SEASONALITY OF BIOIRRIGATION PATTERNS AND ASSOCIATED OXYGEN DYNAMICS IN PERMEABLE MARINE SEDIMENTS

Bioturbation plays a critical role in biogeochemical processes in marine sediments. Of particular importance is the supply of oxygen to anoxic sediments during burrow ventilation with subsequent effects on organic matter mineralization and elemental fluxes. The spatial extent of oxygen supply can be particularly high in permeable sands subject to bulk advective fluid flow, where significant volumes of sediment can be characterized by minute to hour scale, oxic-anoxic oscillations due to intermittent burrow ventilation and bioadvection by infauna. Microenvironmental redox patterns are ultimately controlled by individual animal behaviors, ages (sizes), and abundances, but quantitative data that discern the seasonality of bioirrigation activity and coupled geochemical dynamics are scarce.

We present data from a series of laboratory experiments that assessed the seasonality of bioirrigation activity by a model organism: the head-down, pocket-injection polychaete Clymenella torquata. Planar optode imaging was used to characterize oxygen dynamics induced by worms in temperature-controlled mesocosms with both intact and reconstructed sediments collected during different seasons. Temperature alone was found to be a good predictor of bioirrigation depths (exclusively within the top 10 cm at 21°C, transitioning away from the surface with decreasing temperatures, and exclusively below 5 cm and down to 30 cm depth at 5-6°C) and active irrigation intervals (ranging from ~4 min at 21°C to ~12 min at 5°C). However, temperature was insufficient to explain the lengths of non-irrigating, quiescent intervals, as these differed between summer-collected animals/sediment and winter-collected animals/sediment at winter temperatures (~9-17 min in summer sediment at 5°C, ~22-34 min in winter sediment at 6°C). Therefore, bioirrigation patterns are not exclusively controlled by temperatures, but likely depend on other seasonal variables such as food availability and sedimentary oxygen demand, and behavioral and biochemical acclimation to seasonal conditions. These results underscore the importance of performing laboratory experiments at in-situ temperatures with animals and sediment collected in-season to ensure the most natural possible behavior and resolution of coupled biogeochemical effects.