The trophic level of zooplankton living in an aquatic ecosystem will be

The step or position in a nutritive series like food chain is referred to as trophic level.
There are different trophic levels in an ecosystem and food chain. The producers (photosynthetic organisms) form the first trophic level. The phytoplanktons form the first trophic level (T₁) in an aquatic system and the zooplanktons form the second trophic level (T₂) as they are the consumers that feed on producers.