The Kermadec Trench, located in the southwestern Pacific Ocean, is ~1195km long, 120km wide, ~10,000m deep, and accounts for 3.6% of the New Zealand Exclusive Economic Zone. Formed by the subduction of the Pacific Plate under the Australian Plate, the trench exhibits steep adjacent slopes, that, north of 32°S on the underriding plate, can exceed 40 degrees due to an extensive network of fault escarpments up to 1000m in height and that are laterally extensive over many tens of kilometers. General oceanographic conditions and the characteristics of the fault escarpments, the deep trench axis, and the north (Osborn Seamount) and south (Hikurangi Trough and Rapuhia Scarp intersection) boundaries of the trench habitat are discussed. There is evidence of mass flow deposits, petite spot volcanism, and sedimentary basins that form complex habitats for benthic fauna. Overall data for biological communities that can be statistically compared are limited, but patterns in the distribution of selected hadal communities are described.

Demands are being made of the marine environment that threaten to erode the natural, social, and economic benefits that human society derives from the oceans. Expanding populations ensure a continuing increase in the variety and complexity of marine-based activities—fishing, power generation, tourism, mineral extraction, shipping, etc.

The Bari Canyon System (BCS) is located on the southwestern Adriatic margin, in the Central Mediterranean Sea indenting the continental shelf at approximately 200m. It is characterized by a rough morphology, with two main parallel branches and a channel–levee complex with subvertical flanks, and a variable substrate with rocky outcrops along the canyon flanks. The geological context and the presence of two different dense water masses, the Levantine Intermediate Water (flowing along slope) and the North Adriatic Dense Water (cascading down slope), impacting the canyon act as major drivers for the settlement of valuable biota, such as cold-water corals (CWC), among which Madrepora oculata is the main frame builder. Due to the relevance of its CWC habitat, the BCS has been explored since 2003 by several oceanographic cruises and a huge amount of multidisciplinary data (e.g., geophysical data, sediment samples, ROV images, CTD and mooring deployments) was collected to map the BCS habitat at different scales with different methodologies. The abiotic and biotic components of the BCS benthic habitats were manually mapped and classified, applying a hierarchical scheme (CoCoNet classification scheme) to categorize and integrate at different scales every habitat component to obtain a comprehensive view of the benthoscape. The CWC habitat mapping was also supported by habitat suitability models implemented for the BCS (Ecological Niche Factor Analysis, Maximum Entropy—Maxent; Generalized Linear Models), to infer the habitat extent within the canyon.

Reef morphology varies across the depth gradient of tropical shelves and the controlling drivers operate at different geological timescales. For instance, while Holocene sea level rise drowned wide expanses of offshore reefs, modern disturbance (i.e., hurricanes/storm regime) shapes coral and coralline reef morphology. Here we report a regional scale acoustic mapping using a high-resolution sidescan sonar along the Abrolhos Shelf (South Atlantic, Eastern Brazil) mapping the spatial occurrence of submerged reefs, from 5 to 90m deep. A database comprising photographs and video footage taken by divers, ROV, and drop cameras complemented the acoustic surveys with information on finer-scale reef morphology and benthic communities. Results showed that reef structures were classified as pinnacles, reef banks, and paleovalley edges. An extensive rhodolith bed was also mapped in the outer shelf. Pinnacles and reef banks could also be classified into low- and high-relief structures. Reef morphology follows a cross-shelf trend, with high-relief structures dominating the shallow waters (shallower than 20m), while low-relief structures dominate the offshore area up to 40m deep. The outer shelf is covered by rhodoliths up to the shelf break. The benthic community changes from shallow to deeper reefs. The most significant change is in coral species richness. The transition from a reef to a rhodolith habitat is still a matter of investigation in order to understand the ecological dynamics of this change. Still, rhodoliths are known as hot spots of biodiversity. This means that more than 70% of the Northern Abrolhos shelf encompasses highly complex and diverse habitats that support high biodiversity and provide important ecosystem services, but most of the information about the structure and the dynamics of biological assemblages is restricted to the small portion of emerging reefs.

This case study applied the Coastal and Marine Ecological Classification Standard (CMECS) to initial characterization of a deep-sea seamount by combining observations from a remotely operated vehicle (ROV) and information derived from multibeam sonar bathymetry and backscatter. Spatial segmentation of the multibeam bathymetry was done using algorithms based on defining bathymorphons resulting in six classes: flats, slopes, ridges, valleys, shoulders, and footslopes. These classes were modified to delineate CMECS “Level 1” geoform units for Gosnold Seamount. Further segmentation of landforms was completed using textural analysis of the sonar backscatter mosaic of the seamount to identify segments of the same landform type with similar reflectivity texture. All of the ROV dive video of the seafloor was analyzed manually to create a spreadsheet of 933 georeferenced annotations of organisms and associated substrate types. The dominant sediment type over each 50m segment of the ROV track was also classified using substrate unit terminology from CMECS into four classes: bedrock (10% of ROV track), fine unconsolidated sediments on bedrock (84%), coral rubble (1%), and sand (5%). Eleven genera of corals, two classes of sponges, and four classes of echinoderms were observed along the track, with glass sponges dominating the annotation and abundance counts. Nominal regression revealed that depth, temperature, and sediment type were significant predictors of individual coral along the ROV track (P<.001, P<.001, P<.001, respectively). In contrast, slope, sediment type and dissolved oxygen were significant predictors of sponge distribution along the track. In summary the application of CMECS to Gosnold Seamount provided a useful systematic framework for structuring geoform, substrate, and biotic classification of benthic habitat. Using this standard, in combination with the semiautomated seafloor segmentation approach utilized, can provide a consistent and reproducible habitat classification approach for large regions and facilitate comparison of habitats among features.

This chapter presents a broad synthesis and overview based on the 53 case studies included in Part 2 of this book, and on questionnaires completed by the authors. The case studies covered areas of seafloor ranging from 0.2 to 500,000km2 (average of 16,670km2) and a broad range of geomorphic feature types. The mean depths of the study areas ranged from 1.5 to 8000m, with about half of the studies on the shelf (depth <120m) and half on the slope and at greater depths. Mapping resolution ranged from 0.01 to 1000m (mean of 45m). There is a skewed distribution of studies across the four naturalness categories with most habitats classed in the “very good” (n=20) and “good” (n=24) condition categories. Only eight studies classified their study area as being in a “poor” condition and none were classified as being in “very poor” condition. The majority of case studies (n=34) concluded that the condition was steady, eight case studies reported a declining condition, and only two case studies assessed the condition of their study area was improving. The confidence of the estimates for condition and trend were more evenly spread between high (n=15), medium (n=21), and low (n=10).

The Tricase Canyon is a 20-km-long submarine canyon, incising the southernmost Apulian margin at the boundary between the Adriatic and Ionian Seas, in the Mediterranean basin. The canyon indents the continental margin at depth of –120m and reaches a maximum depth of approximately –900m. The Tricase Canyon is disconnected from direct fluvial system and has no other tributaries. The shallower area of the canyon is generally characterized by a sparse cnidarian occurrence which includes Protoptilum carpenteri, representing the easternmost occurrence of this species known in the Mediterranean basin. The cold-water corals Madrepora oculata, Desmophyllum pertusum, and Desmophyllum dianthus colonize deeper areas (approximately –800m), settling preferentially on blocks interpreted as mass waste deposits.

The Primeiras and Segundas archipelagos are a string of coralline islands in northern Mozambique. The presence of upwelling, clockwise and anticlockwise eddies, and the influence of Zambezi river’s discharge result in nutrient and biodiversity dispersion along the coastline and a connection between Mozambique’s southern and northern reef systems. The fringing spur and groove reef structure and relative substrate distribution are consistent throughout the two archipelagos. However, due to differing anthropogenic pressures, the state of the archipelagos varies. The southern islands are considered to be in good condition, while the northern islands are in poor condition. According to observations gathered in the past decades, the condition in both archipelagos is considered stable. A total of 9 algae species, 8 seagrass species, 103 coral species, and more than 160 fish species have been observed in the region since 1999 and preliminary results indicate that complex, heterogeneous zones could be used as surrogates for species diversity.

The “Sylter Outer Reef” is a special protected area in the German North Sea. The area is characterized by patches of coarse sediments compared to the otherwise sandy North Sea. Sediments coarser than sand size originate from the last glacial period or postglacial sorting processes. One special feature is winnowed gravel lag deposits, which are very rich in species. The features have a sickle-like to elongated shape with a distinct boundary between fine sand and gravel on one side, and a fuzzy boundary with gravel to fine sand on the other side. The sharp boundary indicates an abrupt shift in morphology. Gravel deposits are observed to be morphologically deeper than the surrounding finer sediments. In addition, three soft-bottom communities can be associated with this feature: Tellina fabula on fine to medium sediments; Goniadella–Spisula on medium to coarse sediments; and Goniadella–Spisula on coarse sediments.

An area of 5320km2 in water depths of 30–250m has been mapped on German Bank on the southern Scotian Shelf in Atlantic Canada. The Scotian Shelf is a formerly glaciated continental margin characterized by a topographically rugged inner shelf. Bedrock is exposed at the seafloor on much of German Bank. Ice-contact sediment (till) was deposited beneath or at the margins of the ice sheet directly onto bedrock during the Wisconsinan glaciation and occurs as a widespread sediment blanket. Ice-distal glaciomarine silt overlies the older till and is primarily confined to small basins on the bank. Limited accumulations of postglacial sediments are composed of well-sorted sand, grading to rounded and subrounded gravel. Analysis of seafloor photographs and underwater video revealed broadscale gradients in benthic fauna composition across the bank. Surficial geology, broad biophysical (“benthoscape”), and modeled scallop habitat suitability maps are presented for the area, generated using a variety of methods. The combined use of these different maps is offering benefits for use in fisheries management in this area.