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.

wouldnt it be the scientific method?

explanation:

both anaerobic and aerobic respiration use glucose (or other carbohydrates), proteins and lipids to produce atp and nadh. cell uses those energy molecules, obtained from food and   breaks them down during the chemical reactions of respiration. in anaerobic respiration, nadh is regenerated to nad+ by enzymes lactate dehydrogenase (lactate is produced, e.g. in muscle cells) or by alcohol dehydrogenase where ethanol is produced (alcohol fermentation), while in aerobic respiration, electrons from nadh are deposited in the mitochondria to regenerate nad+ (electron transport chain. anaerobic respiration produces 2 atp per molecule of glucose compared to aerobic respiration where about 38 atp molecules are produced.