Final answer:
Phosphorus is the nutrient most likely to limit primary production in freshwater by causing algal blooms, which can lead to reduced light penetration and oxygen depletion, affecting the survival of aquatic organisms and creating dead zones.
Step-by-step explanation:
The element most likely to limit primary production in freshwater ecosystems is phosphorus. Nitrogen and phosphorus are important limiting nutrients in lakes and ponds, determining the growth of phytoplankton. When there's an overabundance of these nutrients from sources like sewage and agricultural runoff, it leads to a proliferation of algae in a phenomenon known as algal blooms.
These algal blooms can have detrimental effects on aquatic environments, reducing light penetration, causing oxygen depletion when they decompose, and potentially leading to the death of oxygen-requiring organisms, resulting in dead zones. Freshwater bodies like Lake Erie and marine habitats such as the Gulf of Mexico are examples where high levels of phosphorus from storm water runoff present significant environmental challenges.