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Abstract This field study examined how sediment macroinfauna change patterns of sediment oxygen demand (SOD) throughout a diel oxygen cycle. Sediments with a greater faunal presence would be expected to have greater overall SOD, and at night may alter their behavior and influence SOD depending on their response to low-oxygen stress. Dynamic faunal bioturbation or bioirrigation behavior would also result in corresponding variation in SOD values on short time scales. In situ flow-through benthic metabolism chambers were used to measure SOD at a high temporal resolution in discrete sediment patches. Sediments with more macroinfauna had greater average SOD over the diel cycle, consistent with previous studies. Where more macroinfauna were present, they drove greater SOD during nightly low oxygen, presumably by enhancing their burrowing and irrigation activities. SOD was also more variable on a sub-diel timescale in sediments with more macroinfauna. Sediment oxygen demand is dynamic and highly sensitive both temporally, on very short timescales, and spatially, in terms of resident fauna, and their interaction produces heretofore unaccounted complexity in patterns of SOD particularly in shallow coastal systems. Extrapolations of temporally and spatially limited SOD measurements to a system-wide scale that do not account for the short-term and spatially variable effects of fauna may produce imprecise and misleading estimates of this critical ecosystem function. 

Abstract Muddy marine sediments are elastic materials in which bubbles grow and worms extend their burrows by fracture. Bubble growth and burrowing behavior are dependent on the stiffness and fracture toughness (K_Ic) of these muds. This article describes a custom laboratory apparatus to measure the fracture toughness of muddy, cohesive sediments using a bubble injection method. The system induces fracture in sediment samples by incrementally injecting air through a needle inserted into the sediment. The increasing pneumatic pressure is monitored until it drops abruptly, indicating bubble formation. Fracture toughness is then calculated from the peak pressure at which fracture occurred, following cavitation rheology methods developed for soft gels. The system has produced measurements that compare well to previous data but with better spatial resolution, allowing for characterization of spatial heterogeneity on small scales. 

Creating burrows through natural soils and sediments is a problem that evolution has solved numerous times, yet burrowing locomotion is challenging for biomimetic robots. As for every type of locomotion, forward thrust must overcome resistance forces. In burrowing, these forces will depend on the sediment mechanical properties that can vary with grain size and packing density, water saturation, organic matter and depth. The burrower typically cannot change these environmental properties, but can employ common strategies to move through a range of sediments. Here we propose four challenges for burrowers to solve. First, the burrower has to create space in a solid substrate, overcoming resistance by e.g., excavation, fracture, compression, or fluidization. Second, the burrower needs to locomote into the confined space . A compliant body helps fit into the possibly irregular space, but reaching the new space requires non-rigid kinematics such as longitudinal extension through peristalsis, unbending, or eversion. Third, to generate the required thrust to overcome resistance, the burrower needs to anchor within the burrow . Anchoring can be achieved through anisotropic friction or radial expansion, or both. Fourth, the burrower must sense and navigate to adapt the burrow shape to avoid or access different parts of the environment. Our hope is that by breaking the complexity of burrowing into these component challenges, engineers will be better able to learn from biology, since animal performance tends to exceed that of their robotic counterparts. Since body size strongly affects space creation, scaling may be a limiting factor for burrowing robotics, which are typically built at larger scales. Small robots are becoming increasingly feasible, and larger robots with non-biologically-inspired anteriors (or that traverse pre-existing tunnels) can benefit from a deeper understanding of the breadth of biological solutions in current literature and to be explored by continued research. 

Animals with long, skinny bodies are often called “worms,” but there are many kinds of worms—even in the ocean. Annelids (segmented worms) include garden earthworms, but their ocean relatives come in many colors, shapes, and sizes. Some are so small that they live between grains of sand, while others can be longer than a human and eat fish! Marine worms are essential to the ocean food web, as both predators and prey. They help create homes for plants and animals by burrowing and building tubes in ocean sediments. Scientists are still discovering new worm species, and there are still many mysteries about how worms eat, why they live in the places they do, and what roles they play in ocean ecosystems. Worms are a fascinating and important part of ocean communities. 

Abstract Infaunal organisms mix sediments through burrowing, ingestion and egestion, enhancing fluxes of nutrients and oxygen, yet the mechanisms underlying bioturbation remain unresolved. Burrows are extended through muddy sediments by fracture, and we hypothesize that the cohesive properties of sediments play an important but unexplored role in resisting bioturbation. Specifically, we suggest that crack branching, tortuosity, and microcracking are important in freeing particles from the cohesive matrix, and that the sediment properties that affect these processes are important predictors of bioturbation. We use finite element modeling and simplified, mechanics‐based models to explore the relative importance of sediment mechanical properties and worm behaviors in determining crack propagation paths. Our results show that crack propagation direction depends on variability in fracture toughness, and that applying more force to one side of the burrow wall, simulating “steering” behavior, has surprisingly little effect on crack propagation direction. Burrowers instead steer by choosing among crack branches. Paths created by burrowing worms in natural sediments are mostly straight with some crack branching, consistent with modeling results. Crack branching also requires sufficient stored elastic energy to drive two cracks, and worms can exert larger forces resulting in more stored energy in stiffer sediments. This implies that more crack branching and consequently more particle mixing occurs in heterogeneous sediments with low fracture toughness relative to stiffness. Whether sediments with greater potential for crack branching also experience higher bioturbation remains to be tested, but these results indicate that material properties of sediments may be important in resisting or facilitating bioturbation. 

Abstract In shallow coastal systems, sediments are exposed to dramatic and complex variability in environmental conditions that influences sediment processes on short timescales. Sediment oxygen demand (SOD), or consumption of oxygen by sediment‐dwelling organisms and chemical reactions within sediments, is one such process and an important metric of aquatic ecosystem functioning and health. The most common instruments used to measure SOD in situ are batch‐style benthic chambers, which generally require long measurement periods to resolve fluxes and thus do not capture the high temporal variability in SOD that can be driven by dynamic coastal processes. These techniques also preclude linking changes in SOD through time to specific features of the sediment, for example, shifts in sediment faunal activities which can vary on short time scales and can also be affected by ambient oxygen concentrations. Here we present an in situ semi‐flow through instrument to repeatedly measure SOD in discrete areas of sediment. The system isolates patches of sediment in replicate benthic chambers, and measures and records oxygen decrease for a short time before refreshing the overlying water in the chamber with water from the external environment. This results in a sawtooth pattern in which each tooth is an incubation, providing an automated method to produce direct measurements of in situ SOD that can be directly linked to an area of sediment and related to rapid shifts in environmental conditions.