Herbivore Diet Affects Fecal Pellet Settling

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Abstract
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.Key words: fecal pellets, herbivorous zooplankton, Calanus, nutrition, sinking rate, model