Coral reefs are vibrant ecosystems that host a remarkable diversity of marine life. Built by tiny animals over millennia, these biodiverse environments provide shelter and sustenance for countless species. For these delicate structures to thrive, environmental conditions must remain within specific limits, with salinity being a particularly influential factor.
Defining Salinity and Typical Ranges
Salinity refers to the amount of dissolved salts present in water. This measurement is commonly expressed in parts per thousand (ppt) or Practical Salinity Units (PSU). The average salinity of the world’s oceans is approximately 35 ppt. Coral reefs generally flourish within a relatively narrow salinity range, typically between 32 and 42 ppt. However, most reef-building corals are best suited for salinities ranging from 32 to 35 ppt.
Why Salinity Matters for Coral Health
Corals require a stable salinity environment to maintain their biological functions. Their survival depends on regulating water and salt balance within their cells, a process called osmoregulation. Deviations from optimal salinity force corals to expend energy on osmoregulation, diverting resources from other vital processes.
Salinity also directly influences coral calcification, the process by which corals build their hard calcium carbonate skeletons. This skeletal formation is fundamental to reef growth and structural integrity. When salinity levels are outside the preferred range, ion availability for calcification is affected, impeding coral growth and structure maintenance.
Factors Influencing Coral Reef Salinity
Both natural processes and human activities can cause fluctuations in coral reef salinity. Natural factors include freshwater runoff from heavy rainfall or rivers, which decreases salinity in coastal areas; coral reefs often do not develop near large river outflows due to this effect. Conversely, high evaporation rates in arid regions can increase salinity. Ocean currents also transport water masses with varying salt concentrations.
Human activities also alter salinity. Coastal development and land use changes can modify natural freshwater discharge patterns and increase runoff into coastal waters. Discharges from large-scale desalination plants can also locally elevate salinity, though this is less common directly on reefs.
Impacts of Salinity Fluctuations
When coral reefs experience salinity levels outside their optimal range, they face stress. Both hyposalinity (abnormally low salinity) and hypersalinity (abnormally high salinity) can harm corals. Hyposalinity, often caused by severe freshwater flooding, can lead to “freshwater bleaching,” where corals expel their symbiotic algae. This shock disrupts normal cell function, resulting in reduced growth, increased disease susceptibility, and coral mortality.
Juvenile corals are particularly vulnerable to low salinity, and such fluctuations can hinder larval settlement and survival. While less studied, hypersalinity can also induce stress and reduce calcification rates. These salinity changes impact the entire reef ecosystem, altering the diversity, growth, and survival of other marine species.