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

Answer: (c) Mangroves, grasslands, lakes, oceans

Mangroves (nutrient-rich estuaries) > grasslands > lakes > open ocean (nutrient-poor surface water).

The idea

Productivity means how much plant matter one square metre makes in a year. It depends on four supplies: sunlight, water, warmth and nutrients. A mangrove has all four, with rivers and tides bringing fresh nutrients daily. The open ocean has sunlight only in its top layer, and nutrients sink out of that layer, so most of it is a “blue desert”.

Mangrove and estuary: light + water + constant nutrients → highest → Grassland: good soil, seasonal rain → Lake: limited by nutrients and depth → Open ocean: nutrients scarce at the surface → lowest

Why the others are wrong

(a) Puts the ocean first. Its total output is large only because it is vast; per unit area it is the lowest.

The trap

Confusing TOTAL production (oceans are big) with productivity per unit area (oceans are poor).

Also know

Gross primary productivity is all the food plants make; net primary productivity is what is left after their own respiration. The most productive ecosystems per unit area are tropical rainforests, coral reefs, estuaries and swamps.