There is substantial interest in measuring C:P ratios in aquatic systems be
cause these particulate ratios may influence food quality of phytoplankton
to zooplankton grazers. In most analyses C is measured by dry combustion us
ing a CN analyzer requiring that P be measured on a separate sample. We sho
w for marine and freshwater phytoplankton cultures and naturally occurring
seston that C can be measured by a variation of the same wet-persulfate dig
estion method frequently used for P analysis, allowing C and P to be measur
ed on the same filter. Over a large range of C values [1 to 40 mu mol C fil
ter(-1)), the persulfate method for measuring C gave the same C value for l
ake seston, estuarine seston, and marine and freshwater phytoplankton cultu
res as high temperature dry combustion on a Carlo-Erba CN analyzer. For C,
the wet digestion method is much less expensive than the dry combustion met
hod in terms of both equipment and per-sample materials costs. Further, the
persulfate method is more precise (CV = 3 %) than the dry combustion metho
d (CV = 12 %) for the measurement of C. This greater precision of C measure
ment, and the fact that C and P are measured on the same sample, means that
small differences in C:P ratios can more readily be detected.