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

Abstract

There is substantial interest in measuring C:P ratios in aquatic systems because these particulate ratios may influence food quality of phytoplankton to zooplankton grazers. In most analyses C is measured by dry combustion using a CN analyzer requiring that P be measured on a separate sample. We show for marine and freshwater phytoplankton cultures and naturally occurring seston that C can be measured by a variation of the same wet-persulfate digestion method frequently used for P analysis, allowing C and P to be measured on the same filter. Over a large range of C values (1 to 40 μmol C filter-1), the persulfate method for measuring C gave the same C value for lake seston, estuarine seston, and marine and freshwater phytoplankton cultures 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 method in terms of both equipment and per-sample materials costs. Further, the persulfate method is more precise (CV = 3%) than the dry combustion method (CV = 12%) for the measurement of C. This greater precision of C measurement, 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.