BUOYANCY AND GROWTH-CHARACTERISTICS OF 3 POSITIVELY BUOYANT MARINE DIATOMS

The growth rates and buoyancy properties of 3 oceanic diatoms in the g enus Rhizosolenia were examined at light levels from 8 to 211 mu mol q uanta m(-2) s(-1). Maximum growth rates ranged from 0.37 to 0.78 divis ions d(-1) with saturation occurring between 29 and 164 mu mol quanta m(-2) s(-1). Severe growth rate depressions were noted in R. acuminata and R. formosa at irradiance levels above 50 to 155 mu mol quanta m(- 2) s(-1). In all 3 species the percentage of positively buoyant cells was inversely related to light intensity. In R. formosa both growth ra te and tolerance to high light levels decreased substantially as cell size decreased. Batch culture C:chlorophyll ratios (130 to 261) replic ated values found in field Ethmodiscus and Rhizosolenia mats, and sugg est that the elevated C:chlorophyll ratios found in buoyant, oceanic p hytoplankton are typical of healthy cells. Calculations suggest that c arbohydrate ballasting can account for buoyancy changes and that these reserves are adequate to support dark NO3- uptake. Under steady-state conditions in situ, the observed growth and buoyancy properties would lead to subsurface population maxima in all 3 species. However, the d ynamic light-related buoyancy changes probably occur on a shorter time scale than these batch culture experiments. These results indicate th at vertical migration is a property basic to these diatoms Life histor y strategy, and, like multispecies Rhizosolenia mats, solitary Rhizoso lenia chains transport new nitrogen to the euphotic zone in oligotroph ic seas.