Limnol. Oceanogr., 44(2), 1999, 415–424

Using 68Ge : 14C dual-labeling, we investigated the absorption efficiency of diatom carbon for the cold water appendicularian Oikopleura vanhoeffeni. The absorption efficiency of bulk carbon (mean 5 67%) was not influenced by body size or ingestion rate. For the first time for a pelagic tunicate, food and feces were fractionated into their major biochemical constituents (i.e., low-molecular-weight compounds, lipid, protein, and polysaccharide), allowing calculation of absorption efficiencies for each fraction. Low-molecular-weight compounds and proteins were preferentially absorbed over lipids and polysaccharides. However, predicted C : N ratios of the fecal pellets of O. vanhoeffeni were in the lower range of C : N ratios reported for zooplankton feces. The results are relevant for modeling biogeochemical cycles because pelagic tunicates contribute greatly to vertical particulate organic carbon flux. Quantifying organic matter absorption is important to understanding feeding ecology. On an autecological level, the extent to which various organic nutrients are absorbed is important for growth and survival of individuals and may elucidate feeding and digestive mechanisms (Checkley 1980; Houde and Roman 1987; Kiørboe 1989; Roman 1991). On a synecological level, changes in the composition of particulate organic matter caused by ingestion and digestion by major grazers are important for modifying biogeochemical fluxes, since pellets of large zooplankton are a primary mode of particle flux (Suess 1980; Michaels and Silver 1988; Fortier et al. 1994). Pelagic tunicates are among the major grazers that remove a significant fraction of primary production from the water column (Alldredge 1981; Deibel 1988; Knoechel and SteelFlynn 1989); they also produce fast-sinking fecal pellets (Michaels and Silver 1988; Fortier et al. 1994) and, in the case of appendicularians, discard a large number of fastsinking feeding webs or ‘‘houses’’ (Taguchi 1982). Although absorption efficiencies have been studied in great detail for crustacean zooplankton, especially copepods (Conover 1966 a,b) and krill (Lasker 1960), information pertaining to pelagic tunicates is sparse. Gorsky (1980) examined absorption efficiencies in the appendicularian Oikopleura dioica, Andersen (1986) in the salp Salpa fusiformis, and Madin and Purcell (1992) in the salp Cyclosalpa bakeri. Absorption efficiencies have not yet been determined for doliolids and pyrosomids (Madin and Deibel 1997). In the present study, we investigated the absorption efficiencies for bulk carbon of the cold water appendicularian Oikopleura vanhoeffeni feeding on the laboratory-grown diatom Thalassiosira nordenskioldii. We use the term ‘‘absorption’’ instead of ‘‘assimilation’’ because, strictly defined, 1 Present address: Fish Ecology Lab, Biology Department, Queen’s University, Kingston, Ontario, Canada K7L 3N6.