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.
Vertical profile
time series of layer development over 1/2 wave period, T.
Vertical section
time series of layer development. Upper left panel is initial condition.
All other panels are differences from intial condition.
Animation
of the vertical section from above. Velocities are given at right.
Several cases of
the model with different parameters, and some real data, courtesy of Dr.
Tim Cowles, Oregon State University.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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