SIZE-ASCENT RATE RELATIONSHIPS IN POSITIVELY BUOYANT MARINE DIATOMS

Cell size and ascent rate were measured in individual short chains (2- 8 cells) and single cells of positively buoyant marine diatoms from na tural populations and cultures. Maximum ascent rates for Rhizosolenia formosa, Rhizosolenia acuminata, Rhizosolenia castracanei, Rhizosoleni a debyana, and Ethmodiscus rex suggest an upper limit to diatom ascent rates on the order of 7-8 m h(-1). Ascent rates were weakly correlate d with cell diameter, volume, and surface:volume ratio across the pool ed data for all species (n = 105). Among Rhizosolenia spp., ascent rat e was strongly correlated with size-related measures. The Rhizosolenia spp. average diameter was highly correlated with average ascent rate (r(2) = 0.99, n = 4). Calculated cell density was inversely related to cell diameter in Rhizosolenia spp. Cell diameter is a better predicto r of ascent rate than volume in long, cylindrical cells owing to covar iations of density and form resistance. The observed ascent rates are consistent with vertical migration by these large diatoms in oligotrop hic seas.