Main objective: The OceanX 2023 (OCX 2023) is the continuation of our long-term strategy to map deep-sea biodiversity and identify Vulnerable Marine Ecosystems (VMEs) in the Azores. In this cruise, we operated from the RV OceanExplorer and planned to visit some unexplored areas such as the geomorphological structures around the Princesa Alice bank and Dom João de Castro seamount, as well as other areas near the slopes of Pico and Faial islands. As in the MapGES cruises, the objectives were to (i) map benthic communities inhabiting unexplored seamounts, ridges and island slopes, (ii) identify new areas that fit the FAO Vulnerable Marine Ecosystem definition, (iii) determine distribution patterns of deep-sea benthic biodiversity in the Azores, and (iv) determine the condition of benthic communities by looking at evidence of fishing damage to fauna, presence of lost fishing gear and marine litter. Moreover, this cruise aimed to collect biological samples of poorly known or unidentified species observed on video footage. The results of these explorations will directly contribute to the Regional Government of the Azores efforts to declare 30% of the Azores EEZ as marine protected areas of which 10% should be strictly protected. The results of this cruise added to the previous contributions to identify the environmental drivers that determine the spatial distribution of deep-sea benthic biodiversity in the Azores. It also provides valuable information in the context of Good Environmental Status (GES), Marine Spatial Planning (MSP) and new insights on how to sustainably manage deep-sea ecosystems.

Methodology: We performed several underwater video transects using the vehicles available in the RV OceanXplorer. These assets included the Argus ROV Chimaera, rated to 6000 meters depth, and the Triton submersibles Nadir and Neptuno rated to 1000 meters and with similar sampling capabilities as the ROV Chimaera. In each of the areas or geomorphological structures to be explored, we carried out video transects from a depth of about 1 000 m to the shallowest depth of each structure. The objective was to obtain underwater images to characterize the biodiversity along the entire bathymetric gradient and substrate types of each structure and to collect as many samples of unknown species as possible. Since in-situ data of water-mass characteristics is very scarce, one of the goals of the cruise is to collect information that can contribute to the understanding of the water masses distribution in the Azores. Vertical CTD/Rosette profiles could be conducted at each of the video stations at a seabed depth of about 1,000 m in order to measure physical and chemical seawater properties that characterize the dominant water masses described for the Azores: 0-150m, Mixed layer and seasonal thermocline; 150-550 m, North Atlantic Central Water (NACW); 500-1500 m, Mediterranean Outflow Water (MOW) / Subarctic Intermediate Water (SAIW, west of MAR). The North Atlantic Deep Water (NADW; >1500 m) will not be targeted during this cruise. One open-ocean control station should also be considered. Accurate knowledge of seabed characteristics is crucial in order to effectively plan the biological surveys and to better understand the ecological processes that are observed in the video images. Considerable effort has been placed in the last few years in collecting accurate bathymetric data in the Azores region and, therefore, most areas to be visited have already been surveyed with MB. Nevertheless, we suggested conducting surveys in all 3 main areas to improve coverage or complement existing data (Princess Alice bank, D. João de Castro bank and SE Pico area). Additionally, opportunistic MB surveys were conducted during the transits. In order to achieve our communication goals, our media team onboard Ocean Explorer was in permanent contact with the OKEANOS institute media team on land to organize social media posts about the mission. Several science communication activities were organized during the cruise, including full photo and video coverage of the scientific activities.

Cruise summary: During the OXR 2023 survey we visited several areas in the central group of the Azores, namely D. João de Castro seamount and Princesa Alice bank and the slopes of Faial Capelinhos, Pico S Lajes, and Pico Ponta da Ilha (Table 1). Taking advantage of the technological assets onboard RV OceanXplorer, namely the ROV Chimaera and the Submersibles Neptune and Nadir, we were able to conduct 8 manned submarine dives and 12 remotely operated vehicle dives, one ROV D. João de Castro seamount, 3 SUB and 3 ROV dives in Capelinhos, 5 SUB and 6 ROV dives in Princesa Alice (Princesa Alice, Princesa Alice W, Bourée E), and one ROV dive in Pico S Lajes and another one in Ponta da Ilha N. From the 20 ROV and SUB dives, 19 were successful. In general both the ROV and the submersible dives covered a much smaller linear distance (average 952 m) per hour of survey (206 m·h^-1) when compared to the Azores drift-cam. One of the reasons for the shorter distances was the need to collect biological samples to clarify the taxonomic identification of several organisms. During these dives, we were able to collect 268 biological samples belonging to approximately 197 different morphotypes. It is likely that this sampling effort will help solving about 100 taxonomic questions. After each ROV or submersible dive we performed a CTD cast to measure water masses properties and collect water samples for more detailed analyses. Form the 27 CTD stations, 15 were specifically conducted to address deep-sea related questions, while the other stations were performed to address the Pelagic team objectives. From these 15 dedicated CTD casts, we obtained 157 water samples for different types of analyses.

Main achievements: The deep-sea component of the OceanX 2023 expedition revealed that banks and seamounts heavily exploited by bottom longline fishing are still home deep-sea benthic communities with a high natural and ecological value.

1.       During the OXR 2023 we visited 7 different areas, and mapped 3,840 km² of seabed, of which 630 km² is new information to be included in official Portuguese databases. We were able to participate in 8 manned submarine dives and 12 remotely operated vehicle dives, which produced 84:30 hours of deep sea video, over 19 km of bottom, and 268 biological samples. We also performed 27 stations for analysis of water masses, in which 157 samples were collected for analysis of environmental DNA and other parameters.

2.       We observed many benthic communities, some of them quite special. One of the highlights of this campaign was the big basalt outcrops found at around 800 m depth that were densely colonized by many colonies of the black coral Leipathes expansa. Most of the colonies observed were visually healthy, particularly large, probably older than 1.000 years, and with a lot of associated biodiversity. It is possible that before demersal fishing exploited this bank, these arborescent black corals, vulnerable to fishing, were even much more abundant, larger and older. These corals are very slow growing and can live for thousands of years.

3.       We also discovered one of the few hard coral reefs known in the Azores composed by the lace corals Lophelia pertusa and Madrepora occulata. This small reef was observed at around 850 m depth on the Capelinhos area, west of Faial. Hard coral reefs play an important role in the biogeochemical cycles of the deep sea, but are very vulnerable to ocean acidification. Some portions of this reef were not alive, forming a compact coral framework to which many other associated species were attached. A better understanding of their distribution and their susceptibility to climate change are priority lines of research for the future.

4.       We also discovered one of the largest and densest coral garden dominated by the primnoid Callogoria verticillata found in the Azores. This gorgonian forest, found on the Dom João de Castro seamount, is characterized by fan-shaped colonies whose feathery branches resemble palm leaves. Most of the colonies observed were smaller than usual, but possibly the densest aggregation of this species observed so far.

5.       We observed dense aggregations of the largest species of sponges that inhabit the deep sea of the Azores (e.g., Characella pachastrelloides, Haliclona magna), on the Princesa Alice bank. Sponges play a structuring role in the deep sea, increasing ecosystem productivity and creating habitat for other species.

6.       In one of the areas explored with multibeam probes, an unknown canyon (or gorge) of about 15 km long, 340 m wide and with cliffs about 100 m high was found. The canyons and gorges of the Azores are fairly unknown habitats where unique ecosystems can be found.

7.       The OceanX 2023 expedition made it possible to identify that a large part of the benthic communities, including corals and sponges, observed in one of the Azores' main demersal fishing grounds are still in good environmental condition and have a high natural and ecological value. However, some long-lived coral colonies with visible fishing impacts were observed. These in situ observations corroborate the conclusions of previous studies which suggest that well-regulated deep-sea fishing based on hook-and-line gear (preferably handlines) could contribute to a sustainable exploitation of the deep sea.

8.       The manned submarines and the ROV available on the RV OceanXplorer, allowed collecting 268 biological samples belonging to approximately 197 different morphotypes. It is likely that this sampling effort will help solving about 100 taxonomic questions, and some samples could even reveal new species to science.

In summary, we found areas that fit the definition of Vulnerable Marine Ecosystems, and compiled valuable scientific information to inform policies that promote the preservation of the natural heritage, guaranteeing the sustainable use of the deep sea and minimizing the negative impacts on these vulnerable ecosystems.