Coral Reef Growth Requirements

Coral reefs require specific environmental conditions to develop and thrive. These conditions primarily involve light availability, water clarity, temperature, and salinity.

Evaluating Growth Conditions

• 1. Photic Conditions: Corals depend on sunlight because they host symbiotic algae (zooxanthellae) which perform photosynthesis. This process provides essential nutrients for the coral. Therefore, sufficient light penetration is a necessary condition.
• 2. Clean and Sediment-Free Water: Excessive sediment can block sunlight needed by the zooxanthellae, smother the corals, and increase turbidity. Clean, clear water is crucial for reef health.
• 3. Sea Salinity of 6%: Most reef-building corals thrive in marine environments with salinity levels around 30-35 parts per thousand (ppt), which is roughly 3.0-3.5%. A salinity of 6% (60 ppt) is extremely high, and values significantly lower than normal seawater, like 6% (which is very low), are generally unsuitable for coral growth.
• 4. Tropical Sea Water Temperature: Corals are predominantly found in tropical and subtropical waters. They require warm temperatures, typically between $20^\circ C$ and $28^\circ C$ or $29^\circ C$. While the specific range mentioned ($20^\circ C$ to $21^\circ C$) is at the lower end of the optimal range, it falls within the necessary warm, tropical sea water conditions required for growth.

Conclusion on Necessary Conditions

Based on the biological requirements of corals, photic conditions (sunlight), clean water free from sediment, and warm tropical temperatures are essential. Very low salinity levels, such as 6%, are detrimental.

Final Answer Selection

The conditions necessary for coral reef growth are:

• 1. Photic conditions
• 2. Clean and sediment free water
• 4. Tropical sea water with temperature of $20^\circ C$ to $21^\circ C$

Therefore, the correct combination is 1, 2 and 4 only.