What happens to zooplankton faecal pellets? Implications for material flux

Copepod faecal pellets have often been considered as rapid transporters of material out of the euphotic zone. Laboratory experiments on their degradation and sinking rates support this view, but field data on the distribution and flux of pellets through the water colomn present contradictory evidence. We suggest that due to the exclusion of metazoans from previously published degradation experiments, such studies may have little relevance to the natural environments. In 1987/1988 we carried out experiments using adult copepods of mixed species but dominated byCentropages hamatus collected in Kiel Bight (FRG). We have demonstrated that copepods can be highly adept at breaking up their own pellets while ingesting only a small proportion, a behaviour we define as “coprorhexy”. The microbiota is probably unable to cause significant modification to faecal pellets before they are fragmented within a few hours of their production. Thereafter, microbial remineralisation will become important. Many of the “difficult” field data can be readily explained if the process of coprorhexy is taken into account and, indeed, breakage of large particles by crustacean zooplankton may be an important process in modifying material transport in the ocean. Copepods appear to perform coprorhexy by removing the peritrophic membrane with its attached bacterial flora and this may then be ingested. We speculate on the nutritional value of such a behaviour and the possible significance of “ghost” pellets, consisting of a membrane with little or no apparent solid content.