The growth rates and buoyancy properties of 3 oceanic diatoms in the genus Rhizosolenia were examined at light levels from 8 to 211 mu-mol quanta m-2 s-1. Maximum growth rates ranged from 0.37 to 0.78 divisions 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 rate and tolerance to high light levels decreased substantially as cell size decreased. Batch culture C:chlorophyll ratios (130 to 261) replicated values found in field Ethmodiscus and Rhizosolenia mats, and suggest that the elevated C: chlorophyll ratios found in buoyant, oceanic phytoplankton are typical of healthy cells. Calculations suggest that carbohydrate ballasting can account for buoyancy changes and that these reserves are adequate to support dark NO-3- 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 dynamic light-related buoyancy changes probably occur on a shorter time scale than these batch culture experiments. These results indicate that vertical migration is a property basic to these diatoms life history strategy, and, like multispecies Rhizosolenia mats, solitary Rhizosolenia chains transport new nitrogen to the euphotic zone in oligotrophic seas.
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