UC San Diego

Microscale chemical gradients facilitate nutrient uptake by phytoplankton, recycling of organic matter by bacteria, and prey and mate finding by zooplankton. By mediating planktonic interactions, chemical microstructure is an important factor in the productivity of the largest ecosystem on the planet. Two main sources of chemical microstructure have been identified up to this point: planktonic excretions and turbulent stirring of large-scale gradients. The gradient deformation mechanism quantified in this work represents a third, previously unrecognized source of chemical microstructure. Gradient deformation occurs when sinking particles and swimming plankton drag fluid with them, leaving trails of gradient enhancement that extend for hundreds of body lengths and last for minutes.

Numerical simulations of a sphere falling through a linear tracer gradient demonstrate that the gradient deformation primarily depends on the Péclet number. When the diffusivity of the tracer is small compared to the size and speed of the object, Pe > 100, contours of tracer wrap around the sinking sphere, stretching vertically and accumulating horizontally downstream. The maximum tracer contour displacement scales as L_{Def} ~ Pe^{1/3}, the maximum gradient enhancement is G_{Max} ~ Pe^{4/5}, and the amount of tracer flux across a given contour is M ~ Pe^{2/3}.

A novel dyegraph apparatus was developed to measure the tracer disturbance left by sinking spheres and swimming copepods. Dropping spheres through linear gradients of dye and salinity in dyegraph experiments confirms the predictions from numerical simulations. Cruising copepods displace seawater both anteriorly and posteriorly and enhance the background gradient up to a

factor of 190 < G_{Max} < 450. The deformation trail left by a 3 mm copepod is estimated to be at least 1 m long and increases the flux of chemical by a factor of 40 through a surface roughly the size of the organism. While the bulk flux of tracer induced by this mechanism in the ocean is minimal, the combined gradient enhancement of all migrating zooplankton and sinking particles may have a significant impact on phytoplankton productivity, particularly in the vast oligotrophic regions of the world’s oceans.