［[Question] Why does the degree of reduction change for each group of organisms?

Except for special environments such as hydrothermal deposits, the source of energy in the ocean is mostly photosynthesis using sunlight. Sunlight for photosynthesis penetrates up to 200 m above the sea surface. The biomass of all taxa is high near the surface because of the reach of the energy-producing sunlight.

Different taxa differ in how biomass is depleted from the surface layer, depending on how each taxa obtains energy.For example, heterotrophic bacteria use dissolved organic matter as energy, but it is known that the decrease in dissolved organic matter with increasing depth is not so large. This is why heterotrophic bacteria decrease biomass slowly with increasing depth.

On the other hand, unicellular animals that feed on heterotrophic bacteria show a slightly steeper decrease in biomass with increasing depth. This is a reflection of the loss of energy due to uneaten food when having an eat-eaten relationship, and the loss due to assimilation and metabolism .