Hydraulic activities by ghost shrimp Neotrypaea californiensis induce oxic-anoxic oscillations in sediments

Porewater pressure sensing, time-lapse photography, and planar optode imaging of oxygen were applied to investigate hydraulic activities by thalassinidean ghost shrimp Neo trypaea californiensis and the associated dynamics of oxygen in and around their burrows. Ghost shrimp were hydraulically active 40% of the time. For >90% of this time the sediment was pressurized, which caused transport of oxygenated water into the burrow and the surrounding sediment, and transport of anoxic water through the sediment-water interface. Sediment blow-outs and irrigation episodes with the shrimp in head-down orientation caused flow reversals and porewater transport towards the burrow. At sediment permeabilities >2 x 10(-1)3 m(2) these porewater pressure dynamics were detected throughout the entire sediment volume within the shrimp tanks (6 l) even if a second burrow opening was established. Intermittent burrow irrigation performed at different locales within the burrow was sufficient to maintain oxic conditions during >90% of the time in burrow parts frequented by the shrimp. In contrast, sediment surrounding the burrows and surficial sediment were characterized by irregular oxygen supply, with oxic and anoxic conditions alternating on the time-scale of 1 to 1.5 times h(-1). The oscillatory regions around the burrows were restricted to a few millimeters in muddy sediments, but extended up to 4 cm away from the burrow in more permeable sediments. The oscillatory character of porewater chemistry in the presence of hydraulically active organisms is expected to have significant effects on microbial diversity and biogeochemical processes in marine sediments.