Oxygen consumption rate of copepod fecal pellets : variations among copepod species, prey types and prey nutritional values

Hypoxia has increasingly become a global issue due to increasing input of anthropogenic nutrients. Hypoxic bottom water is often formed from high algal biomass and subsequent sinking and bacterial decomposition. More research is starting to focus on the contribution of zooplankton fecal pellets to hypoxia. In this study, I focus on the degradation processes and oxygen consumption of zooplankton fecal pellets produced in laboratory cultures. First, fecal pellets of three copepod species, Acartia spinicauda, Paracalanus sp. and Centropages orsinii, feeding on the diatom Thalassiosira pseudonana were used to test the oxygen consumption rate. The results showed fecal pellets from different copepods were degraded at different rates, with fecal pellets from Centropages orsinii being the highest (95.47 fmol O2 μm-3) and that from Acartia spinicauda being the lowest (17.72 fmol O2 μm-3). Next, oxygen consumption rates of fecal pellets produced by Acartia spinicauda fed with 3 different algal prey, a diatom Thalassiosira pseudonana, a crytophyte Rhodomonas sp. and a chlorophyte Dunaliella sp., were investigated. The results showed fecal pellets produced from the diatom diet were degraded at the lowest rate (19.5 fmol O2 μm-3 d-1) whereas that from a chlorophyte diet were degraded at the highest rate (40 fmol O2 μm-3 d-1). The fecal pellets from the diatom diet degraded at the lowest rate due to their relatively low nutritional value (high C: N ratio). However, because of their fast sinking rates, they can consume significant dissolved oxygen in the bottom layer of the water column or at the surface sediment during a spring bloom that is typically dominated by diatoms. Furthermore, fecal pellets of Acartia spinicauda fed with a diatom Thalassiosira pseudonana growing in medium that is potentially nitrogen or phosphorus limited were used to test the oxygen consumption rate. The results showed that fecal pellets from an N and P balanced diet were degraded at the highest rate (48.3 fmol O2 μm-3 d-1) whereas that from N-limited diet were degraded at the lowest rate (24.1 fmol O2 μm-3 d-1), but some of the results are difficult to explain. Nevertheless, it was clear that the oxygen consumption rate of fecal pellets from N and P balanced growth condition consumed the most oxygen, whereas pellets from either N or P deficient diets were lower, indicating both N and P contents determine the degradability of the fecal pellets. My findings suggest that fecal pellets in eutrophic estuarine coastal waters will consume more dissolved oxygen, which may contribute to the formation of coastal hypoxia, or the so called “dead zone”