Sequence of ecosystems by productivity — UPSC Prelims PYQ 2013

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

Oceans, lakes, grasslands, mangroves
Mangroves, oceans, grasslands, lakes
Mangroves, grasslands, lakes, oceans
Oceans, mangroves, lakes, grasslands

Explanation

Option (c) is correct

Productivity = production/unit area/unit time. It depends on the number and diversity of producers.
Ecotones (Mangroves, estuaries, and grasslands) have greater productivity than the surrounding ecosystems.
Tropical Rainforests and coral reefs are an exception, as they have productivity comparable to wetlands because of their rich diversity of primary producers.
Note: Grasslands are not transitional all the time. E.g., Steppe and Savanna grasslands, which are very vast, are non-transitional and have very low productivity because of the minimal diversity of primary producers. (Grasslands become transitional only when they are narrow). So, the order of decreasing productivity will be like Mangroves, ……, ……, …….. .
Oceans are very deep, and hence productivity is limited to the surface only (deep below in the aphotic zone, productivity is negligible. The Aphotic zone in oceans is a few kilometres thick!).
Also, ocean surface water is abysmal (very poor) in nutrients (except near the shoreline).
Nutrient-rich cold water flows as a sub-surface flow in the aphotic zone. Sunlight and nutrients are far apart, so primary productivity is very low except in regions with an upwelling of nutrient-rich cold water (cold and warm ocean current mixing zones).
So, among the options, the open ocean ecosystem has the least productivity. (Desert ecosystem also has very low productivity, lesser than the oceans). • So, the answer will look like Mangroves, ………., ………, Oceans. The only such option is c). Lakes, just like oceans, have low productivity. However, due to some plants in the photic zone, lakes have productivity slightly greater than that of oceans.