Mud-Dwelling Seafloor Animals Quietly Steer Carbon Storage in the North Sea

Beneath the gray, churning waters of the southeastern North Sea lies a stretch of fine-grained mud that scientists increasingly view as one of the shelf’s most important carbon vaults. A new study published in the journal Biogeosciences reveals that the animals burrowing through this sediment are not passive residents but active managers of the carbon cycle, and that their influence shifts dramatically with depth, salinity, and the force of tidal currents. Drawing on 171 macrofaunal taxa collected from 50 stations across a muddy depocenter off the island of Helgoland, a team led by Chueh-Chen Tung of the Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research has produced one of the most detailed portraits yet of how seafloor biodiversity governs the processes that determine whether organic carbon is buried or released back to the atmosphere.

The study area, a region of fine-grained sediment known as a mud depocenter, sits in water depths of roughly 7 to 51 meters and has recently been identified as a hotspot for carbon burial, with burial efficiency reported to be about twice the global average. Such depocenters form where hydrodynamic conditions trap and retain fine particles delivered by rivers, allowing organic matter to accumulate rapidly. While the geochemistry of these deposits has attracted considerable attention, the biological side of the equation has remained poorly resolved. The new research addresses that gap by asking how the composition, traits, and diversity of bottom-dwelling invertebrates vary across the depocenter and how those variations translate into ecosystem functions tied to carbon cycling.

Sampling was carried out in July 2023 aboard the research vessel Heincke, using a van Veen grab at each of the 50 stations. The team identified 31,086 individual animals spanning 9 phyla and 89 families, ranging from brittle stars and heart urchins to bivalves and bristle worms. Biomass across the stations varied more than twentyfold, from less than 0.4 grams to more than 10 grams of ash-free dry mass per square tenth of a meter. When the researchers clustered the communities by their biomass composition, four distinct assemblages emerged, arranged in a striking east-west pattern: two clusters dominated the deeper western reaches of the study area, while the other two occupied the shallower eastern zone, with all four overlapping in a transitional band between 20 and 30 meters of water depth.

Statistical modeling revealed that this spatial structure was not random. A distance-based redundancy analysis showed that environmental variables explained 44.1 percent of the variation in community composition, with sediment sorting, skewness, kurtosis, median grain size, bottom salinity, and bottom shear stress all emerging as significant predictors. The deeper communities were associated with higher salinity and calmer bottom waters, whereas the shallower communities endured stronger tidal shear stress and fresher, more variable conditions. In effect, the seafloor fauna of the depocenter is sorted along two intersecting gradients: one physical, driven by the friction of currents on the bed, and one chemical, driven by the mixing of saline North Sea water with freshwater runoff from the coast.

The functional contrast between deep and shallow assemblages proved even more telling than the taxonomic one. In the deeper clusters, biomass was dominated by mobile biodiffusors and subsurface filter feeders, most notably the brittle star Amphiura filiformis, which alone accounted for nearly 59 percent of total macrofaunal biomass in those communities on average. Laboratory studies cited by the authors show that this species can mix sediments at rates of roughly 35 square centimeters per year, actively transporting particles both downward to its mouth and upward to the surface. In the shallower clusters, by contrast, the fauna was dominated by less mobile, surface-modifying bivalves such as Abra species and tube-dwelling polychaetes, organisms that redistribute particles only at the sediment surface and ventilate their burrows infrequently.

These trait differences translated directly into divergent ecosystem functions. The team calculated four functions for each community: bioturbation potential, bioirrigation potential, secondary production, and respiration. Bioturbation and bioirrigation, the processes by which animals rework sediment particles and pump oxygenated water into their burrows, were roughly two to two and a half times higher in the deep clusters than in the shallow ones, with mean bioturbation potential of 284.0 versus 151.2 and mean bioirrigation potential of 223.2 versus 86.6. Both differences were highly significant statistically. The pattern reversed for energy-related functions: community respiration averaged 505.9 milligrams of carbon per square meter per day in the shallow clusters against 347.6 in the deep ones, a significant difference, while secondary production showed the same trend at 135.9 versus 107.5 milligrams of carbon per square meter per day.

The biodiversity-functioning relationships that emerged were, in places, counterintuitive. Secondary production and respiration both increased with taxonomic and functional richness, with correlation coefficients around 0.54, consistent with the idea that a greater variety of species brings a greater variety of trait combinations and enhances niche complementarity. But bioturbation and bioirrigation potentials were negatively correlated with Shannon diversity, Simpson diversity, and community evenness, with the strongest negative correlation, r equal to minus 0.55, linking evenness to bioturbation. The explanation, the authors argue, lies in a selection effect: sediment reworking in this system is disproportionately driven by a few highly efficient key species, above all Amphiura filiformis, rather than by the collective action of a diverse community. Where one dominant bioturbator reigns, evenness falls and reworking potential soars.

Crucially, the study shows that these biodiversity-functioning relationships are not fixed but bend along environmental gradients. Interaction regressions and simple slope analyses revealed that bottom shear stress and salinity significantly modulated the sign and strength of the correlations. In the deeper, high-salinity, low-shear habitats, the negative relationships between bioturbation or bioirrigation and diversity were pronounced; in the shallower, high-shear, low-salinity zone, they weakened or even turned slightly positive, though not significantly. The relationship between functional redundancy and energy-related functions was likewise modulated by shear stress, salinity, and sediment kurtosis, a measure of the peakedness of the grain-size distribution that the authors suggest may serve as a proxy for habitat stability. In stable, high-kurtosis sediments, functionally redundant communities appeared to prioritize persistence over peak performance, hinting at a trade-off between resilience and functioning efficiency.

The implications extend beyond the German Bight. Because bioturbation and bioirrigation control the delivery of oxygen and electron acceptors such as nitrate into deeper sediment layers, they regulate the microbial processes that determine whether organic matter is remineralized to carbon dioxide or preserved in the deposit. The finding that deeper communities combine high biomass with intense reworking suggests that carbon and nutrient cycling in the heart of the depocenter is strongly biologically driven, whereas in the shallower, physically disturbed margins, carbon dynamics are governed more by hydrodynamic forces than by fauna. As the authors note, a direct measurement linking macrofaunal communities to carbon sequestration is still missing, and closing that gap will be essential for predicting how these systems respond to change.

What the study delivers, meanwhile, is a clear warning wrapped in an opportunity. Biodiversity loss in shelf seas may not simply erode ecosystem functions uniformly; it may rewire them, suppressing some pathways while leaving others intact, depending on which species disappear and where. Preserving functional diversity, and in particular the keystone bioturbators that dominate the deep muds, appears essential for sustaining the benthic machinery that allows coastal depocenters to act as carbon reservoirs. As climate policy increasingly looks to natural carbon sinks, this work makes the case that the humble invertebrates of the seafloor, from brittle stars to burrowing clams, deserve a seat at the table alongside forests and wetlands in the accounting of the planet’s carbon budget.

Subject of Research: Benthic macrofaunal biodiversity and its role in carbon cycling ecosystem functions in a North Sea mud depocenter

Article Title: Linking marine benthic biodiversity and ecosystem functions related to carbon cycling in a continental mud depocenter

Article References: Linking marine benthic biodiversity and ecosystem functions related to carbon cycling in a continental mud depocenter. (n.d.). https://doi.org/10.5194/bg-23-7129-2026

Image Credits: AI Generated

DOI: 10.5194/bg-23-7129-2026

Keywords: benthic macrofauna, biodiversity, carbon cycling, bioturbation, bioirrigation, North Sea, mud depocenter, ecosystem functioning, sediment biogeochemistry, functional traits, carbon sequestration, shelf sea