Here's why:

* Abiotic factors are non-living components of an ecosystem. They include things like temperature, sunlight, water, and minerals.

* Biotic factors are living organisms within an ecosystem, such as plants, animals, and microorganisms.

Dissolved oxygen is simply oxygen that is dissolved in water. It's not a living organism, so it falls under the category of an abiotic factor.

While the presence of dissolved oxygen is crucial for the survival of many aquatic organisms (biotic), oxygen itself is not alive.