A model of pycnocline thickness modified by the rheological properties of phytoplankton exopolymeric substances

Abstract

We model how phytoplankton-produced exopolymeric substances (EPS) may change pycnocline thickness δz through control by Richardson number Ri. Shear stress τFORCED is imposed across the pycnocline, giving a shear rate δu/δz, where δu is cross-pycnocline velocity difference, modulated by viscosity η. In natural waters, viscosity is composed of two components. The first is Newtonian, perfectly dispersed viscosity due to water and salts. The second is non-Newtonian, and depends on phytoplankton abundance raised to an exponent between +1.0 and +1.5. It is also generally shear-thinning, depending on (δu/δz)P, where P is negative. In suspensions of microbial aggregates, viscosity depends also on (length scale)d. Published measurements of EPS rheology in a culture of Karenia mikimotoi are input. These measurements were made at 0.5 mm, so they can be scaled to δu/δz if d is known for this EPS over the appropriate range of length scales. The model shows that δu/δz is very sensitive to d. At values of d <31 million L-1) have no significant effect on δz at ambient values (>1 cm). Future investigations of pycnocline dynamics should include measurements of rheological properties, and particularly d. © The Author 2010.