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

The disposal site in question has long received sewage sludge from the Dan Region Wastewater project, known as SHAFDAN, the municipal treatment system serving the densely populated Tel Aviv metropolitan area. Sludge disposal at sea is a practice with a long and contested history worldwide, and the Israeli site has been monitored for decades precisely because the eastern Mediterranean shelf is an unusually demanding environment for such activity. The Levantine basin is warm, oligotrophic, and increasingly stressed by seawater warming, so any additional organic load delivered to the seabed lands on an ecosystem already near its tolerance limits. Earlier work by the same research group and by colleagues at the Israel Oceanographic and Limnological Research had documented the chemical and biological footprint of the sludge, but the new study spans the full transition from active disposal to its complete cessation in March 2017, giving scientists an unusually clean before-and-after comparison.

The technical heart of the study is a twelve-year benthic time series built from box-corer samples collected aboard the research vessels Shikmona and Mediterranean Explorer. Each spring and fall, the team quantified macrofaunal abundance, the composition of taxa present, taxa richness, and the total organic carbon content of the sediment, commonly abbreviated TOC. TOC serves as a chemical proxy for organic enrichment: the more organic material, whether from sludge, phytoplankton debris, or other sources, has settled into the sediment, the higher the reading. Macrofauna, the worms, mollusks, crustaceans and other small animals living within the sediment, act as the biological counterpart to that chemical signal. Because different species have very different tolerances for organic loading and low oxygen, the identity and abundance of the animals present integrate the environmental history of the site over months to years. Reading the two signals together, chemistry and biology, is the standard logic of benthic impact monitoring, and it is what allowed the researchers to disentangle the effects of sludge from those of natural seasonality and regional warming.

During the active disposal period, before March 2017, the picture was stark. The community was overwhelmingly dominated by one or more opportunistic species of capitellid polychaetes, a family of segmented worms famous in marine ecology as classic indicators of disturbed, organically enriched sediments. Capitellids thrive where other animals cannot: they tolerate the hypoxic, sulfide-rich conditions that develop when excess organic matter fuels bacterial respiration in the seabed. Their monopolization of the disposal area is textbook ecological succession in reverse, a community pushed back to the earliest, most degraded stage described in the Pearson-Rosenberg model of organic enrichment. In practical terms, the sea floor beneath the disposal zone had been converted from a diverse shelf community into a monoculture of pollution-tolerant worms, a biological signature of chronic organic loading that persisted as long as the sludge kept arriving.

Perhaps the most striking finding from the disposal years is that the impact was not static but rhythmic. The researchers identified an annual cycle driven by the interplay between waste delivery and winter hydrodynamics. Sludge accumulated on the seabed through the summer months, and then winter storms dispersed it, scouring and redistributing the organic layer across the shelf. This accumulation-and-dispersion cycle produced a matching annual cycle in macrofaunal abundance, with the fall samples, taken after the summer accumulation phase, showing the strongest suppression. By fall, the accumulated sludge overrode every other environmental factor the team could evaluate, prominently attenuating the abundance of the macrofaunal community. TOC levels, meanwhile, remained stable and relatively high year-round during the disposal period, indicating that the sediment’s organic burden never fully cleared between dumping seasons. The system, in other words, was locked into a perpetual seasonal pulse of enrichment and partial recovery, never allowed to escape the disposal footprint.

Then came a twist that makes the study more than a routine impact assessment. Between 2012 and 2017, well before the disposal actually stopped, the community began to change in complex, gradual ways. The dominance of the opportunistic capitellids attenuated, a wider variety of other taxa entered the samples, and taxa richness shifted accordingly. Crucially, these changes were not timed with the cessation of sludge disposal, which did not occur until March 2017. The researchers initially hypothesized that the driver was the warming of the eastern Mediterranean seawater, a trend well documented for the Levantine basin and increasingly implicated in ecological upheavals across the region, from multi-species collapses at the warm edge of the sea to shifts in sponge and fish populations. Whatever the precise mechanism, the episode is a cautionary tale for monitoring programs everywhere: community change in a polluted area does not necessarily mean the pollution has changed, and attributing temporal shifts to a single management action requires careful timing analysis.

The cessation itself, when it finally came, produced a response that was both delayed and decisive. In 2018, one year after disposal ended, the disposal area settled into a new steady state that was unmistakably different from the disposal-era baseline. Macrofaunal abundance settled at low levels, but taxa richness was relatively high, the taxonomic composition stabilized in a new configuration, and TOC levels dropped to background levels comparable to unaffected shelf sediments. The chemical recovery was remarkably fast by marine standards; organic enrichment that had persisted year-round during the disposal period was effectively flushed from the system within a year once the source was cut. The biological community, however, did not simply revert to some pre-disposal original state. Instead, it reorganized into a novel assemblage, a reminder that recovered ecosystems are not always restored ecosystems, and that legacies of decades of disturbance can shape community structure long after the pressure is removed.

The study’s spatial dimension adds further texture to the story. By sampling multiple stations across the disposal area and its surroundings, the team could distinguish the core impact zone, where sludge effects were strongest, from peripheral areas where the annual accumulation-dispersion cycle and the post-cessation recovery played out differently. This spatial gradient is what makes the dataset valuable beyond the Israeli coast. Similar disposal sites and wastewater outfalls around the world, from the New York Bight to Australian waters to the Gulf of Lions, have produced comparable patterns of opportunistic dominance and slow recovery, and the Israeli series confirms that the same ecological principles operate in the ultra-oligotrophic, rapidly warming eastern Mediterranean. It also demonstrates the value of sustained, twice-yearly sampling with consistent methods; a shorter or less frequent program could easily have missed either the pre-cessation community shift or the one-year chemical rebound, and drawn the wrong conclusion about cause and effect.

For marine managers, the takeaways are concrete. First, cessation works: cutting off the sludge source allowed the sediment chemistry to reset within roughly a year and released the macrofaunal community from its capitellid monoculture. Second, recovery is not instantaneous or symmetrical; the biological reorganization took years and produced a new steady state rather than a return to the past. Third, disentangling multiple stressors requires long time series, because climate-driven change can mimic, mask, or interact with local pollution signals in ways that short-term monitoring cannot resolve. As coastal nations worldwide phase out sea disposal of sewage sludge under international conventions, the Israeli shelf offers one of the clearest documented examples of what the other side of that decision looks like: a sea floor that, given the chance, reassembles itself into something more diverse, more stable, and far more representative of what a healthy Mediterranean shelf should be.

Subject of Research: Long-term effects of sewage-sludge disposal and its cessation on benthic macrofaunal communities of the Israeli Mediterranean shelf

Article Title: Effects of sewage-sludge disposal and its cessation on benthic macrofaunal community on the Israeli Mediterranean shelf

Article References: Lubinevsky, H., & Tom, M. (2026). Effects of sewage-sludge disposal and its cessation on benthic macrofaunal community on the Israeli Mediterranean shelf. Environmental Monitoring and Assessment, 198(10), Article 1122. https://doi.org/10.1007/s10661-026-15967-x

Image Credits: AI Generated

DOI: 10.1007/s10661-026-15967-x

Keywords: sewage sludge, benthic macrofauna, Mediterranean Sea, Israeli shelf, polychaetes, capitellids, total organic carbon, sediment monitoring, marine pollution, ecological recovery, Levantine basin, environmental monitoring