In oxygen-based estimates of lake primary production, what does net ecosystem production (NEP) represent?

• A. Gross production only.
• B. Respiration only.
• C. The balance between gross production and respiration.
• D. Nutrient uptake rate.

Answer: (C)

Net ecosystem production in oxygen-based lake estimates represents the net result of photosynthesis and respiration—the balance between gross production and respiration. It is the difference between the rate at which organisms produce organic material (gross production) and the rate at which they consume it for energy (respiration). This tells you whether the lake is gaining or losing organic carbon: a positive NEP means more production than respiration (net biomass growth), while a negative NEP means respiration outweighs production (net loss of organic matter). It’s not just gross production, not just respiration, and not nutrient uptake rate.