Thermal Waste Discharge

Changes in seawater temperature can significantly affect photosynthesis, aerobic respiration, growth, reproduction, metabolism, and survival of marine organisms. According to the well-established Q10 Rule, the rate of most biochemical reactions approximately doubles for every 10°C increase in temperature, although this increase generally represents the upper physiological tolerance limit for many organisms. Temperature changes also influence numerous microbial processes, including nitrogen fixation, nitrification, and denitrification. As a result, aquatic organisms can survive only within a limited range of temperature conditions.

When seawater temperatures rise or fall beyond the tolerance limits of marine species—including fish, aquatic insects, benthic invertebrates, zooplankton, phytoplankton, and microorganisms—their survival, reproduction, and ecosystem functions may be severely compromised. Coral reefs provide a well-known example because they are highly sensitive to even small temperature anomalies, which can trigger coral bleaching. Temperature changes also indirectly degrade marine habitats by altering dissolved oxygen concentrations and the solubility of calcium carbonate (calcite and aragonite), both of which are essential for reef-building organisms. In addition, seawater temperature influences the solubility and bioavailability of heavy metals and other toxic substances, thereby affecting the physiological processes of marine organisms. Temperature is also one of the primary environmental factors controlling the survival and distribution of marine viruses.

Seawater temperature directly influences water density, electrical conductivity, and pH throughout the water column. Increasing temperature also reduces the solubility of dissolved gases, particularly dissolved oxygen (DO) and carbon dioxide (CO₂). Under elevated temperature conditions, oxygen demand increases because of enhanced bacterial respiration while the water's capacity to retain dissolved oxygen decreases. Consequently, marine waters may become hypoxic or even anoxic, creating stressful or lethal conditions for aquatic life.

Potential indicators of seawater temperature change can be grouped into three categories: