Biological Redistribution of Lake Sediments by Tubificid Oligochaetes: Branchiura sowerbyi and Limnodrilus hoffmeisteri/Tubifex tubifex

Mechanisms and rates of sediment mixing by the largest oligochaete in Lake Erie, Branchiura sowerbyi, have been quantitatively investigated using a multiple ¹³⁷Cs tracer layer microcosm technique and compared with a mixture of the dominant tubificids, Limnodrilus hoffmeisteri + Tubifex tubifex. These worms feed head down in the sediment (up to 20 cm for B. sowerbyi and up to 10 cm for L. hoffmeisteri/T. tubifex) on organic-rich particles and deposit fecal pellets at the sediment-water interface (conveyor-belt feeding). Obliteration of tracer layers by these worms was attributed to mixing by both diffusive- and feeding-style (advective) processes. The downward velocities were 2.87 to 3.66 cm/d/100,000 indiv/m² for the cells with B. sowerbyi (～10 cm body length, 13 mg body mass) and 0.33 to 0.49 cm/d/100,000 indiv/m² for the cells with L. hoffmeisteri / T. tubifex (～5 cm body length, 1 mg body mass). These downward velocities correspond to sediment fluxes across the sediment-water interface of 66.4 to 86.4 g dry sediment/indiv/m²/yr in cells with B. sowerbyi and 5.91 to 9.09 g dry sediment/indiv/m²/yr in the cells with L. hoffmeisteri / T. tubifex). The differences between species was due to differences in biomass, with recycling rates of 5.11 to 6.65 and 5.91 to 9.09 g dry sediment/mg biomass/m²/yr for B. sowerbyi and L. hoffmeisteri/T. tubifex, respectively. Similarly, biomass normalized downward velocities were 806 to 1,028 cm/yr/kg biomass/m² and 1,205 to 1,789 cm/yr/kg biomass/m² for B. sowerbyi and L. hoffmeisteri/T. tubifex, respectively. Both B. sowerbyi and L. hoffmeisteri / T. tubifex feed selectively on organic-rich fine-grained particles and showed an increase in particle selectivity with an increase in population density. The particle selectivity factor values ranged from 1.0 to 2.5. Food competition at a higher population density might force these organisms to selectively feed on a smaller size range of sediments. The maximum feeding rate for B. sowerbyi (4,000 to 8,000 indiv/m²) ranged from 9.10 to 13.9 per yr at depths between 11.7 and 13.6 cm while the maximum biodiffusion coefficient, D_b, ranged from 0.78 to 1.02 cm²/yr at depths between 1.6 and 2.3 cm. The maximum feeding rate for L. hoffmeisteri / T. tubifex (20,000 to 40,000 indiv/m²) was 8.13 to 13.1 per yr at depths between 5.21 and 5.27 cm and D_b ranged from 0.20 to 0.72 cm²/yr at depths between 0.87 and 2.0 cm.