All plants, whether they are tomatoes in your garden, trees in the forest, or phytoplankton in the ocean require three things to grow – water, sunlight, and nutrients. In the ocean, as there is no shortage of water, the dominant factors impacting phytoplankton growth are sunlight and nutrients. Often in the open ocean, however, nutrients are at their highest concentrations deep below the sunlit surface layer of the ocean. In these areas, phytoplankton growth tends to be limited to the thin layer at the bottom of the surface mixed layer that has enough light to facilitate growth and abuts the nutrient-rich deep waters. If the surface mixed layer is too deep, by the time the phytoplankton cells are deep enough to reach the nutrients they need for growth, phytoplankton production becomes light limited rather than nutrient limited. The same processes that limit phytoplankton growth in the open ocean can lead to highly productive coastal margins as upwelling brings these nutrient rich deep waters to the surface – check out the Coastal Upwelling nugget.
This nugget explores water column profiles of two OOI Regional Cabled Array sites in the NE Pacific – Oregon Slope Base at the base of the continental slope and Axial Base to the east, 500 km offshore. At both sites, there is a clear surface mixed later with limited nutrients, though this layer is more pronounced at Axial Base. The highest nutrient concentrations are in the deeper waters. An interesting feature in the Axial Base profiles is the peak in oxygen concentration at the base of the mixed layer due to a thin layer of primary production where nutrients are accessible but the phytoplankton are still in an area with enough light to reproduce.