Final answer:
Plankton, specifically phytoplankton, account for a larger percentage of the ocean's biomass because of their high turnover rate and rapid reproduction. They serve as the primary producers in the marine ecosystem, providing nutrients for other organisms through photosynthesis.
Step-by-step explanation:
The reason why plankton account for a larger percentage of the ocean's biomass than benthos and nektons combined is due to their high turnover rate and rapid reproduction. Plankton, specifically phytoplankton, are tiny photosynthetic organisms that live in the ocean's surface waters. They are the primary producers in the marine ecosystem, converting sunlight into food through photosynthesis.
Phytoplankton serve as the base of the food chain, and are a rich source of nutrients for other organisms including zooplankton, small fishes, and shrimp. These organisms feed on the phytoplankton, and in turn, are consumed by larger predators like nektons. However, because of the rapid consumption of phytoplankton by primary consumers, their biomass is minimized at any given point in time. Yet, their quick reproduction allows them to continuously support the entire marine ecosystem.
On the other hand, bentos are organisms that dwell at the sediments of the ocean, and nektons are free-swimming aquatic animals. While both contribute to the ocean's biomass, plankton, particularly phytoplankton, have a greater contribution due to their high abundance, rapid growth, and role as the primary producers in the marine food web.