  2. Melting Ice: The rapid influx of freshwater from melting terrestrial glaciers (Himalayas, Alps) and polar ice sheets (Greenland, Antarctica). Note: Melting of floating sea ice (like the Arctic Ocean) does not directly raise the sea level.
• Impacts: Complete inundation of low-lying island nations (Tuvalu, Maldives), intensified coastal erosion, destructive storm surges, and saltwater intrusion into freshwater aquifers and deltaic agricultural lands (e.g., Sundarbans).