Bioturbation rates in muddy sediments are thought to be due primarily to the reworking activities of benthic deposit feeders. However, current mathematical models of bioturbation do not explicitly link rates of particle mixing with realistic biological reworking mechanisms. To address this problem, I present a transition-matrix model of bioturbation that quantitatively links the reworking activities of individual organisms and community-level particlemixing rates. Solutions to the model are presented for two kinds of tracers; particle-reactive radionuclides with a constant input flux and conservative tracers added to the sediment as a pulse. The model was used to predict the vertical profiles of excess 234 Th and 210 Pb in the field. The model parameters were determined from benthic community-structure data. Model predictions were then compared to measured profiles of these tracers. On the basis of this comparison, I inferred that maldanid polychaetes at the study site were collecting sediment at the sedimentwater interface and depositing it at depth. This transport mechanism had a large effect on the predicted tracer profiles. A sensitivity analysis of the model indicated that deposit feeding by the two most abundant species, Mediomastus ambiseta and Nucula annulata, was the most important process determining the burial rate of the tracers. The model results also indicated that the combined effects of deposit feeding and sedimentation were sufficient to determine the vertical distributions of excess 234 Th and 210 Pb at the study site.

Coastal marine sediments are important sites of methylmercury (MMHg) production, and dissolved efflux provides an important source of MMHg to near-shore, and possibly offshore, water columns and food webs. We measured the flux of MMHg across the sediment-water interface at four stations in Boston Harbor that span a range of infaunal population densities and bioirrigation intensities. At each station we carried out total MMHg flux measurements using core incubations and collected near-surface pore waters to establish MMHg gradients for diffusive flux calculations. The flux cores were also imaged by CT scanning to determine the distribution of infaunal burrows, and pore-water sulfide and 222Rn profiles were measured. Total MMHg fluxes, measured using core incubations, ranged from -4 to 191 pmol m(-2) d(-1), and total MMHg fluxes were strongly correlated with burrow densities at the stations. Estimated diffusive fluxes, calculated based on MMHg concentration gradients below the sediment-water interface, were much lower than total fluxes at three of the stations, ranging from 2-19 pmol m(-2) d(-1). These results indicate that MMHg exchange may be significantly enhanced over molecular diffusion in bioturbated sediments. Furthermore, burrow density provides a strong predictor of total MMHg flux. Pore-water exchange of both dissolved MMHg and 222Rn, a naturally occurring pore-watertracer, increased across the range of observed burrow densities, suggesting that the presence of burrows enhances both MMHg production and flux.

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The deposition of surficial sediments many centimeters below the sediment-water interface due to the reworking activities of organisms is a potentially important but easily overlooked process in marine sediments. This kind of downward particle transport is difficult to observe in the laboratory or in the field but it has important consequences for bioturbation rates and sediment geochemistry. It is also much more likely to be size dependent than other sediment-mixing mechanisms, such as conveyor-belt feeding, and may also explain some subsurface maxima observed in sediment chemical profiles.We examined the mechanisms behind downward particle transport in Boston Harbor. Laboratory observations indicated that a large cirratulid polychaete, Cirriformia grandis, collected particles (glass beads) near the sediment surface and deposited them at depth. Furthermore, particle collection by this species was size dependent. C. grandis preferred smaller particles in the 16-to 32-m size range relative to larger particles.A mathematical model was developed to simulate the feeding and burrowing mechanisms of C. grandis and to predict the vertical profiles of tracer particles of assorted sizes in the field. The model was tested by comparing predicted profiles with profiles of glass beads measured at the field site. These glass beads were deployed in replicated patches on the bottom of Boston Harbor. Vertical distributions of the beads after 99 d were compared to profiles predicted by the model. Good agreement between predicted and measured profiles indicated that the feeding and burrowing mechanisms of C. grandis were sufficient to determine observed patterns of size-dependent bioturbation rates at this site.

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