What group of organisms are the most important primary producers in the marine aquatic food web? How deep down in the water column can they be found?

The phytoplankton are the dominant primary producers in the Marine habitat.

Explanation:

Primary producers are organisms such as green plants, algae, bacteria that can make their own food by converting sunlight energy or chemical energy into organic compounds.

The primary producers in the Marine habitat are the dominant photosynthetic organism called phytoplankon. Phytoplankon are single cell prokaryotes, the trap sunlight energy and convert them to chemical energy a process called photosynthesis.

Phytoplankon are found within the average ocean depth is 4000 meters but they are limited near top of the ocean.

the answer is 100% density i've answered this question before and done the quiz myself.