N2 production and fixation in deep-tier burrows of Squilla empusa in muddy sediments of Great Peconic Bay

Abstract Global marine N budgets often show deficits due to dominance of benthic N 2 production relative to pelagic N 2 fixation. Recent studies have argued that benthic N 2 fixation in shallow water environments has been underestimated. In particular, N 2 fixation associated with animal burrows may be significant as indicated by high rates of N 2 fixation reported in muddy sands populated by the ghost shrimp, Neotrypaea californiensis (Bertics et al., 2010). We investigated whether N 2 fixation occurs at higher rates in the burrow-walls of the deep-burrowing (~ 0.5–4 m) mantis shrimp, Squilla empusa , compared to ambient, estuarine muds and measured seasonal in-situ N 2 concentrations in burrow-water relative to bottom-water. Acetylene reduction assays showed lower N 2 fixation in burrow-walls than in un-populated sediments, likely due to inhibitory effects of O 2 on ethylene production. Dissolved N 2 was higher in burrow-water than proximate bottom-water at all seasons, demonstrating a consistent balance of net N 2 production relative to fixation in deep-tier biogenic structures.