Three large diatoms, Stephanopyxis palmeriana (Greville) Grunow, Pseudoquinardia recta von Stosch, and Navicula sp. (cell volumes 1.15 x 10(5) to 3.83 x 10(5) mum3), were isolated from the Sargasso Sea and cultivated in batch cultures under low irradiance. Growth rates mu of each species occurred in 2 phases, an exponential phase where mu varied from 0.72 to 1.12 d-1 and a much slower transition phase that lasted from 3 to 6 d. We suspect that diffusion limitation of nutrient transport controlled growth rates during this latter phase. Exponential growth rates were rapid enough to meet the requirements of a bloom scenario whereby total annual new production in a locale such as the Sargasso Sea could be met in a single 21 d bloom, The C:N:P ratio of all 3 species was close to the Redfield proportions during exponential growth. Uncoupling between photosynthesis and nutrient acquisition was evident in 1 species, S. palmeriana, with carbon accumulation, both in the form of phytoplankton carbon and dissolved organic carbon, continuing well into the stationary phase, long after nutrients were depleted from the growth medium. In fact, 50% of particulate organic carbon production occurred after the culture entered the stationary phase. Clearly there is ambiquity in the traditional definition of new production which implies that over a relatively short time scale a balance exists between new nitrogen entering the euphotic zone and new carbon production, assuming a Redfield stoichiometry between cellular carbon and nitrogen. The excess carbon, both particulate and dissolved, could lead to large discrepancies in estimates of new production.
