Which nutrient is commonly used in fertilizers that can become limiting in aquatic ecosystems?

Nitrogen is a key nutrient that plays an essential role in the growth and development of aquatic plants and algae. In many aquatic ecosystems, nitrogen can often be a limiting factor, meaning that its availability can restrict the growth of these organisms. When nitrogen levels are low, it can limit the primary production of phytoplankton and aquatic plants, which are foundational to the food web and ecosystem health.

In agricultural and horticultural practices, nitrogen is commonly added to fertilizers to enhance plant growth. However, excessive nitrogen can lead to eutrophication when it enters water bodies, promoting overly rapid growth of plants and algae. This growth can deplete oxygen in the water, which is detrimental to aquatic life. Therefore, managing nitrogen levels is critical for maintaining balance in aquatic ecosystems.

The other options listed, such as selenium, krypton, and lithium, are either not commonly associated with fertilizers or do not have the same significant impact on aquatic ecosystems as nitrogen does. Selenium can be found in trace minerals and does have environmental impacts, but it is not typically a limiting nutrient in aquatic systems. Krypton is an inert gas and does not play a role in nutrient cycling, and lithium is mainly utilized for other purposes, including pharmaceuticals and batteries rather than as a fertilizer