Bioirrigation modeling in experimental benthic mesocosms

Burrow irrigation by benthic infauna affects chemical mass transfer regimes in marine and estuarine sediments, The bioirrigation facilitates rapid exchange of solutes between oxygenated overlying water and anoxic pore water, and thus promotes biogeochemical reactions that include degradation of sedimentary organic matter and reoxidation of reduced species. A comprehensive understanding of chemical mass transfer processes in aquatic sediments thus requires a proper treatment of bioirrigation. We investigated bioirrigation processes during early diagenesis using laboratory benthic mesocosms. Bioirrigation was carried out in the mesocosms by Schizocardium sp., a funnel-feeding enteropneust hemichordate that builds and ventilates a U-shaped burrow. Interpretation of the laboratory results was aided by a two-dimensional multicomponent model for transport and reactions that explicitly accounts for the depth-dependent distribution of burrows as well as the chemical mass transfers in the immediate vicinity of burrow walls. Our study shows that bioirrigation significantly affects the spatial distributions of pore water solutes. Moreover, bioirrigation promotes burrow walls to be the site of steep geochemical gradients and rapid chemical mass transfer. Our results also indicate that the exchange function, alpha, widely used in one-dimensional bioirrigation modeling, can accurately describe the bioirrigation regimes if its depth attenuation is coupled to the depth-dependent distribution of burrows. In addition, this study shows that the multicomponent 2D reaction-transport model is a useful research tool that can be used to critically evaluate common biogeochemical assumptions such as the prescribed depth dependencies of organic matter degradation rate and C/N ratio, as well as the lack of macrofaunal contribution of metabolites to the pore water.