Why are nitrate and phosphate concentrations usually low in sunlit surface waters?

Nitrate and phosphate concentrations are typically low in sunlit surface waters because they are rapidly consumed by phytoplankton during photosynthesis.12

In the ocean's upper layers, where light is abundant, these organisms utilize dissolved nutrients to convert solar energy into chemical energy, forming the foundation of the marine food web. Because these nutrients are essential for growth, they are quickly depleted by the high biological productivity of the surface zone.132

Once these organisms die or are consumed, the nutrients may be transported to deeper waters or undergo bacterial breakdown. In many regions, the replenishment of these surface nutrients depends on physical processes such as currents or upwelling, which bring nutrient-rich water from deeper, darker layers back to the surface.14