Question

Upper part of sea/aquatic ecosystem contains
A. Plankton
B. Nekton
C. Plankton and Nekton
D. Benthos

Answer 1

Hint:
$71\% $of the Earth is covered by water. Water exists as in rivers, ponds, lakes, sea, etc. Ecosystem is the group of community in an area interacting with each other and the environment. The living organisms that live in the water bodies and are dependent on water for survival are the aquatic organisms. Aquatic ecosystems include oceans, lakes and rivers. It has different layers such as littoral, limnetic, profundal and benthic.

Complete answer:
The word plankton comes from planktos which means drifter. These do not swim on their own. They are drifted by water in tides, currents, etc.
The upper part of an aquatic ecosystem of the sea contains plankton. These are passive floaters in upper water.
Unlike Planktons, Nektons are aquatic animals that are able to swim freely as the word Nekton means to swim. These are active swimmers which are not affected by the water currents. These are not found in the upper part of the sea and can be vertebrates, squids, crabs. The term was proposed by ‘Ernst Haeckel’.
Moving towards the ocean floor or the bottom the organisms that are found are the benthos or benthic organisms. Some of the examples include bacteria, diatoms, amoeba, etc. They can live in standing or running water and require a substrate. They rely on inorganic matter and dead organisms on the ocean floor.

Hence option A is the correct answer.

Note: The marine ecosystem has the greatest biodiversity. Phytoplankton provide the maximum of oxygen on earth. Marine ecosystem is very important as $90\% $ of the life lives in the ocean thus we should take measures to reduce water pollution thereby saving the beautifully diversified ecosystem as these provide feed for livestock, raw materials to prepare medicine, etc.