If phytoplankton in an ocean is completely destroyed for some reason, the following outcomes are likely to occur:

The ocean as a carbon sink would be adversely affected: Phytoplankton plays a crucial role in the carbon cycle by absorbing carbon dioxide from the atmosphere through photosynthesis and storing it in the deep ocean. If phytoplankton were completely destroyed, the ocean`s ability to absorb carbon dioxide from the atmosphere and act as a carbon sink would be severely affected.

The food chains in the ocean would be adversely affected: Phytoplankton forms the base of the marine food chain and is the primary source of food for many marine organisms, including fish, whales, and zooplankton. If phytoplankton were completely destroyed, the entire marine ecosystem would be adversely affected, and many species would struggle to find food, leading to a collapse of the food chain.

The density of ocean water would not drastically decrease: While phytoplankton does contribute to the production of oxygen in the ocean, its absence would not drastically decrease the density of ocean water.

Therefore, the correct answer is 1 and 2 only.