Concept:Productivity in aquatic habitats is limited by factors that are scarce or variable, while water itself is always abundant there.
Explanation:In aquatic ecosystems, the main factors that restrict productivity are sunlight penetration, nutrient availability, dissolved oxygen, and temperature.
Sunlight is needed for photosynthesis by aquatic plants and algae.
Food (nutrients) and temperature also directly affect the growth and metabolic rates of aquatic organisms.
However, water is not a limiting factor because aquatic organisms are already surrounded by water.
Therefore, among the options given, water is the factor that is not a major limiting factor of productivity in an aquatic habitat.
Answer:C. Water