ABSTRACT: Copepod fecal pellet carbon was measured using a high temperature combustion (HTC) method that permitted the analysis of individual pellets. Fecal pellets were directly injected into the HTC system along with either 50 or 100 µl of low dissolved organic carbon seawater. The standard deviation around seawater blanks was <5% of the total organic carbon content of the fecal pellets. In comparison, the standard deviation around blanks for the 'traditional' CHN analysis method was 12 to 25% of the fecal pellet carbon. Therefore, the HTC method offers less uncertainty in the measurement. Rapid freezing of fecal pellets for storage did not affect the carbon/volume relationships (i.e. slope or the y-intercept of the regression line between pellet carbon and volume). At a fixed chlorophyll concentration (0.6 g µl^-1) in a laboratory culture, the carbon/volume ratio (slope of the regression line) was the same regardless of the type of phytoplankton fed to the copepods. The carbon/volume ratio of the copepod pellets increased approximately linearly with increasing chlorophyll concentration, indicating that changes in food concentration affected the rate of change in carbon per unit volume of the fecal pellets. The y-intercept was not significantly different from zero for all diets in the laboratory studies. This result suggests that changing between diatom diets (Phaeodactylum tricornutum or Thalassiosira weissflogii) and a chlorophyte diet (Dunaliella tertiolecta) did not significantly affect the carbon concentration per unit volume within fecal pellets and that the method was adequately correcting for blank errors. Copepod fecal pellets were also collected as part of the U.S. JGOFS Arabian Sea study during March-April 1995 and were analyzed with the HTC method. In the field study, there was a significant difference in carbon content per unit volume among the different pellet colors (food types). These differences between laboratory and field results may be due either to zooplankton species differences or to a heterotrophic diet in the field. The slope of the carbon/volume regression line was similar for all pellet types yet the concentration of carbon in the pellets varied in the field study. The field results suggest that a constant carbon/volume value cannot be used for fecal pellets to calculate their potential particulate organic carbon flux.

KEY WORDS: Copepod · Fecal pellet · Carbon · High temperature combustion method

Published in MEPS Vol. 171 (1998) on October 1
ISSN: 0171-8630. Copyright © Inter-Research, Oldendorf/Luhe, 1998