The correct answer is "1 and 2 only" because the complete destruction of phytoplankton would severely disrupt the ocean's role as a carbon sink, as phytoplankton are essential for carbon fixation through photosynthesis, thus limiting the ocean's ability to absorb atmospheric CO2. Additionally, phytoplankton serve as the foundational base of marine food webs; their removal would collapse the food chains, affecting myriad marine organisms, from zooplankton to larger predators. The statement regarding the density of ocean water (option 3) is not correct, as the overall salinity and temperature, rather than phytoplankton populations, primarily influence water density.