A method for the measurement of particulate C and P on the same filtered sample

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.