Floating microscopic heterotrophs are mostly grouped as

Explanation

Phytoplankton are microscopic marine organisms that sit at the bottom of the food chain, most phytoplankton are buoyant and float in the upper part of the ocean. However, they are not heterotrophic like the zooplanktons.

microbes are living organisms around us that we can't see with our naked eyes.

nektons are organisms that are able to swim and move independently of current.

There is an explanation video available .