In most freshwater lakes, the algal productivity is limited by the availability of which of the following inorganic ions?

(a) Carbon

(b) Nitrogen

(c) Phosphorus

(d) All of the above

(c) Phosphorus

In the majority of freshwater lakes, phosphorus is the primary limiting nutrient for algal productivity.

A limiting nutrient is an essential element that is in the shortest supply relative to the needs of an organism. Even if other nutrients like nitrogen and carbon are abundant, algae cannot grow beyond the amount allowed by the scarcity of phosphorus.

Phosphorus is a crucial component of DNA, RNA, and ATP (the cell's energy currency), but it is naturally present in very low concentrations in most freshwater systems. When excess phosphorus is introduced into a lake, often from sources like fertilizer runoff or sewage, it removes this natural limit. This leads to a rapid, massive growth of algae known as an algal bloom, a process called eutrophication, which can severely harm the lake's ecosystem.