Modeling lugworm irrigation behavior effects on sediment nitrogen cycling

Benthic infauna in marine sediments have well-documented effects on biogeochemical cycling, from individual to ecosystem scales, including stimulation of nitrification and nitrogen removal via denitrification. However, the effects of burrowing depth and irrigation patterns on nitrogen cycling have not been as well described. Here we examined the effects of lugworm behavior on sediment nitrogen cycling using a reaction-transport model parameterized with literature and laboratory data. Feeding pocket depth and pumping characteristics (flow rate and pattern) were varied, and rates of nitrification, denitrification, and benthic exchange fluxes were computed. As expected, more intense burrow irrigation stimulated denitrification and coupled nitrification-denitrification. At high pumping rates and low sediment oxygen consumption rates (similar to 10(-6) mol m(-3) s(-1)), simulation results showed a decrease in rates of nitrification and denitrification with decreasing burrow depth due to incomplete consumption of injected oxidants. Model results also suggest that discontinuous irrigation leads to temporal variability in sediment nitrogen cycling, but that the time-averaged rates do not depend on the irrigation pattern. We identify (1) the poorly constrained chemical composition of lumen fluid injected into sediments and (2) the response of microbial activity/distribution to oscillating redox conditions as critical knowledge gaps affecting estimates of sediment nitrogen removal.