The amount of zooplankton biomass that supports fish productivity and carbon sequestration is referred to as harvestable secondary production (HSP). HSP is an important aspect of the marine nitrogen (N) cycle and affects the growth of zooplankton based on primary production, which is fertilized by N sources from outside the euphotic zone (e.g., through upwelling or inputs from rivers), also known as new production. Currently, there is no approach for directly quantifying zooplankton growth that can be applied to the entire zooplankton community, as is the case with primary production in the oceans. For this reason, HSP is the least well-known process in biogeochemical models. More than 50 years after the first method for quantifying new production in the ocean was developed, recent advances in isotope ecology now allow us to empirically quantify HSP at the community level using the trophodynamic approach for the first time. The HSP-Amazon project is the first to systematically pursue this approach in order to close fundamental knowledge gaps about secondary production in the ocean, using the Amazon plume with its habitat-specific changes in the N cycle as a natural laboratory.