We have carried out a series of experiments designed to allow comparison of sediment irrigation rates determined simultaneously using two methods: the measurement of Rn-222/Ra-226 disequilibrium in pore waters, and the measurement of distributions of a tracer, Br-, which was added to the water overlying sediments at the start of incubation experiments. The experiments were carried out on fine-grained sediments from Buzzards Bay, MA. We made irrigation rate measurements on sediments in their natural state, as well as on sediments that had been treated to alter macrofaunal abundance nd diversity. The range of irrigation rates measured was similar for both tracers, and was similar to rates measured at the study site previously by Martin and Sayles (1987). Furthermore, the two tracers gave similar patterns of irrigation rate variability between cores and with depth below the sediment-water interface. On the other hand, comparisons of individual cores showed significant differences in the absolute rates measured using the different tracers in particular, the Rn-222/Ra-226 disequilibrium method yielded more rapid irrigation rate estimates at depths exceeding 10 cm below the sediment-Water interface. These differences could be due to the inherent limitations on the sensitivity of the methods, to artifacts in measurement procedures, to differences in the permeability of burrow walls to the two tracers (Rn and Br-), or to differences in the time-scales on which the two tracers record irrigation events. Irrigation rates determined by the Br tracer method were roughly correlated with the abundance of Nephtys incisa in the sediments, but were not related to abundances of the other numerically important deposit feeders, Nucula annulata and Mediomastus ambiseta.
