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

Global marine N budgets often show deficits due to dominance of benthic N₂ production relative to pelagic N₂ fixation. Recent studies have argued that benthic N₂ fixation in shallow water environments has been underestimated. In particular, N₂ fixation associated with animal burrows may be significant as indicated by high rates of N₂ fixation reported in muddy sands populated by the ghost shrimp, Neotrypaea californiensis (Bertics et al., 2010). We investigated whether N₂ 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 N₂ concentrations in burrow-water relative to bottom-water. Acetylene reduction assays showed lower N₂ fixation in burrow-walls than in un-populated sediments, likely due to inhibitory effects of O₂ on ethylene production. Dissolved N₂ was higher in burrow-water than proximate bottom-water at all seasons, demonstrating a consistent balance of net N₂ production relative to fixation in deep-tier biogenic structures.