Degradation of faecal pellets in Pegea confoederata (Salpidae, Thaliacea) and its implication in the vertical flux of organic matter

Abstract

Faecal pellets of P. confoederata were maintained in suspension for 10 days and changes in their morphological, microbiological and fatty acid characteristics as well as their resulting sinking rates were followed during this period. Among the morphological characteristics measured, the perimeter, the surface area and the longest dimension decreased linearly with time but their form remained unchanged. The weight decreased. Microbiological concentrations showed three stages, a rapid increase was followed by a rapid decrease and then finally a small re-increase for heterotroph populations and stabilization for autotrophs. Total fatty acid concentration followed a similar pattern to that of microbial concentrations with some exceptions for individual fatty acids. These three stages in degradation were attributed to incubation conditions as well as to trophic relationships. The sinking rates, measured in different turbulent conditions, varied from 2706 to 3646 m day −1 for the fresh faecal pellets and decreased 30% after 10 days incubation whatever the turbulent kinetic energy dissipation rate. For fresh faecal pellets, the observed sinking rates were greater in highly turbulent conditions than in moderately turbulent or in calm conditions. At the beginning of incubation, minimal sinking rates were observed in moderately turbulent conditions and after four days in calm conditions. This was attributed to the state of degradation of the faecal pellets. Considering the wind induced turbulence for the 0 to 200 m layer, high wind speeds delay sinking rates and increase residence times when compared to moderate or low wind speeds. According to our calculations, the residence times in this superficial layer of faecal pellets of P. confoederata, an oceanic salp species, never exceed 100 min (calculated for wind speeds varying from 0 to 24 m s −1).