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Methane-rich fluids arising from organic matter diagenesis in deep sediment layers sustain chemosynthesis-based ecosystems along continental margins. This type of cold seep develops on pockmarks along the Congo margin, where fluids migrate from deep-buried paleo-channels of the Congo River, acting as reservoirs. Similar ecosystems based on shallow methane production occur in the terminal lobes of the present-day Congo deep-sea fan, which is supplied by huge quantities of primarily terrestrial material carried by turbiditic currents along the 800 km channel, and deposited at depths of up to nearly 5000 m. In this paper, we explore the effect of this carbon enrichment of deep-sea sediments on benthic macrofauna, along the prograding lobes fed by the current active channel, and on older lobes receiving less turbiditic inputs. Macrofaunal communities were sampled using either USNEL cores on the channel levees, or ROV blade cores in the chemosynthesis-based habitats patchily distributed in the active lobe complex. The exceptionally high organic content of the surface sediment in the active lobe complex was correlated with unusual densities of macrofauna for this depth, enhanced by a factor 7–8, compared with those of the older, abandoned lobe, whose sediment carbon content is still higher than in Angola Basin at same depth. Macrofaunal communities, dominated by cossurid polychaetes and tanaids were also more closely related to those colonizing low-flow cold seeps than those of typical deep-sea sediment. In reduced sediments, microbial mats and vesicomyid bivalve beds displayed macrofaunal community patterns that were similar to their cold-seep counterparts, with high densities, low diversity and dominance of sulfide-tolerant polychaetes and gastropods in the most sulfidic habitats. In addition, diversity was higher in vesicomyid bivalve beds, which appeared to bio-irrigate the upper sediment layers. High beta-diversity is underscored by the variability of geochemical gradients in vesicomyid assemblages, and by the vesicomyid population characteristics that vary in density, size and composition. By modifying the sediment geochemistry differently according to their morphology and physiology, the different vesicomyid species play an important role structuring macrofauna composition and vertical distribution. Dynamics of turbiditic deposits at a longer temporal scale (thousands of years) and their spatial distribution in the lobe area also resulted in high heterogeneity of the “cold-seep-like communities”. Dynamics of chemosynthetic habitats and associated macrofauna in the active lobe area resembled those previously observed at the Regab pockmark along the Congo margin and rapid succession is expected to cope with high physical disturbance by frequent turbiditic events and huge sedimentation rates. Finally, we propose a model of the temporal evolution of these peculiar habitats and communities on longer timescales in response to changes in distributary channels within the lobe complex. [ABSTRACT FROM AUTHOR]

In this study, "artificial reef" (AR) impacts of offshore windfarms (OWFs) on the surrounding soft-sediments were investigated. Benthic grab samples were collected at nearby (37.5 m) and distant (500 or 350 m) positions from turbines of two Belgian OWFs (Belwind: monopiles and C-Power: jackets). Higher macrobenthos abundance and species richness were found nearby jacket foundations of C-Power compared to distant positions and differences were most pronounced within deeper sediments (i.e., gullies between sandbanks) at intermediate levels of fine sand fractions (10–20%) and total organic matter (0.5–0.9%). Strong benthic enrichment (>1000 ind. m−2, >20 spp. sample−1) was also linked with higher fine sand fractions (>20%) near the jackets. Moreover, nearby sediments showed higher occurrences of coastal species and habitat diversification was promoted by Mytilus edulis shell debris and alive organisms ("biofouling drop-offs"). The lack of similar results around monopiles (Belwind) confirms that the extent of detectable AR-effects depends on site- and turbine specific factors. • "Artificial reef" (AR) effects influence soft-sediment benthic communities nearby (37.5 m) jacket turbines. • Nearby sediments contained more abundant, richer and compositionally different communities compared to distant sediments. • Strongest benthic enrichment was observed in deeper sediments (i.e., sandbank gullies) and was linked with sediment fining. • Mytilus edulis - associated biofouling drop-off products enhanced habitat diversification nearby the jackets. • The detectability and extent of AR-effects strongly depends on site - and turbine specific factors. [ABSTRACT FROM AUTHOR]

Highlights: [•] Do dune-recovery interventions with geotextiles have long-term ecological effects? [•] We assessed community and behavioural features as bioindicators of long-term effects. [•] The indicators did not significantly differ between impact and control sites. [•] The impacted site had lower talitrids’ abundance with respect to the control one. [•] T. saltator abundance can highlight long-term impacts of dune-recovery interventions. [Copyright &y& Elsevier]

Abstract: The recovery of macrofaunal communities after marine aggregate dredging was assessed using both traditional indices (abundance, biomass and species diversity), and functional analysis techniques (Somatic Production, Taxonomic Distinctness, Infaunal Trophic Index, Biological Traits Analysis and Rao''s Quadratic Entropy). A previously dredged area (Area 222), located off the southeast coast of England was selected for this investigation. Area 222 was split into sites that had been subjected to relatively high dredging intensity, relatively low dredging intensity, and undisturbed reference areas. Both traditional and functional analyses indicated that macrofauna at the low dredging intensity site had fully recovered at least 7 years after the dredging ceased. Recovery times at the high intensity site had a greater variability and most of the techniques recorded the recovery had yet to take place even 11 years after the dredging had ceased. Since Area 222 was dredged for a long period of time (approx. 25 years), it is suggested that a longer time series of study be carried out so that the definitive recovery period in this high intensity site can be determined. While a longer time series study is not always a realistic or cost effective, the present study could be useful to facilitate the selection of metrics to support in the assessment of macrofaunal recovery. [Copyright &y& Elsevier]