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

Abstract

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 ind−1 h−1 and 3.8 pellets ind−1 h−1 and was significantly higher with T. weissflogii than with the other food sources. Average pellet size varied between 2.2 ×105 µm3 and 10.0 ×105 µm3. 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 µm−3 d−1, 2.86 fmol O2 µm−3 d−1, and 0.73 fmol O2 µm−3 d−1 in pellets produced on Rhodomonas sp., T. weissflogii, and E. huxleyi, respectively. The average carbon‐specific respiration rate was 0.15 d−1 independent on diet (range: 0.08‐0.21 d−1). Because of ballasting of opal and calcite, sinking velocities were significantly higher for pellets produced on T. weissflogii (322 ± 169 m d−1) and E. huxleyi (200 ± 93 m d−1) than on Rhodomonas sp. (35 ± 29 m d−1). 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.