Bottom trawling is one of the most destructive fishing methods currently in use, with acute impacts on benthic ecosystems and chronic impacts on macrofauna communities. However, the long-term effects of chronic bottom trawling on smaller components of benthic communities and on sediment biogeochemistry are less well understood. To address this knowledge gap, we examined the effects of bottom trawling and environmental variables (bottom water and sediment properties) on alpha diversity and community structure of prokaryotes, meiofauna (including metazoans and foraminifera), and macrofauna over a spatial gradient of commercial bottom trawling, including a marine protected area which has been unfished for 12 years after >100 years of chronic trawling. Our results showed that chronic trawling affected the four organism groups in different ways. Prokaryote and foraminifera diversities were slightly higher at sites with higher trawling intensities, due to a greater number of rare species. Community composition was affected by trawling in all groups except meiofaunal metazoans. Sedimentary carbon played a significant role in shaping all four communities, as well as carbon degradation rates, but was not itself affected by trawling. Our results highlight that the complex interactions between environmental variables and disturbances from bottom trawling affect different components of the benthic fauna in different ways. Differences in organism size, population turnover rates, metabolic and ecological plasticity, feeding traits, and sensitivity to physical disturbance probably explain these differences.

Land permafrost thaw transfers increasing amounts of organic matter and nutrients to the Arctic Ocean. These nutrients could stimulate primary production directly, or indirectly following remineralization in sediments. Projections of this effect are limited by scarce observations and poor understanding of the underlying controls. Here, we focus on the Kara, Laptev, and East Siberian Sea shelves that receive strong input from large rivers and coastal erosion, linking ship‐board measurements of sediment–water nutrient fluxes to environmental parameters associated with land input. Ammonium and nitrite releases were positively related to high concentration and low decomposition state of terrigenous organic matter, based on biomarkers. Nitrate release was related to O 2 penetration depth. Phosphate and silicate release were highest at stations with strong marine influence. Our findings suggest that changes in environmental conditions, such as land input, might alter the nutrient balance in the Siberian Arctic Ocean, with implications for ecological and biogeochemical processes.

Meiofauna (all invertebrates smaller than 1 mm) are not only sensitive to environmental changes but also contribute significantly to nutrient cycling and energy transfer to higher trophic levels. Despite their importance, meiofauna distribution and ecology in the Siberian seas remain understudied. Here, we employ sediment environmental DNA metabarcoding to characterize meiofauna diversity across the unexplored Siberian seas. We show that meiofauna community structure is primarily driven by river discharge and coastal erosion, which are heavily influenced by climate change, rather than geographical distinctions between the seas. We observed higher meiofauna diversity in nearshore areas where river plumes promoted colonizer nematode communities that are resilient to disturbances. Yet, their dominance may lead to decreased ecosystem stability in the future. This study provides a valuable baseline for meiofauna diversity in remote Siberian seas undergoing rapid environmental change, which will be useful for assessing the future direction and pace of benthic ecological trajectories.

Sediments polluted with hydrophobic organic contaminants (HOCs) and metals can pose environmental risks, yet effective remediation remains a challenge. We investigated a new composite sorbent comprising granular activated carbon (GAC) and a calcium-silicate (Polonite (R), PO) for thin-layer capping of polluted sediment, with the aim to sequester both HOCs and metals. Box cores were collected in polluted Oskarshamn harbor, Sweden, and the sediments were treated with GAC and/or Polonite in a 10-week mesocosm study to measure endpoints ranging from contaminant immobilization to ecological side effects on native fauna and biogeochemical processes. The GAC particle size was 300-500 mu m to reduce negative effects on benthic fauna (by being non-ingestible) and of biogenic origin (coconut) to have a small carbon footprint compared with traditional fossil ACs. The calcium-silicate was a fine-grained industrial by-product used to target metals and as a carrier for GAC to improve the cap integrity. GAC decreased the uptake of dioxins (PCDD/Fs) in the bivalve Macoma balthica by 47 % and the in vitro bioavailability of PCB by 40 %. The composite cap of GAC + Polonite decreased sediment-to-water release of Pb < Cu < Ni < Zn < Cd by 42-98 % (lowest to highest decrease) and bioaccumulation of Cd < Zn < Cu in the worm Hediste diversicolor by 50-65 %. Additionally, in vitro bioavailability of Pb < Cu < Zn, measured using digestive fluid extraction, decreased by 43-83 %. GAC showed no adverse effects on benthic fauna while Polonite caused short-term adverse effects on fauna diversity and abundance, partly due to its cohesiveness, which, in turn, can improve the cap integrity in situ. Fauna later recovered and bioturbated the cap. Both sorbents influenced biogeochemical processes; GAC sorbed ammonium, Polonite decreased respiration, and both sorbents reduced denitrification. In conclusion, the side effects were relatively mild, and the cap decreased the release and bioavailability of both HOCs and metals effectively, thus offering a promising sustainable and cost-effective solution to remediating polluted sediments.

On September 26th, 2022, the detonations at the gas pipelines Nord Stream 1 and 2 resulted in some of the largest non-natural releases of methane known. The distribution of methane in the surrounding seawater and the possible effects were not apparent. To trace the pathways of methane we recorded CH4 concentrations and the isotopic signal (δ13C-CH4) in seawater, and air. A week post-explosion, we detected methane concentrations up to 4 orders of magnitude above the natural Baltic Sea background. The released fossil methane created a distinct plume with δ13C-CH4 ratios differing from natural background values. The strong water stratification preserved the distribution pattern initiated by the explosion, shown by the laterally strong concentration gradient within the plume. Our analysis encompasses three stages of the explosion's impact; the initial sea-air methane release, measurements taken during our research expedition one week later, and a third stage triggered by the shift from summer to winter conditions as an outlook on how winter mixing and microbial activity will influence the plume.

Interstitial meiofauna, organisms smaller than 500 μm that live between sediment grains, are the most abundant animals on Earth. They play crucial roles in biogeochemical cycles, but their responses to microplastics (MPs) remain understudied. Due to their size, meiofauna may be particularly vulnerable to MPs. We quantified how realistic levels of MP contamination affect bioturbation, oxygen penetration depth (OPD), and diffusive oxygen uptake (DOU) in sediment mesocosms over thirteen days. Bioturbation depth and OPD increased, while DOU decreased across all treatments. However, sediments containing MPs had lower bioturbation depth and slightly higher OPD compared to controls. The reduction in bioturbation was likely due to meiofauna stress, while the highest MP contamination caused increased bioturbation depth, likely due to evasion responses. Increased OPD over time was likely due to reduced labile organic matter. This study highlights how bioturbation, OPD, and DOU shift with MP pollution, confirming MPs' impacts on ecosystem functions.

Coastal ecosystems dominate oceanic methane (CH4) emissions. However, there is limited knowledge about how biotic interactions between infauna and aerobic methanotrophs (i.e. CH4 oxidizing bacteria) drive the spatial-temporal dynamics of these emissions. Here, we investigated the role of meio- and macrofauna in mediating CH4 sediment-water fluxes and aerobic methanotrophic activity that can oxidize significant portions of CH4. We show that macrofauna increases CH4 fluxes by enhancing vertical solute transport through bioturbation, but this effect is somewhat offset by high meiofauna abundance. The increase in CH4 flux reduces CH4 pore-water availability, resulting in lower abundance and activity of aerobic methanotrophs, an effect that counterbalances the potential stimulation of these bacteria by higher oxygen flux to the sediment via bioturbation. These findings indicate that a larger than previously thought portion of CH4 emissions from coastal ecosystems is due to faunal activity and multiple complex interactions with methanotrophs.

Meiofauna, organisms smaller than 1 mm, are the most abundant and diverse invertebrates inhabiting the world's ocean floor but their contribution to benthic oxygen demand is still poorly constrained. This knowledge is crucial for understanding seabed respiration, global marine carbon, and oxygen cycles, which are relevant to all nutrient cycling and energy flows in the ecosystem. It is common to predict meiofauna respiration based on their biomass or volume, which are difficult to quantify, and thus meiofauna are rarely included in biogeochemistry studies. In addition, it is still unknown how well the generalized allometric relations describe all meiofauna respiration. Therefore, we used a novel approach specially developed for single meiofauna respiration measurements to derive the respiration rates of 10 meiofauna groups in two marine and one brackish coastal muddy environments under oxic and hypoxic conditions, representing natural sediment conditions. Our estimates suggest that large ostracods and juveniles of macrofauna (e.g., bivalves, trumpet worms, and priapulids) had the highest individual respiration rates. Meiofauna community as a whole contributed 3–33% to sediment oxygen uptake. However, the most important contributors to the overall sediment oxygen uptake were nematodes and foraminifera which had lower respiration rates but were highly abundant. Therefore, out of more than 22 meiofauna phyla, we recommend that nematode and foraminifera respiration, which contributes 3–30% (total 3–33%) to sediment oxygen uptake, should be taken into consideration in any estimations of benthic oxygen and carbon cycles.

Identifying and quantifying nitrogen pools is essential for understanding the nitrogen cycle in aquatic ecosystems. The ubiquitous diatoms represent an overlooked nitrate pool as they can accumulate nitrate intracellularly and utilize it for nitrogen assimilation, dissipation of excess photosynthetic energy, and Dissimilatory Nitrate Reduction to Ammonium (DNRA). Here, we document the global co-occurrence of diatoms and intracellular nitrate in phototrophic microbial communities in freshwater ( n = 69), coastal ( n = 44), and open marine ( n = 4) habitats. Diatom abundance and total intracellular nitrate contents in water columns, sediments, microbial mats, and epilithic biofilms were highly significantly correlated. In contrast, diatom community composition had only a marginal influence on total intracellular nitrate contents. Nitrate concentrations inside diatom cells exceeded ambient nitrate concentrations ∼100–4000-fold. The collective intracellular nitrate pool of the diatom community accounted for <1% of total nitrate in pelagic habitats and 65–95% in benthic habitats. Accordingly, nitrate-storing diatoms are emerging as significant contributors to benthic nitrogen cycling, in particular through Dissimilatory Nitrate Reduction to Ammonium activity under anoxic conditions.

Meiofauna (invertebrates that pass through a 1‐mm mesh sieve, but are retained on a 40‐µm mesh) represent the most abundant and diverse animal group on Earth, but empirical evidence of their role in benthic respiration, production and carbon cycling across ecosystems is not well documented. Moreover, how meiofauna respond to changing oxygen conditions is poorly understood. We further developed an incubation system, in which oxygen and temperature conditions are easily controlled and single meiofaunal nematode respiration is resolved in glass capillary tubes, using Clark‐type oxygen microsensor. We performed the respiration measurements after exposing nematodes to different ambient oxygen concentrations, which resulted in 3–60 µM O 2 during hypoxic and 80–210 µM O 2 during oxic incubations in close proximity to the respective nematodes. Individual nematode respiration rates ranged from 0.02 to 1.30 nmol O 2 ind. −1 day −1 and were 27% lower during hypoxic than oxic incubations. Rates derived from established allometric relations were on average fourfold higher than our direct measurements. The presented method is suitable for single nematode respiration measurements and can be adapted to a wide range of experimental conditions. Therefore, it can be used to assess meiofauna contribution to ecosystem processes and investigate species‐specific responses to changing environmental conditions, for example, oxygen stress, increasing water temperature.