Foraging in Turbulent Flow: Bridging Individual Motility and Meter-Scale Phytoplankton Patchiness

In the ocean, turbulent motions stir phytoplankton layers into complex spatial structures, increasing patchiness and gradients of phytoplankton biomass. At scales of a few meters, the patchiness of phytoplankton affects the efficiency by which motile zooplankton are able to forage.

Defining patchiness as the slope of the phytoplankton variance spectrum, a simple numerical ecosystem model was used to generate phytoplankton of constant, prescribed patchiness. Individual zooplankton are then modeled to forage on phytoplankton distributions with tactic and kinetic behavioral responses.

For a given motility pattern, a level of patchiness can be identified for which zooplankton foraging is optimal. With increasing patchiness, gradients in phytoplankton increase and typical length scales of patches decrease. Phytoplanktonic features become too small to be exploited by zooplankton of certain perceptive and motoric ranges.
With decreasing patchiness, gradients in phytoplankton weaken and typical length scales of patches increase. Tactic zooplankton travel along phytoplankton gradients and if those gradients weaken, the directionality of movement is lost.

Zooplankton grazing itself modifies phytoplankton distributions such that patchiness and gradients can be intensified as well as derogated. Grazers therefore shape their food environment and influence their own grazing efficiency.

Patchiness-motiliy coupling is an important mechanism that is necessary to better constrain carbon fluxes from primary producers toward higher trophic levels and needs to be considered as a subgrid-scale process in biogeochemical models.