Diversity of Growth Rates Maximizes Phytoplankton Productivity in an Eddying Ocean

In the subtropical gyres, phytoplankton rely on eddies for transporting nutrients from depth to the euphotic zone. But, what controls the rate of nutrient supply for new production? We show that vertical nutrient flux both depends on the vertical motion within the eddying flow and varies nonlinearly with the phytoplankton growth rate. Flux is maximized when the growth rate matches the inverse of the decorrelation timescale for vertical motion. Using a three-dimensional ocean model and a linear nutrient uptake model, we find that phytoplankton productivity is maximized for a growth rate of 1/3 day(-1), which corresponds to the timescale of submesoscale dynamics. Variability in the frequency of vertical motion across different physical features of the flow favors phytoplankton production with different growth rates. Such a growth-transport feedback can generate diversity in the phytoplankton community structure at submesoscales and higher net productivity in the presence of community diversity.

Automated summary

This study investigates the factors influencing the nutrient supply rate for new phytoplankton production in the subtropical gyres. It shows that the vertical nutrient flux is dependent on the vertical motion within the eddying flow and varies nonlinearly with the phytoplankton growth rate. The flux is maximized when the growth rate matches the inverse of the decorrelation timescale for vertical motion. The findings suggest that phytoplankton productivity is maximized at a growth rate of 1/3 day(-1), corresponding to the timescale of submesoscale dynamics. Variability in the frequency of vertical motion across different physical features of the flow promotes phytoplankton production with different growth rates. This growth-transport feedback can contribute to diversity in the phytoplankton community structure and enhance net productivity in the presence of community diversity.