Sponges form an important component of benthic ecosystems from shallow littoral to hadal depths. In the deep ocean, beyond the continental shelf, sponges can form high-density fields, constituting important habitats supporting rich benthic communities. Yet these habitats remain relatively unexplored. The oil and gas industry has played an important role in advancing our knowledge of deep-sea environments. Since its inception in the 1960s, offshore oil and gas industry has moved into deeper waters. However, the impacts of these activities on deep-sea sponges and other ecosystems are only starting to become the subject of active research. Throughout the development, operation and closure of an oil or gas field many activities take place, ranging from the seismic exploration of subseafloor geological features to the installation of infrastructure at the seabed to the drilling process itself. These routine activities and accidental releases of hydrocarbons during spills can significantly impact the local marine environment. Each phase of a field development or an accidental oil spill will therefore have different impacts on sponges at community, individual and cellular levels. Legacy issues regarding the future decommissioning of infrastructure and the abandonment of wells are also important environmental management considerations. This chapter reviews our understanding of impacts from hydrocarbon exploration and exploitation activities on deep-sea sponges and the habitats they form. These impacts include those (1) at community level, decreasing the diversity and density of benthic communities associated with deep-sea sponges owing to physical disturbance of the seabed; (2) at individual level, interrupting filtration owing to exposure to increased sedimentation; and (3) at cellular level, decreasing cellular membrane stability owing to exposure to drill muds. However, many potential effects not yet tested in deep-sea sponges but observed in shallow-water sponges or other model organisms should also be taken into account. Furthermore, to the best of our knowledge, no studies have shown impact of oil or dispersed oil on deep-sea sponges. To highlight these significant knowledge gaps, a summary table of potential and known impacts of hydrocarbon extraction and production activities combined with a simple "traffic light" scheme is also provided.

Studies in terrestrial and shallow-water ecosystems have unravelled the key role of interspecific interactions in enhancing biodiversity, but important knowledge gaps persist for the deep sea. Cold-water coral reefs are hotspots of biodiversity, but the role of interspecific interactions and “habitat cascades” (i.e. positive effects on focal organisms mediated by biogenic habitat formation) in shaping their biodiversity is unknown. Associations between macrofaunal hosts and epifauna were examined in 47 stations at the Mingulay Reef Complex (northeast Atlantic). In total, 101 (group level) and 340 (species level) unique types of facultative associations formed by 43 hosts and 39 epifaunal species were found. Molluscs and empty polychaete tubes had higher values for the type and number of host-epifaunal associations, the Shannon–Wiener (H) and Margalef (d) indices of the epifauna than the rest of the taxonomic groups (p < 0.05). Hosts’ body size, orientation, surface smoothness, and growth form explained a significant amount of variability (32.96%) in epifauna community composition. Epifaunal species richness (S), H and d were 27.4 (± 2.2%), 56.2 (± 2.8%) and 39.9 (± 2.3%) of the respective values for the total sessile communities living on coral framework. This is intriguing as coral framework is orders of magnitude larger than the size of macrofaunal hosts. It is suggested that bivalves, tunicates and empty polychaete tubes increase habitat heterogeneity and enhance biodiversity through “habitat cascades”, in a similar way that epiphytes do in tropical rainforests. Most macrofaunal habitat suppliers in the studied cold-water coral reef are calcified species and likely susceptible to ocean acidification. This indicates that the impacts of climate change on the total biodiversity, structure and health of cold-water coral reefs may potentially be more severe than previously thought.

Biogenic habitats often form hot spots of biodiversity. However, the role of epibiosis and the ‘habitat cascades’ phenomenon in enhancing structural heterogeneity and biodiversity in biogenic habitats in remote and difficult-to-access areas is little known. In this work, we provide the first insight by exploring epibiosis across remote habitats that often support high levels of biodiversity, i.e., cold-water coral reefs and marine caves. The present study acts as a stepping-stone for the further exploration of ‘habitat cascades’ in habitats where scientific knowledge about this phenomenon is limited.

We examined the isotopic signatures (δ¹³C, δ¹⁵N) of fauna living in association with the sponge Spongosorites coralliophaga colonizing coral rubble on cold-water coral reefs in the northeast Atlantic – the shallow inshore (122–131 m collection depth) Mingulay 01 area and the deep offshore (683–800 m) Logachev 02 mound. The δ¹⁵N signatures of suspended particulate organic matter and three primary consumers, i.e. Spongosorites coralliophaga, Reteporella beaniana and Parazoanthus anguicomus were used as trophic baselines and the resulting trophic structure was compared. In both regions four trophic levels were distinguished. However, the use of S. coralliophaga or R. beaniana as baselines resulted in a skewed trophic structure due to the enriched δ¹⁵Ν signatures of these two species on the Logachev 02 mound. Using suspended particulate organic matter and P. anguicomus as baselines, the Mingulay 01 area communities were characterized by elevated relative biomass of lower trophic levels compared to the Logachev 02 mound. Relative biomass of suspension/filter feeders was also higher at the Mingulay 01 area. The two regions differed significantly with regard to the prevailing environmental conditions: apart from the difference in depth and distance from shore, the Mingulay 01 area was characterized by higher primary production in surface waters, tight pelagic–benthic coupling and higher velocity of bottom currents, and it is hypothesized that these characteristics were the main drivers of the observed differences. This study highlighted that multiple trophic baselines can provide a better interpretation of food-web structure and that the use of sponges or bryozoans as baselines across bathymetric gradients should be avoided.

Aim: We assessed the effects of regional oceanographic shifts on the macrofaunal biodiversity and biogeography of cold‐water coral reefs (CWCRs). CWCRs are often hotspots of biodiversity and ecosystem services and are in the frontline of exposure to multiple human pressures and climate change. Almost nothing is known about how large‐scale atmospheric variability affects the structure of CWCRs’ communities over ecological timescales, and this hinders their efficient conservation. This knowledge gap is especially evident for species‐rich macrofauna, a key component for ecosystem functioning.Location: The Mingulay Reef Complex, a protected biogenic ecosystem in the northeast Atlantic (120–190 m).Methods: A unique time series (2003–2011) at 79 stations was used to make the first assessment of interannual changes in CWCRs’ macrofaunal biodiversity, biogeography and functional traits. We quantified the impacts of interannual changes in North Atlantic Oscillation Index (NAOI)—the major mode of atmospheric variability in the North Atlantic, bottom temperature and salinity alongside static variables of seafloor terrain and hydrography.Results: Environmental gradients explained a significant amount of community composition ( = 26.7%, p < .01) with interannual changes in bottom temperature, salinity and NAOI explaining nearly twice as much variability than changes in terrain or hydrography. We observed significant differences in community composition, diversity and functional traits but not in species richness across interannual variability in bottom temperature. In warmer years, the biogeographic composition shifted more towards a temperate and subtropical affinity.Main Conclusions: Our findings highlight the necessity for thorough investigations of faunal communities in CWCRs as they may be sensitive to interannual changes in regional oceanography. Considering the scientific consensus on the substantial warming of North Atlantic by 2100, we recommend the establishment of programmes for the monitoring of CWCRs. This will support an advanced understanding of CWCRs’ environmental status over time and will serve their conservation for the future.