A pond fills with organic matter during what?

A pond fills with organic matter during eutrophication.

Explanation:

Eutrophication takes place when an environment like aquatic environment in the form of lakes, ponds, sea etc is filled and enriched with organic matter and nutrients.

This is a huge concern for aquatic or marine organisms as the excess nutrients give rise to algal bloom due to production of huge amounts of phytoplanktons. These phytoplanktons will use and deplete the reserves of oxygen present in the water, leaving very little or none for other aquatic or marine organisms. This leads to destruction of such organisms which require oxygen to survive and finally their endangerment and extinction.

Natural eutrophication occurs due to gradual and slow sedimentation process occurring over very long periods of time.

Artificial or man-made eutrophication due to sedimentation of organic waste from solid wastes, industrial effluents increases the nutrient content like nitrogen in the water bodies resulting in rapid and mass eutrophication disturbing the entire ecological system.

A pond fills with organic matter when "pond ages from oligotrophic to mesotrophic".

Explanation:

The ponds are basically ranging from youngest to oldest in the given categories: Oligotrophic, Mesotrophic, or Eutrophic phases of live. The oligotrophic ponds are known as fresh or young waterbody. Oligotrophic bodies are low in nutrient quantities, like nitrogen and phosphorus, so donot encourage algae and aquatic plants productivity. The middle ages mesotrophic aquatic bodies are carrier of more nutrients and thus more readily the development of plants and algae take place than oligotrophics, but less than eutrophic production. As aging take place from oligotrophic to mesotrophic, the pond sides continue to slope less and the base of the pond starts to fill in with organic matter.