From Sediment to Sequestration: Linking Bioturbation to Carbon Cycling

Bioturbation by benthic and sediment-dwelling species significantly influences organic carbon preservation, accounting for 4% of the variation in sediment mixing depth. Yet our understanding of how different species, communities or functional bioturbation roles influence water-sediment carbon fluxes or contribute to long-term carbon burial is poorly understood. This is a key knowledge gap for assessing the impacts that seabed disturbance, such as bottom trawling, may have on carbon processes or for determining the potential carbon benefits of better seabed protection.  As part of the Convex Seascape Survey, we have been investigating the effect of faunally mediated sediment mixing and burrow ventilation on water-sediment carbon processes and associated nutrient fluxes. In a series of mesocosm experiments, we assembled 17 macrofaunal invertebrate species in monoculture and in a three-species mixture. In all experiments, individuals were collected using a van veen grab from the Firth of Clyde or Loch Etive, Scotland, and were placed in sediment-filled mesocosm aquaria (6 x 6 x 23 cm) with overlying seawater. Fluorescent sediment particles were added after 24 hours to quantify sediment reworking, and experiments were maintained for up to 10 days. Using Carbon-13 labelled algae, we quantified the movement of particulate organic carbon into the sediment. Overlying water parameters (e.g., pH, DIC, oxygen consumption) and sediment mixing and burrow ventilation activities were measured to assess inter- and intra-species contributions to nutrient cycling and carbon flux.Our data reveal that different groups of sediment mixers (predominantly deep burrowers versus surficial modifiers) perform distinct functional roles in the cycling of nutrients and carbon. We find, for example, that deep burrowers and active bioturbators promote higher levels of nitrite (NO₂⁻) and nitrate (NO₃⁻) release into the overlying water. This suggests that their burrow formation and ventilation enhance microbial nitrification, converting ammonium into nitrite and then nitrate. In contrast, surficial modifiers were associated with elevated levels of phosphorus and ammonium in the overlying water. This pattern likely reflects the dominance of ammonification, where organic matter decomposition releases ammonium and remineralisation releases phosphorous in surface layers with moderate oxygenation. Dissolved inorganic carbon release and concomitant alkalinity changes produced by individuals in our studies are species-specific and can be quite pronounced in relation to bioturbation function. The amount of particulate organic carbon redistributed by bioturbation is also species-specific and our labelled algae both help understand this redistribution of carbon in the surface sediments beyond more traditional methods of measuring bioturbation but also call into scrutiny categorical methods of grouping bioturbating organisms.These findings highlight that functional traits of bioturbating animals matter more than taxonomic species identity in regulating carbon fluxes and burial at the water-sediment interface. They reveal a divergence of roles across sediment depths, emphasising the potential for functional loss and ecosystem degradation from depth-specific disturbances like bottom trawling.