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The degree to which biotic communities are regionally enriched or locally saturated, and roles of key structuring processes, remain enduring ecological questions. Prior studies of reef-building corals of the Indo-West Pacific (IWP) found consistent evidence of regional enrichment, a finding subsequently questioned on methodological grounds. Here we revisit this relation and associated relations between richness and abundance (as "effective number of species"), and coral cover, used as a proxy for disturbance and competition. From 1994 to 2017, we sampled > 2,900 sites on shallow (typically < 8-10 m depth below reef crest) and deeper reef slopes in 26 coral ecoregions, from Arabia to the Coral Triangle, Eastern Australia, Micronesia and Fiji, for a total pool of 672 species. Sampling intensity varied among ecoregions but always approached asymptotic richness. Local coral communities on both shallow and deep reef slopes were, on average, comprised of 25% of regional pools, ranging from 12 to 43% for individual ecoregions. The richest individual shallow and deep sites, averaged across all ecoregions, comprised 42 and 40% of regional pools, ranging from 30 to 60%, the highest in environmentally marginal ecoregions. Analyses using log-ratio regression indicated that IWP coral communities on deeper reef slopes were intermediate between regionally enriched and locally saturated. Communities on shallow reef slopes showed more evidence of regional enrichment, consistent with these being most susceptible to disturbance. Unimodal curvilinear relations between local richness and coral cover provide support for disturbance mediation and competitive exclusion. IWP coral communities are clearly dynamic, shaped by biological, ecological, and oceanographic processes and disturbance regimes that influence reproduction, dispersal, recruitment, and survival. Yet there is also evidence for a degree of local saturation, consistent with a niche-neutral model of community assembly. The r

This study represents the first ROV-based exploration of the Perth Canyon, a prominent submarine valley system in the southeast Indian Ocean offshore Fremantle (Perth), Western Australia. This multi-disciplinary study characterizes the canyon topography, hydrography, anthropogenic impacts, and provides a general overview of the fauna and habitats encountered during the cruise. ROV surveys and sample collections, with a specific focus on deep-sea corals, were conducted at six sites extending from the head to the mouth of the canyon. Multi-beam maps of the canyon topography show near vertical cliff walls, scarps, and broad terraces. Biostratigraphic analyses of the canyon lithologies indicate Late Paleocene to Late Oligocene depositional ages within upper bathyal depths (200-700 m). The video footage has revealed a quiescent 'fossil canyon' system with sporadic, localized concentrations of mega- and macro-benthos (∼680-1,800 m), which include corals, sponges, molluscs, echinoderms, crustaceans, brachiopods, and worms, as well as plankton and nekton (fish species). Solitary (Desmophyllum dianthus, Caryophyllia sp., Vaughanella sp., and Polymyces sp.) and colonial (Solenosmilia variabilis) scleractinians were sporadically distributed along the walls and under overhangs within the canyon valleys and along its rim. Gorgonian, bamboo, and proteinaceous corals were present, with live Corallium often hosting a diverse community of organisms. Extensive coral graveyards, discovered at two disparate sites between ∼690-720 m and 1,560-1,790 m, comprise colonial (S. variabilis) and solitary (D. dianthus) scleractinians that flourished during the last ice age (∼18 ka to 33 ka BP). ROV sampling (674-1,815 m) spanned intermediate (Antarctic Intermediate Water) and deep waters (Upper Circumpolar Deep Water) with temperatures from ∼2.5 to 6°C. Seawater CTD profiles of these waters show consistent physical and chemical conditions at equivalent depths between dive

Marine hardgrounds are common features during the Phanerozoic and hold significant sedimentological and economic importance. Intriguingly, previous reports of marine hardgrounds are concentrated in Calcite Seas, despite elevated seawater CaCO3 saturation in Aragonite Seas. This bias remains unclear in origin and requires more hardground information, especially from Aragonite Seas, for clarification. Well-developed Holocene marine hardgrounds at Abu Dhabi provide such a good opportunity. This study focused on a widespread and well-developed Holocene marine hardground layer at Abu Dhabi and analyzed its chronostratigraphy, petrology, mineralogy, and geochemistry. The results show that the studied hardground layer can be divided into lower and upper parts, characterized by planar upper surface and no borings nor encrustations. The lower part (with a 14C age of 6945−6368 cal yrs BP) formed during a sea-level transgression, and is laterally traceable along both a seaward and a landward direction. The upper part (with a 14C age of 5871−5452 cal yrs BP) formed during following sea-level transgression and/or stillstand, and disappears along a landward direction. Compared with the lower part, the upper hardground part is higher in δ13Ccarb and δ18Ocarb, supporting formation within more evaporated seawater settings depositing more high-Mg calcite. Both parts consist mainly of aragonite and high-Mg calcite in both carbonate grains and intergranular early-marine cement, but the lower hardground part contains more protodolomite within the early-marine cement. Moreover, an inverse relation in contents indicates a diagenetic transition from aragonite to dolomite during hardground formation and early diagenesis. Further, in combination with previous studies, the findings of this study confirm the rapidity, lateral diachronicity, and composite nature of Holocene marine hardgrounds with mineralogy controlled by sea-level changes. Similar hardgrounds may also be well developed i