A Low-Reynolds number conundrum How fast should diatoms sink

Meeting Abstract

P3.46  Saturday, Jan. 5  A Low-Reynolds number conundrum: How fast should diatoms sink? MIKLASZ, K.A.*; DENNY, M.W.; Hopkins Marine Station of Stanford University, Pacific Grove; Hopkins Marine Station of Stanford University, Pacific Grove kmiklasz@stanford.edu

As one of the ocean�s primary producers, diatoms are an important source of fixed carbon. Consequently, it is important to understand the movement and flux of diatoms through the water column, or how fast diatoms of different sizes sink. This involves finding a relationship of the form V~rn, where r is a general size measurement and V is sinking speed. Fortunately, fluid dynamic theory predicts that for small objects such as diatoms, this relationship should be V~r2. Unfortunately, empirical data collected for diatoms over the last fifty years suggest an exponent of anywhere between 1 and 1.5, but not 2. This study reconciles this discrepancy between empirical data and fluid dynamic theory. A diatom�s mass is mostly concentrated in its outer frustrule, whereas its inner cytoplasm is nearly neutrally buoyant. Since the diatom�s frustrule is relatively thin and heavy, the diatom�s density scales as a surface area instead of a volume. If a surface area scaling law is incorporated into the fluid dynamic theory and several assumptions are made about the diatom�s geometry, then the relationship becomes V~r1. If the assumptions about geometry are not exactly true, the exponent can be anywhere between 1 and 2. Preliminary data comparing the sinking speed of cleared and fixed diatoms has confirmed that this modification to the fluid dynamic theory is the answer to the discrepancy. As a result, this study offers an equation that estimates a diatom�s sinking speed based on its geometry and size characteristics, which can be measured under a light microscope.

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