Unialgal cultures were used to investigate relationships between cell volume and the carbon and nitrogen content of nondiatomaceous marine nanophytoplankton. Cell dimensions were determined by image-analyzed epifluorescence microscopy and particulate C and N by high-temperature dry combustion. Volumes were calculated by direct integration with published algorithms (biovolume), but could be estimated equally well from linear dimensions as prolate spheres. Preservation with 0.5% glutaraldehyde reduced cell volumes 29% on average. Correlations were highly significant between biovolume of preserved cells and C and N contents. Nonlinear regression models appeared most appropriate because smaller cells contained more C and N per unit volume than did larger cells. Suggested general C densities for estimating cell C from preserved volume were 0.36 pg pm-3 for 10' pm3 cells, 0.24 pg pm-3 for lo2 pm3 cells, and 0.16 pg C pm-3 for lo3 pm3 cells. Previous regression models substantially underestimated the C densities of nanophytoplankton of lo*-lo3 l.cm3. The explanation for these differences includes the method of determining mean population volumes, the use of preservatives, and the occurrence of significant vacuolar volume in larger phytoplankton.

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