Seemingly inconsequential changes in sea temperature due to global warming eventually result in declines in fish and seabird populations. A rise of just two degrees prevents the vertical mixing of seawater from different strata. This restricts the availability of upwelling nutrients to phytoplankton. Since zooplankton, which feed upon phytoplankton, feed the rest of the food chain, the declines are inevitable. Which one of the following most accurately describes the role played in the argument by the statement that zooplankton feed upon phytoplankton? ['It helps show how global temperature changes affect larger sea animals indirectly.', 'It is intended to provide an example of the ways in which the vertical mixing of seawater affects feeding habits.', 'It is offered in support of the idea that global warming poses a threat to all organisms.', 'It is offered as one reason that global warming must be curtailed.']