Sinking characteristics of dinoflagellate cysts1

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Abstract
This study reports the first direct measurements of the density and sinking rates of marine dinoflagellate cysts. Gyrodinium uncatenum, Gonyaulax tamarensis, and Scrippsiella trochoidea cysts had densities of 1.14, 1.24, and 1.32 g cm−3. No significant difference between cultured and natural cyst density was observed. Measured settling velocities ranged from 0.008 to 0.013 cm s−1 (6–11 m d−1). Settling rates calculated using the measured densities and reasonable assumptions about shape and orientation during descent were within 10–20% of measured sinking rates, confirming that cyst sinking can be described by a modification of Stokes’ law for nonspherical particles in a viscous medium. The three types of cysts examined are more dense and fall faster than most vegetative phytoplankton cells. Removal of an outer layer of short calcite spines from S. trochoidea cysts reduced both cell density and radius by 7% and the sinking rate by 37%. The faster settling velocity of the spiny cyst is not considered a general effect common to all ornamented cysts but presumably applies only to those with numerous short spines or processes. Morphological features can thus markedly affect the rate of cyst deposition, but the adaptive significance of surface ornamentation remains unknown.