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Burrowers such as thalassinideans remobilize sediment in benthic ecosystems, altering granulometry, enhancing organic matter cycling and oxygenation. We characterized the distribution of the mud shrimp Upogebia noronhensis and the associated macroinfauna along a depth and granulometric gradient in a shallow subtidal area in the southern Brazilian coast. Mud shrimp densities were estimated by burrow opening count using 0.25 m2 quadrats in three sediment zones: sand, sand-mud transition and mud. Macroinfaunal community descriptors and sediment granulometric characteristics were assessed. U. noronhensis average density varied from 0.6 to 145 individuals m−2 and was highest in the transition zone and lowest in the sand zone. Macrofauna in the sand zone was at least three times more abundant and 1.2 times richer, averaging 436.3 organisms and 39 species, while the transition and mud zones were more even in species distribution (species evenness > 0.7). The shrimp presence seems to be linked to a coarse sand content <80% and clay and silt <40% in the sediment, a compromise between gallery construction ability and filter-feeding suitability. High densities and aggregated distribution of mud shrimp, combined with alteration of sediment grain composition and organic matter, are likely to affect macroinfaunal abundance nearby the burrows.

Due to large losses of seagrass meadows worldwide, restoration is proposed as a key strategy for increasing coastal resilience and recovery. The emergence of a seagrass meadow is expected to substantially amplify biodiversity and enhance benthic metabolism by increasing primary productivity and respiration. Nevertheless, open questions remain regarding the metabolic balance of aging seagrass meadows and the roles benthic communities within the seagrass ecosystem play in overall metabolism. To address these questions, we investigated a chronosequence of bare sediments and adjacent Zostera marina meadows of 3 and 7 years since restoration alongside a natural meadow located within a high-temperate marine embayment in Gåsö, Sweden. We combined continuous measurements of O2 fluxes using underwater eddy covariance with dissolved inorganic carbon (DIC) and O2 fluxes from benthic chambers during the productive season (July). Based on the ratio between O2 and DIC, we derived site-specific photosynthetic and respiratory quotients, enabling the conversion of eddy covariance fluxes to DIC. We assessed benthic diversity parameters as potential drivers of metabolic flux variability. We observed high rates of gross primary productivity (GPP) spanning −18 to −82 mmolDICm-2d-1, which increased progressively with meadow age. Community respiration (CR) mirrored the GPP trend, and all meadows were net heterotrophic (GPP < CR), with net community productivity (NCP) ranging from 16 to 28 mmolDICm-2d-1. While autotrophic biomass did not increase with meadow age, macrophyte diversity did, elucidating potential effects of niche complementarity among macrophytes on community metabolism. These findings provide valuable insights into how community composition and meadow development relate to ecosystem functioning, highlighting potential tradeoffs between carbon uptake and biodiversity.

We identify how ecosystem functioning in shallow estuaries is affected by shifts in benthic fauna communities. We use the shallow estuary, Odense Fjord, Denmark, as a case study to test our hypotheses that (1) shifts in benthic fauna composition and species functional traits affect biogeochemical cycling with cascading effects on ecological functioning, which may (2) modulate pelagic primary productivity with feedbacks to the benthic system. Odense Fjord is suitable because it experienced dramatic shifts in benthic fauna community structure from 1998 to 2008. We focused on infaunal species with emphasis on three dominating burrow-dwelling polychaetes: the native Nereis (Hediste) diversicolor and Arenicola marina, and the invasive Marenzelleria viridis. The impact of functional traits in the form of particle reworking and ventilation on biogeochemical cycles, i.e. sediment metabolism and nutrient dynamics, was determined from literature data. Historical records of summer nutrient levels in the water column of the inner Odense Fjord show elevated concentrations of NH4+ and NO3- (DIN) during the years 2004-2006, exactly when the N. diversicolor population declined and A. marina and M. viridis populations expanded dramatically. In support of our first hypothesis, we show that excess NH4+ delivery from the benthic system during the A. marina and M. viridis expansion period enriched the overlying water in DIN and stimulated phytoplankton concentration. The altered benthic-pelagic coupling and stimulated pelagic production may, in support of our second hypothesis, have feedback to the benthic system by changing the deposition of organic material. We therefore advice to identify the exact functional traits of the species involved in a community shift before studying its impact on ecosystem functioning. We also suggest studying benthic community shifts in shallow environments to obtain knowledge about the drivers and controls before exploring deep-water environments.

Coastal and marine ecosystems provide a wide range of ecosystem services, including habitats for 24 rich biodiversity, improved water quality, coastal protection against erosion, carbon storage, and 25 cultural benefits such as recreation, aesthetics, and human well-being. Drawing from terrestrial 26 ecology, the concept of rewilding to degraded coastal and marine ecosystems has the potential to 27 address the interconnected crises of climate change, biodiversity loss, and pollution. While the 28 understanding and implementation of rewilding in terrestrial systems is relatively advanced, \"coastal 29 and marine rewilding\", is still in its infancy (Cornerier 2024) We thank all contributing authors for joining this Research Topic. We hope that the papers will 85 inspire and contribute to the development of coastal rewilding approaches within the framework of 86 nature-based solutions. 87 88

Managed realignment by deliberate flooding of coastal areas is an adaptation to sea level rise but may risk enriching the coastal zone with nutrients when seawater floods agricultural soil. This study focuses on the early development of macroalgae and their sources of nitrogen (N) in Gyldensteen Coastal Lagoon, Denmark. The lagoon was claimed for agricultural purposes in 1871 and reflooded by managed realignment 143 yr later (2014). Our hypotheses were: (1) that nutrients of agricultural origin from the newly flooded soil initiate opportunistic macroalgal blooms; and (2) that the isotopic composition of green algae rapidly reflects the origin of nutrient sources. We monitored macroalgal cover and conducted stable isotope (δ 15N) analyses to assess the origin of N sources. Intense green macroalgal blooms occurred during the first summer after flooding and diminished in the 2 following years as a result of rapid water exchange. Low δ 15N in macroalgae in the first year (mean ± SE, 4.2 ± 0.3‰) increased significantly in the next year (8.0 ± 0.1‰). A laboratory experiment tested the δ 15N response of opportunistic green macroalgae (Ulva spp.) exposed to organic manure and synthetic inorganic fertilizers. Higher δ 15N (11.1 ± 0.1‰) characterized manure-treated algae compared to fertilizer-treated algae (2.7 ± 0.2‰). Based on these field and laboratory results, we accept both hypotheses and conclude that the major N source supporting macroalgal growth in 2014 was derived from synthetic fertilizers; however, rapid tidal flushing during the following years resulted in nutrient limitation and lower macroalgal growth.

Bioturbation has major impacts on sediment biogeochemistry, which can be linked to the functional traits of the macrofauna involved. Nereis (Hediste) diversicolor and Marenzelleria viridis are 2 functionally different bioturbating polychaetes that strongly affect the ecology and biogeochemistry of coastal sediments. However, the different effects of these polychaetes on the activity and composition of microbial communities and on chemoautotrophic bacteria have not been extensively studied. We performed experiments with sediment aquaria that contained each species separately as well as a non-bioturbated control. Bacterial communities in different sediment zones (surface, burrow, subsurface) were characterized by phospholipid-derived fatty acid analysis combined with stable isotope labeling (13C bicarbonate) to quantify the dark CO₂ fixation by chemoautotrophic bacteria. Pore water chemistry (ΣH₂S and dissolved inorganic carbon) was additionally assessed in each treatment. The strong ventilation but low bioirrigation capacity in the open-ended burrows of N. diversicolor resulted in enhanced aerobic chemoautotrophic activity, potentially by sulfur oxidizing and nitrifying bacteria along the burrow. In contrast, slower ventilation and higher irrigation by M. viridis induced an advective mode of pore water transport. This promotes anaerobic chemoautotrophy around the blind-ended burrow and within the subsurface sediment. Sulfate-reducing bacteria were the dominant anaerobic chemoautotrophs that probably disproportionate sulfur. In conclusion, our analysis shows that bioturbating fauna influence the microbial community and chemoautotrophic activity in sediments, but that the effect strongly depends on the structure of the burrow and on species-specific ventilation behavior and irrigation capacity.

The polychaete Marenzelleria viridis is an invasive species and often replaces the native Nereis diversicolor. This shift leads to more reduced conditions and changes in the biogeochemical function of the sediments. By combining imaging techniques for O₂ (planar optodes) and irrigation patterns (rhodamine WT and brilliant blue), we investigated the relationship between irrigation and O₂ dynamics in burrows of M. viridis. The investigated animals shifted between 2 modes of ventilation: ciliary pumping for 77% of the time and muscular pumping for 23% of the time. On average, muscular pumping was induced every 0.4 h. During ciliary pumping, oxic water was pumped into blind-ended burrows and into the surrounding sediment, inducing an upward porewater transport of O₂-depleted water. This pattern was reversed during muscular pumping. The 2 pumping modes induced oscillating O₂ penetration along the burrow wall and along the primary sediment-water interface. The average net downward irrigation rate, including both pumping modes, amounted to 11.1 × 10−3 ± 2.4 × 10−3 ml min−1. The estimated average oxic sediment volume was 2.1 ± 0.5 cm³ per burrow, and the burrow-specific O² consumption was 45.6 ± 18.1 nmol min−1. M. viridis burrows and the ambient sediment are relatively O₂ depleted, with intensified rates of O₂ consumption, compared to similar-sized native N. diversicolor. The complex O₂ dynamics induced a unique microenvironment that must favor meiofauna and microbial communities that are tolerant to oxic-anoxic oscillations or that have the ability to migrate along with the pulsing oxic-anoxic interface.