Coral reefs are intricate underwater structures built over centuries by tiny organisms called coral polyps. These polyps are small invertebrates that secrete a hard external skeleton made of calcium carbonate. As generations build upon one another, they form the massive, complex architecture of a reef, providing habitat for about 25% of all marine life. The distribution and survival of these ecosystems are governed by non-living environmental conditions known as abiotic factors. These factors include the physical and chemical characteristics of the seawater, determining precisely where corals can settle, grow, and calcify effectively.
Temperature Range for Coral Survival
The temperature of the surrounding seawater is the most significant physical constraint on reef-building corals, which thrive in a narrow thermal window typically between $23^\circ\text{C}$ and $29^\circ\text{C}$. Corals are sensitive to prolonged heat stress, which triggers a breakdown in their biological systems. The accepted benchmark for stress is a temperature just $1^\circ\text{C}$ above the Maximum Monthly Mean (MMM) Sea Surface Temperature (SST) for a given region.
When water temperatures exceed this threshold, the coral experiences thermal stress, disrupting the symbiotic relationship it maintains with single-celled algae called zooxanthellae. These algae live within the coral’s tissue and provide up to 90% of the coral’s energy through photosynthesis. Under high heat, the zooxanthellae’s photosynthetic machinery becomes damaged and begins to produce toxic reactive oxygen species.
In a self-protective measure, the coral polyp expels these now-toxic algae from its tissues, resulting in the coral losing its color and appearing stark white, a process known as bleaching. A bleached coral is not dead, but it is starving and vulnerable to disease. Its survival depends entirely on whether the water temperature drops soon enough for it to re-acquire its symbionts; if the heat stress persists, the coral will die.
The Role of Light and Water Clarity
Light penetration is an absolute requirement for the survival of reef-building corals because of the photosynthetic needs of their zooxanthellae symbionts. Reefs must form within the photic zone, the shallow layer of the water column where sunlight is strong enough to fuel the algae’s energy production. This generally limits the formation of extensive reefs to depths shallower than 70 meters, with the most robust growth occurring much closer to the surface.
The clarity of the water directly controls how much light reaches the coral polyps, making low turbidity a necessity for reef health. Turbidity refers to the cloudiness of the water caused by suspended particles, such as fine sediments or excessive plankton blooms. When the water is turbid, light is scattered and blocked, which starves the zooxanthellae of the energy they need to produce food.
Suspended sediments pose a direct physical threat to the coral polyps themselves. Sedimentation, the settling of these particles onto the coral surface, can physically smother the organisms, covering their tissues and inhibiting their ability to feed and respire. Corals must expend energy to shed this layer of sediment, often by producing mucus, which drains their energy reserves and increases the risk of infection.
Essential Water Chemistry Parameters
The chemical composition of seawater provides strict boundaries for reef development. Corals require a specific concentration of salt, typically 30 to 40 parts per thousand (ppt), to maintain their internal osmotic balance. Significant fluctuations in salinity, such as those caused by freshwater runoff after heavy rain, disrupt the coral’s physiological processes and cause stress.
A pervasive chemical constraint is the water’s acidity, measured by its $\text{pH}$. Ocean acidification, a result of the ocean absorbing excess atmospheric carbon dioxide ($\text{CO}_2$), lowers the $\text{pH}$ of seawater. This change in chemistry directly impacts the coral’s ability to perform calcification, the process of building its protective calcium carbonate skeleton.
Coral skeletons are built using aragonite, a specific mineral form of calcium carbonate. Acidification reduces the availability of carbonate ions in the water, which are the fundamental building blocks corals need. Scientists quantify this availability using the aragonite saturation state ($\Omega_{\text{arag}}$), which measures how easily aragonite will precipitate or dissolve. As the saturation state declines due to falling $\text{pH}$, corals must expend more energy to build and maintain their skeletons, often resulting in thinner, weaker structures.
Physical Forces and Substrate Foundation
The movement of water, or hydrodynamics, plays a dual role in shaping the reef environment by providing sustenance and destructive force. Moderate currents and wave action are beneficial because they facilitate the mass transfer of materials, delivering plankton for feeding and flushing away metabolic waste products. This constant movement enhances the efficiency of the coral’s biological processes, supporting higher growth rates.
Conversely, excessive hydrodynamic forces from large storms or powerful wave surge represent a physical stressor that can cause direct structural damage to the reef. While some robust coral forms are adapted to high-energy environments, extreme wave action can break apart large coral colonies and reduce complex reef structures to rubble.
The physical foundation upon which a reef is built is the final constraint, as coral polyps cannot settle and grow on soft, shifting bottoms. Reefs require a hard, stable substrate, such as rock or pre-existing reef structure, to anchor the initial larval settlement and support the massive weight of the growing colony.