Correct Option (A):
Phytoplankton are microscopic marine organisms that form the base of most oceanic food webs and are critical for various ecosystem services. They perform photosynthesis, absorbing carbon dioxide (CO₂) from the atmosphere.
- Statement 1 is correct: Phytoplankton are significant primary producers and absorb large quantities of CO₂ from the atmosphere, thus playing a crucial role in the ocean's function as a carbon sink. Their destruction would severely impair this carbon sequestration capacity, leading to increased atmospheric CO₂.
- Statement 2 is correct: As primary producers, phytoplankton are the foundational trophic level in marine food chains. Their complete destruction would lead to the collapse of these food chains, impacting zooplankton, fish, and ultimately all higher trophic levels in the ocean due to the loss of the primary energy source.
Incorrect Options:
- Statement 3 is incorrect: The density of ocean water is primarily determined by its temperature and salinity. While phytoplankton biomass can have a negligible local effect, their complete destruction would not drastically decrease the overall density of ocean water.
- Option (B) is incorrect because it omits the significant impact on the ocean's role as a carbon sink (Statement 1).
- Option (C) is incorrect because Statement 3 is factually inaccurate.
- Option (D) is incorrect because it includes the incorrect Statement 3.