Presenter: Mara Freilich (MIT-WHOI Joint Program)

Description:
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? What is the role of submesoscale dynamics as compared with mesoscale dynamics? We show that vertical nutrient flux depends both on the vertical motion within the eddying flow and varies nonlinearly with the growth rate of the phytoplankton itself. Vertical transport of nutrient depends on the time-integrated covariance between vertical velocity and the nutrient's anomaly from its areal mean concentration. 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 subintertial submesoscale dynamics. The spatial structuring in the frequency of vertical motion experienced by water parcels in different physical features of the flow suggests a growth-transport feedback mechanism for favoring different phytoplankton growth rates that can generate diversity in phytoplankton community structure and increase the net nutrient flux in the presence of biological diversity. We use this theoretical advance to interpret observations of net community production in subtropical gyres. One implication of this work is that increasing numerical resolution in ocean models modifies the non-diffusive physical timescales, and requires consideration of the biological uptake timescales in ecosystem models for accurate assessment of the rate and composition of primary production in nutrient-limited ocean regions.