The Planktonic Streaking Hypothesis

Phytoplankton growth in the open ocean is typically constrained by the gradient of nutrients at the surface of their cells. Nutrient concentration is often low and large-scale vertical gradients of nutrients are weak, leaving phytoplankton surrounded by a nutrient-depleted layer that limits uptake. Turbulence and phytoplankton motility can shear the nutrient-depleted layer, increasing local gradients and the diffusive flux. However, turbulence is relatively weak in the deep phytoplankton biomass maximum found in most oceans. We hypothesize that migrating zooplankton deform the weak, ambient nutrient gradients, forming nutrient "streaks" that can benefit phytoplankton. We investigate the stirring of a chemical gradient in tank experiments with calanoid copepods, a ubiquitous migrating zooplankton. Both swimming and sinking copepods enhance weak, ambient chemical gradients, leaving streaks that persist for minutes. Phytoplankton embedded in these streaks have their nutrient-depleted layers sheared and the gradients surrounding the cell are enhanced. Numerical models demonstrate that uptake is increased for individual phytoplankton in the streak, and is particularly enhanced for larger phytoplankton. The fraction of the phytoplankton population that benefits from zooplankton streaking during migrations is expected to divide more rapidly, and maintain the whole population via a "compound interest" effect. Using a microshear dataset representing each ocean basin, we show that zooplankton streaking is not erased but rather enhanced by the levels of turbulence that most plankton are likely to experience. Planktonic streaking may provide an important mechanism driving enhanced phytoplankton uptake of limited nutrients, and maintaining diversity throughout the world's oceans.