Final answer:
Net primary productivity is measured by the total carbon fixed through photosynthesis minus respiration and growth. Chlorophyll concentration and radioactive carbon are direct methods used to gauge this productivity, while tow nets collect organisms for biomass estimates.
Step-by-step explanation:
Net primary productivity is a measure of the energy accumulation within an ecosystem. It is calculated as the total amount of carbon fixed by photosynthesis per year minus the amount that is oxidized during cellular respiration. To measure this productivity on land or in the ocean, scientists often use chlorophyll concentrations as an indirect measure, utilize radioactive carbon to track carbon fixation during photosynthesis, and sometimes employ tow nets in aquatic environments to collect organisms for biomass estimation.
Specifically, net primary productivity includes factors such as the energy incorporated by photoautotrophs (such as plants, algae, and photosynthetic bacteria), minus the energy expended on their respiration and growth. This leftover energy or biomass is then available to consumers higher up in the food web. Therefore, while tow nets might be used to collect organisms and estimate biomass in marine systems, it's the quantification of chlorophyll and utilization of radioactive carbon that directly relate to measuring primary productivity, as they give insights into the biological processes involved.