If the hypothesis mentioned in the second sentence of the passage is correct, which of the following would be the most immediate result of fertilizing the waters of the Southern Ocean with iron?

Fertilizing with iron is unlikely to introduce new marine species, as the strategy primarily aims to boost existing phytoplankton populations, not to alter the species composition drastically. The introduction of new species typically requires more significant ecological changes than simply adding a nutrient.

Iron fertilization does not directly cause a decrease in water temperature. While enhanced phytoplankton growth can influence local carbon cycling and potentially affect temperature indirectly over time, the immediate effect would not be a temperature decrease; rather, it would be increased biological activity.

Although increased phytoplankton photosynthesis could lead to long-term reductions in atmospheric carbon dioxide, this effect would not be immediate. The most immediate result of iron fertilization would be the enhanced growth of existing phytoplankton, which could eventually contribute to carbon reduction but not right away.

This option contradicts the premise of the hypothesis, which suggests that phytoplankton concentrations are currently low due to iron deficiency. Fertilizing the ocean with iron would aim to increase, not decrease, phytoplankton levels.