Which of the following ecosystems would you expect to have the lowest net productivity per cubic meter? 

• A. The open ocean (0m-10m deep).
• B. An estuary.
• C. The open ocean (20m-50m deep).
• D. A coral reef.

Answer: (C)

Verified step by step guidance

1. Understand the concept of net primary productivity (NPP), which is the rate at which all the plants in an ecosystem produce net useful chemical energy. It is typically measured in units of energy per area per time (e.g., kcal/m²/year).
2. Consider the factors that influence NPP in aquatic ecosystems, such as light availability, nutrient availability, and water depth. Light availability decreases with depth, and nutrient availability can vary significantly between different types of aquatic ecosystems.
3. Analyze the characteristics of each ecosystem mentioned: the open ocean (0m-10m deep), an estuary, the open ocean (20m-50m deep), and a coral reef. Note that the open ocean generally has lower nutrient availability compared to estuaries and coral reefs, which are often nutrient-rich and support diverse and productive communities.
4. Recognize that the open ocean (20m-50m deep) is likely to have lower light penetration and nutrient availability compared to shallower waters or more nutrient-rich environments like estuaries and coral reefs. This typically results in lower NPP.
5. Conclude that among the given options, the open ocean (20m-50m deep) would have the lowest net productivity per cubic meter due to its lower light and nutrient availability compared to the other ecosystems listed.