Thin layers of phytoplankton

Recent advances in technology have allowed us to measure fluctuations in phytoplankton concentration in the ocean over vertical scales < 2 cm. These fluctuations may be short-lived micropatches, or long-lived pancake-shaped layers. Although there is not much supporting evidence, it appears that many of the vertical fluctuations in biomass are associated with thin (30 cm) layers of phytoplankton. How these layers form, how long they persist, and how the zooplankton respond to them are still relatively unknown.

One hypothesis is that the layers form by the interaction of vertical shear created by near-inertial waves, with horizontal patchiness of phytoplankton. If the top of a patch is moved to the left, while the bottom is moved to the right, a layer will form. The thickness of this layer is inversely proportional to the amount of vertical shearing. The more shear, the thinner the layer. If there are numerous patches initially, there will be many layers after the shearing.

I developed a model that included these dynamics. The model supported the hypothesis that near-inertial wave shear can generate thin layers of phytoplankton. A diagnostic for this mechanism in the ocean is that the layers should be inclined across isopycnals. The model also gives approximate duration times for the layers, and the horizontal scales of patchiness necessary to create layers of a given thickness.

Franks, P.J.S. 1995. Thin layers of phytoplankton: a model of formation
     by near-inertial wave shear. Deep-Sea Research I 42:75-91

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Peter J.S. Franks
Scripps Institution of Oceanography
University of California, San Diego
La Jolla, CA 92093-0218