The second trophic level in an aquatic ecosystem is:

The correct answer is Option (4) → Zooplanktons

In a pond or lake, the first trophic level is the producers: the phytoplankton and other plants. The second trophic level is the herbivores that eat them, the zooplankton. Fishes feed on these and come higher.

Option (1) Phytoplanktons — Incorrect. They are the producers, the first trophic level.

Option (2) Fishes — Incorrect. Fishes that eat zooplankton are at the third level.

Option (3) Large fishes — Incorrect. Large fishes eat smaller fishes, so they are at a still higher level.

Option (4) Zooplanktons — Correct. In an aquatic ecosystem, the phytoplankton are the producers (first trophic level). The zooplankton feed on them, so they are the primary consumers at the second trophic level.

NCERT Extract:
Ecosystem – Structure and Function – “… A pond is a shallow water body in which all the above mentioned four basic components of an ecosystem are well exhibited. The abiotic component is the water with all the dissolved inorganic and organic substances and the rich soil deposit at the bottom of the pond. The solar input, the cycle of temperature, day-length and other climatic conditions regulate the rate of function of the entire pond. The autotrophic components include the phytoplankton, some algae and the floating, submerged and marginal plants found at the edges. The consumers are represented by the zooplankton, the free swimming and bottom dwelling forms.”