While phytoplankton rely on photosynthesis to produce sugar for energy, they still need other nutrients to grow and reproduce 7. These nutrients are typically phosphorus, nitrogen and iron, though some species also require silicon, calcium and other trace metals 11,13. The more nutrients (particularly phosphorus) that are present in a body of water, the more algae and phytoplankton that will grow 7. An increase in the nutrient concentration of a body of water is called eutrophication 13. Eutrophication is often an indicator of agricultural runoff, which can raise phosphorus and nitrogen concentrations to very high levels. If there are too many nutrients, the algae will form a bloom, which can be very detrimental to water quality and aquatic health 7.
The lack of iron in the open ocean limits phytoplankton growth 10. Nitrogen and phosphorus are also scarce away from coastlines, and can be limiting factors as well 13. However, ocean circulation can cause an upwelling, which moves deep, nutrient-rich water up into the photic (sunlight zone), replacing the nutrient-depleted surface water 30. Upwelling, seasonal ice melts and agricultural runoff can all increase nutrient levels, leading to an increase in phytoplankton populations.