Which one of the following is the correct sequence of ecosystems in the order of decreasing productivity?

The correct answer is C. The sequence is mangroves, grasslands, lakes, oceans, which is the order of decreasing net primary productivity (NPP).

• Mangroves are among the most productive ecosystems, with NPP of roughly 2000 g/m²/yr, because warm, nutrient-rich tidal waters and constant supply of organic matter support dense growth.
• Grasslands come next, at roughly 500 to 600 g/m²/yr, depending on whether they are temperate or tropical.
• Lakes and streams are around 250 g/m²/yr, limited by nutrient and light availability.
• Oceans are the least productive per unit area, at roughly 125 g/m²/yr. Their vast size is offset by nutrient-poor open waters. Option A and D put oceans at the top, which reverses the real pattern.

The core concept is NPP per unit area, not total biomass or area covered. This ties into the Environment and Ecology section of the syllabus and to Mains questions on carbon sequestration and the ecological role of mangroves and wetlands.

Trap: Aspirants assume the oceans, being the largest ecosystem, must be the most productive. Productivity here is per unit area, and open oceans are among the least productive.