Herbivore Diet Affects Fecal Pellet Settling

The sinking rates of fecal pellets produced by herbivorous zooplankton grazing upon phytoplankton stocks composed primarily of diatoms and flagellates were studied and found to be significantly (P < 0.01) different. Under diatom diet, fecal pellets of Calanus sp. showed ranges of volume and sinking rate of 0.38–2.51∙106 μm3 and 70–171 m∙d−1, respectively; under flagellate-based diet the pellet volumes and sinking rates ranged from 0.42 to 3.52∙106 μm3 and 51 to 152 m∙d−1, respectively. Sinking rates for the two sets of fecal pellets were related to compositional density, and significantly (P < 0.01) correlated with the excess density (Δρ = ρp − ρsw) between the particle and the fluid. No significant (P < 0.05) correlation was found between the sinking rates and volumes of fecal pellets examined in these two sets. Calculated pellet densities (ρp) under diatom and flagellate-based nutrition averaged 1.174 and 1.114 g∙cm−3, and conferred average sinking rates of 123.3 and 87.9 m∙d−1, respectively. A mathematical treatment of the functional parameters relevant to the vertical flux of excretory materials is presented, and by application of steady-state turnover theory, provides a framework to estimate the downward transfer rate per unit area of any given pellet constituent considered.