What is phosphorus thought to limit in aquatic habitats?

Phosphorus is a crucial nutrient that plays a significant role in aquatic ecosystems, and its availability is often a critical factor in limiting plant productivity, specifically the growth of phytoplankton and aquatic plants. In many freshwater systems, phosphorus is the limiting nutrient because its concentration is lower than what could support maximum growth rates for these photosynthetic organisms. When phosphorus levels are insufficient, it can restrict the overall productivity of the aquatic environment, leading to reduced growth of aquatic plants and phytoplankton.

This limitation on plant productivity can have cascading effects throughout the food web, as these primary producers form the foundation of the aquatic food chain. If phytoplankton growth is limited by phosphorus, it can impact the populations of herbivorous organisms that feed on them, ultimately affecting higher trophic levels like fish and other aquatic animals.

In contrast, while phosphorus can influence bacterial activity and water clarity, these are generally secondary effects stemming from its direct impact on plant productivity. Fish growth is also affected by various factors, not solely phosphorus, so it is not considered a direct limitation like plant productivity is. Therefore, phosphorus's role as a limiting nutrient in aquatic habitats primarily relates to its influence on the growth and productivity of plant life.