Eutrophication can be defined as:

The correct answer is the gradual increase in the concentration of phosphorus, nitrogen, and other plant nutrients in an aging aquatic ecosystem such as a lake.

Explanation:
Eutrophication refers to the natural or artificial enrichment of an aquatic ecosystem, such as a lake, pond, or reservoir, with essential plant nutrients—most notably phosphorus and nitrogen.

Key Concepts and Process of Eutrophication:
1. Nutrient Accumulation: As an aquatic ecosystem ages (or as a result of human activities like agricultural runoff carrying fertilizers and wastewater discharge), the concentration of nutrients like nitrogen and phosphorus increases in the water body.
2. Algal Blooms: The excess nutrients act as fertilizers, fueling excessive and rapid growth of phytoplankton and algae on the water surface, creating dense layers known as algal blooms.
3. Sunlight Barrier and Oxygen Depletion: Thick algal growth on the surface prevents sunlight from penetrating deeper into the water, causing submerged aquatic plants to die due to lack of photosynthesis.
4. Decomposition and Hypoxia: When algae and plants die, aerobic decomposers (microorganisms) consume massive amounts of dissolved oxygen to break down the organic matter. This severely depletes dissolved oxygen levels in the water (a state known as hypoxia or anoxia).
5. Ecosystem Degradation: The lack of oxygen leads to large-scale mortality of fish and other marine organisms, resulting in a loss of biodiversity and overall degradation of the aquatic ecosystem.

Therefore, eutrophication is defined as the gradual increase in the concentration of phosphorus, nitrogen, and other plant nutrients in an aging aquatic ecosystem such as a lake.