[Question] Why do heterotrophic bacteria and unicellular animals prefer in the subtropical region?

The large difference in the ratio of each taxonomic group to the total plankton biomass between the subarctic and subtropical regions reflects the difference in the constituent organisms. In the subarctic zone, there are many large multicellular zooplankton (copepods with a dormant period) that eat phytoplankton that proliferates in the spring on the surface layer and spend the rest of the season in the deep sea. In the summer months when we studied, they predominate especially in the middle layers (200-1000 m). Dormant copepods can only dive to depths of up to 2000 m, and their occupancy decreases sharply in deeper layers. In addition, even in the same subarctic zone, the occupancy rate of dormant copepods is lower in the sea area closer to the subtropical zone (39N, C).

On the other hand, in the subtropical zone, there is few dormant copepods in the deep sea. This is thought to reflect the poor seasonality of biological production. In the subtropical zone, sedimented particles that fall from the upper layer become dissolved organic matter, and heterotrophic bacteria that feed on it increase, and single-celled animals that feed on the heterotrophic bacteria have biomass following the bacteria's. Single-celled animals serve as food for multicellular zooplankton, but there is a relationship between eating and being eaten for  single-celled animals and the amount of them decreases, so the proportion of multicellular animals is extremely small.