The effect of food concentration on fecal pellet size in marine copepods1

Abstract

Three species of copepods, Temora turbinata, Eucalanus pileatus, and Neocalanus plumchrus, were fed cultures of the diatom, Thalassiosira weissflogii, at concentrations ranging from 0.3 to 14.5 µ g Chl liter −1 to determine whether fecal pellet size was related to food concentration. In all three copepods, pellet size increased with food concentration up to about 3 µ g Chl liter −1 . At higher concentrations pellet size was constant. Pellets were less compact and appeared to be more fragile at low food concentrations. The level of gut contents in these copepods was also related to food concentration up to about 3 µ g Chl liter −1 . Below this concentration, ingestion and defecation were balanced in such a way that the gut did not fill, and therefore pellet size was smaller. Food concentrations sufficient to allow a copepod to fill its gut result in the production of fecal pellets of maximum size. It is concluded that fecal pellets produced by large copepods under conditions of low food availability are less likely to sink out of the euphotic zone than pellets produced by the same copepods under conditions of higher food availability.