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 exc hange of solutes between oxygenated overlying water and anoxic pore water, and thus promotes biogeochemical reactions that include degradation of sedi mentary 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 bioirrigatio n processes during early diagenesis using laboratory benthic mesocosms. Bio irrigation was carried out in the mesocosms by Schizocardium sp., a funnel- feeding enteropneust hemichordate that builds and ventilates a U-shaped bur row. Interpretation of the laboratory results was aided by a two-dimensiona l multicomponent model for transport and reactions that explicitly accounts for the depth-dependent distribution of burrows as well as the chemical ma ss transfers in the immediate vicinity of burrow walls. Our study shows tha t bioirrigation significantly affects the spatial distributions of pore wat er 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-dimensi onal bioirrigation modeling, can accurately describe the bioirrigation regi mes if its depth attenuation is coupled to the depth-dependent distribution of burrows. In addition, this study shows that the multicomponent 2D react ion-transport model is a useful research tool that can be used to criticall y evaluate common biogeochemical assumptions such as the prescribed depth d ependencies of organic matter degradation rate and C/N ratio, as well as th e lack of macrofaunal contribution of metabolites to the pore water.