Transport of diatom frustules by copepod fecal pellets to the sediments of Lake Michigan1

Zooplankton fecal pellets cxpellcd near the surface of lakes provide a mechanism for removal of diatoms to deeper water. Scanning electron micrographs show that copepod fecal pellets, collected from Lake Michigan, contained fractured specimens of 21 species of diatoms. Few intact diatoms were observed and many frustules were reduced to small fragments during ingestion by the copepods. The peritrophic membrane surrounding the pellets is composed of polysaccharides, one of which is chitin. This membrane is broken down by bacterial decomposition in 6-14 days as the pellets settle at an average velocity of 4.7 m d”. As a result, pellets released near the surface in water more than 70 m deep do not transport diatom fragments directly to the sediment. In a large proportion of Lake Michigan the transport of diatoms to the sediments via pellets originating near the surface is unlikely, since about 60% of the lake is >70 m deep. Recent studies have indicated the potential importance of planktonic crustacean fecal pellets as a downward transport mechanism in marine waters (Osterberg et al. 1963; Lowman et al. 1971; Cherry et al. 1975; Schrader 1971). A similar mechanism has not been described in freshwater systems. The purpose of our study was to determine the fate of zooplankton fecal pellets and their role in the sedimentation and dissolution of diatom frustules in Lake Michigan. Forster (1953) showed that the peritrophic membrane of feces in many crustaceans is composed of chitin. Gould (1957) suggested that the membrane of Calanoid copepods may also be chitinous; using a method developed by Campbell ( 1929)) he found no evidence of chitin but thought this probably due to the small amount of material used in the test. The length of time for which a pellet remains intact depends on the rate at which the peritrophic membrane is decomposed. We therefore investigated the chemical nature, mode, and rate of membrane degradation of zooplankton fecal pellets. We acknowledge the assistance of P. 1 Work performed uader the auspices of the U.S. Energy Research and Development Administration. Harris for analysis of the peritrophic membrane and G. Chubb for aid at the Biology Electron Microscope Center.