Effects of diet on dimensions, density and sinking rates of fecal pellets of the copepod Acartia tonsa

The effects of food type and concentration on fecal pellet characteristics of the calanoid copepod Acartia tonsa were examined in the laboratory. Copepods were fed several autotrophic and heterotrophic diets, including the diatoms Thalassiosira weissflogii and Chaetoceros neogracile, the photosynthetic flagellates Rhodomonas lens and Tetraselmis sp., the heterotrophic dinoflagellate Oxyrrhis marina, the heterotrophic flagellates Cafeteria sp. and Oikomonas sp., and the scuticociliate Uronema sp. Copepods were fed both a low (similar to 100-300 mu g C l(-1)) and a high (similar to 500-1500 mu g C l(-1)) concentration of these diets. Length, width and density of the resulting fecal pellets were measured. Sinking rates were calculated from a semi-empirical model based on these parameters. In general, diets that resulted in large pellets also resulted in the least dense pellets. Ciliate and diatom diets produced the largest pellets and resulted in the fastest estimated sinking rates. Heterotrophic flagellate and heterotrophic dinoflagellate diets resulted in the most dense and slowest sinking pellets. Within a diet, significant differences in pellet characteristics were often found between food concentrations, but there was no consistent pattern of increasing or decreasing pellet size or density with an increase in food concentration. The coefficient of variation of pellet sinking rates across all diets in this study was nearly 40%. This indicates the uncertainty in estimated sinking rates if diet is not considered. Combining the sinking rates from this study with published diet-specific fecal pellet degradation rates, we define an L-ratio, the fraction of pellet degradation per unit length of sinking. The L-ratio may be useful in predicting the degree of recycling of pellets within the mixed layer. Diatoms show the lowest L-ratios and photosynthetic flagellates the highest L-ratios.