Production, oxygen respiration rates, and sinking velocity of copepod fecal pellets: Direct measurements of ballasting by opal and calcite

Production, oxygen uptake, and sinking velocity of copepod fecal pellets egested by Temora longicornis were measured using a nanoflagellate (Rhodomonas sp.), a diatom (Thalassiosira weissflogii), or a coccolithophorid (Emiliania huxleyi) as food sources. Fecal pellet production varied between 0.8 pellets ind21 h21 and 3.8 pellets ind21 h21 and was significantly higher with T. weissflogii than with the other food sources. Average pellet size varied between 2.2 3105 mm3 and 10.0 3105 mm3. Using an oxygen microsensor, small-scale oxygen fluxes and microbial respiration rates were measured directly with a spatial resolution of 2 mm at the interface of copepod fecal pellets and the surrounding water. Averaged volume-specific respiration rates were 4.12 fmol O2 mm23 d21, 2.86 fmol O2 mm23 d21, and 0.73 fmol O2 mm23 d21 in pellets produced on Rhodomonas sp., T. weissflogii, and E. huxleyi, respectively. The average carbon-specific respiration rate was 0.15 d21 independent on diet (range: 0.08– 0.21 d21). Because of ballasting of opal and calcite, sinking velocities were significantly higher for pellets produced on T. weissflogii (322 6 169 m d21) and E. huxleyi (200 6 93 m d21) than on Rhodomonas sp. (35 6 29 m d21). Preservation of carbon was estimated to be approximately 10-fold higher in fecal pellets produced when T. longicornis was fed E. huxleyi or T. weissflogii rather than Rhodomonas sp. Our study directly demonstrates that ballast increases the sinking rate of freshly-produced copepod fecal pellets but does not protect them from decomposition. The biological carbon pump of the ocean is driven by sedimentation of phytoplankton aggregated in marine snow and fecal pellets. Through this mechanism, a substantial fraction of CO2 assimilated by phytoplankton is exported from the upper mixed surface waters into the deep ocean and sediments (Turner 2002). Sinking velocities of these particles range between 5 m d21 and 400 m d21 (Alldredge and Gotschalk 1988; Turner 2002). Fecal pellet sinking velocity is largely determined by its composition and size (Komar 1981). Fecal pellet size and density depend on copepod species, food concentrations, and food source (Feinberg and Dam 1998). The contribution of zooplankton fecal pellets to the overall vertical flux varies from a few percent up to 99% in the ocean (Turner 2002). A key feature determining potential vertical fluxes in the water column is the carbon-specific degradation rate of organic matter relative to its sinking velocity. Zooplankton fecal pellets produced on a dinoflagellate diet show high degradation rates and low sinking velocities leading to fast turnover in the water column (Hansen et al. 1996; Thor et al. 2003). In contrast, pellets produced on coccolithophorids show high sinking velocities, and a large fraction of these pellets reach depths below 1,000 m in the ocean (Harris 1994; Knappertsbusch and Brummer 1995). Fecal pellet degradation rate and sinking velocity is thus largely determined by the phytoplankton available as food source. Copepods are ubiquitous in the ocean and in neritic seas. Copepods play an important and dual role in mediating vertical carbon flux as well as nutrient retention within the 1 To whom correspondence should be addressed. Present address: Alfred Wegener Institute for Polar and Marine Research, Am Handelshafen 12, D 27570 Bremerhaven, Germany.