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The deep ocean absorbs vast amounts of heat and carbon dioxide, providing a critical buffer to climate change but exposing vulnerable ecosystems to combined stresses of warming, ocean acidification, deoxygenation, and altered food inputs. Resulting changes may threaten biodiversity and compromise key ocean services that maintain a healthy planet and human livelihoods. There exist large gaps in understanding of the physical and ecological feedbacks that will occur. Explicit recognition of deep-ocean climate mitigation and inclusion in adaptation planning by the United Nations Framework Convention on Climate Change (UNFCCC) could help to expand deep-ocean research and observation and to protect the integrity and functions of deep-ocean ecosystems.

As plastic waste pollutes the oceans and fish stocks decline, unseen below the surface another problem grows: deoxygenation. Breitburg et al. review the evidence for the downward trajectory of oxygen levels in increasing areas of the open ocean and coastal waters. Rising nutrient loads coupled with climate change—each resulting from human activities—are changing ocean biogeochemistry and increasing oxygen consumption. This results in destabilization of sediments and fundamental shifts in the availability of key nutrients. In the short term, some compensatory effects may result in improvements in local fisheries, such as in cases where stocks are squeezed between the surface and elevated oxygen minimum zones. In the longer term, these conditions are unsustainable and may result in ecosystem collapses, which ultimately will cause societal and economic harm. Science , this issue p. eaam7240 Oxygen is fundamental to life. Not only is it essential for the survival of individual animals, but it regulates global cycles of major nutrients and carbon. The oxygen content of the open ocean and coastal waters has been declining for at least the past half-century, largely because of human activities that have increased global temperatures and nutrients discharged to coastal waters. These changes have accelerated consumption of oxygen by microbial respiration, reduced solubility of oxygen in water, and reduced the rate of oxygen resupply from the atmosphere to the ocean interior, with a wide range of biological and ecological consequences. Further research is needed to understand and predict long-term, global- and regional-scale oxygen changes and their effects on marine and estuarine fisheries and ecosystems.

The deep sea, the largest ecosystem on Earth and one of the least studied, harbours high biodiversity and provides a wealth of resources. Although humans have used the oceans for millennia, technological developments now allow exploitation of fisheries resources, hydrocarbons and minerals below 2000 m depth. The remoteness of the deep seafloor has promoted the disposal of residues and litter. Ocean acidification and climate change now bring a new dimension of global effects. Thus the challenges facing the deep sea are large and accelerating, providing a new imperative for the science community, industry and national and international organizations to work together to develop successful exploitation management and conservation of the deep-sea ecosystem. This paper provides scientific expert judgement and a semi-quantitative analysis of past, present and future impacts of human-related activities on global deep-sea habitats within three categories: disposal, exploitation and climate change. The analysis is the result of a Census of Marine Life--SYNDEEP workshop (September 2008). A detailed review of known impacts and their effects is provided. The analysis shows how, in recent decades, the most significant anthropogenic activities that affect the deep sea have evolved from mainly disposal (past) to exploitation (present). We predict that from now and into the future, increases in atmospheric CO(2) and facets and consequences of climate change will have the most impact on deep-sea habitats and their fauna. Synergies between different anthropogenic pressures and associated effects are discussed, indicating that most synergies are related to increased atmospheric CO(2) and climate change effects. We identify deep-sea ecosystems we believe are at higher risk from human impacts in the near future: benthic communities on sedimentary upper slopes, cold-water corals, canyon benthic communities and seamount pelagic and benthic communities. We finalise this review with a short discussion on protection and management methods.

The effect of Ocean Acidification (OA) on marine biota is quasi-predictable at best. While perturbation studies, in the form of incubations under elevated pCO(2), reveal sensitivities and responses of individual species, one missing link in the OA story results from a chronic lack of pH data specific to a given species' natural habitat. Here, we present a compilation of continuous, high-resolution time series of upper ocean pH, collected using autonomous sensors, over a variety of ecosystems ranging from polar to tropical, open-ocean to coastal, kelp forest to coral reef. These observations reveal a continuum of month-long pH variability with standard deviations from 0.004 to 0.277 and ranges spanning 0.024 to 1.430 pH units. The nature of the observed variability was also highly site-dependent, with characteristic diel, semi-diurnal, and stochastic patterns of varying amplitudes. These biome-specific pH signatures disclose current levels of exposure to both high and low dissolved CO(2), often demonstrating that resident organisms are already experiencing pH regimes that are not predicted until 2100. Our data provide a first step toward crystallizing the biophysical link between environmental history of pH exposure and physiological resilience of marine organisms to fluctuations in seawater CO(2). Knowledge of this spatial and temporal variation in seawater chemistry allows us to improve the design of OA experiments: we can test organisms with a priori expectations of their tolerance guardrails, based on their natural range of exposure. Such hypothesis-testing will provide a deeper understanding of the effects of OA. Both intuitively simple to understand and powerfully informative, these and similar comparative time series can help guide management efforts to identify areas of marine habitat that can serve as refugia to acidification as well as areas that are particularly vulnerable to future ocean change.

Controversy pervaded the June 2022 UN Ocean Conference, with partisan alliances forming around burgeoning environmental and social issues. Yet, out of the talks, emerged strong aspirations across UN states and other stakeholders to restore and protect the ocean.

The abundant small taxa on hard substrates in deep-sea environments remain understudied and therefore poorly understood. Abundant seepage, tectonic activity, upwelling, and high productivity generate a diverse array of hard substrates including basalts, carbonate rocks, phosphorites, ferromanganese crusts, and sedimentary rocks throughout the Southern California Borderland (SCB), and wood falls are common given the region's proximity to land. A comparative experimental approach was used to examine the influence of hard substrate type on patterns of macrofaunal colonization under different environmental conditions, and particularly revealed the outsized influence of the wood-boring bivalve Xylophaga on macrofaunal colonization both on and near wood substrates. The experimental substrates--wood (to assess the influence of organic falls), carbonate rock (to assess faunal affinities for seep-related substrates outside of a seep environment), and other rocks (ferromanganese, phosphorites, and sedimentary rocks, which are common throughout the SCB)--were deployed for ten months at two sites (San Juan Seamount, 40-Mile Bank), each at two water depths (~700 m, ~ 1100 m) in the SCB. The settlement of juvenile Xylophaga on wood and nearby substrates drove overall density trends and contributed to the distinct community composition observed among substrate types. Experimental substrates exhibited significantly higher densities than natural rocks, possibly representing the influence of different substrate or earlier successional stages. Diversity was highest on natural rock substrates and lowest on experimental wood substrates, which also exhibited the lowest evenness due to dominance by wood specialists such as Xylophaga, dorvilleid polychaetes, and the ampharetid polychaete Decemunciger sp. Juvenile Xylophaga were found on both experimental wood substrates and nearby rock substrates, and likely provided other species with food, and on wood, with modified (engineered) habitat. These findings suggest that natural wood falls could have a shadow effect on macrofauna of adjacent hard substrates in this region, which merits future study.

Larvae have been difficult to study because their small size limits our ability to understand their behavior and the conditions they experience. Questions about larval transport focus largely on (a) where they go [dispersal] and (b) where they come from [connectivity]. Mechanisms of transport have been intensively studied in recent decades. As our ability to identify larval sources develops, the consequences of connectivity are garnering more consideration. Attention to transport and connectivity issues has increased dramatically in the past decade, fueled by changing motivations that now include management of fisheries resources, understanding of the spread of invasive species, conservation through the design of marine reserves, and prediction of climate-change effects. Current progress involves both technological advances and the integration of disciplines and approaches. This review focuses on insights gained from physical modeling, chemical tracking, and genetic approaches. I consider how new findings are motivating paradigm shifts concerning (1) life-history consequences; (2) the openness of marine populations, self-recruitment, and population connectivity; (3) the role of behavior; and (4) the significance of variability in space and time. A challenge for the future will be to integrate methods that address dispersal on short (intragenerational) timescales such as elemental fingerprinting and numerical simulations with those that reflect longer timescales such as gene flow estimates and demographic modeling. Recognition and treatment of the continuum between ecological and evolutionary timescales will be necessary to advance the mechanistic understanding of larval and population dynamics.

Sediments associated with hydrothermal venting, methane seepage and large organic falls such as whale, wood and plant detritus create deep-sea networks of soft-sediment habitats fueled, at least in part, by the oxidation of reduced chemicals. Biological studies at deep-sea vents, seeps and organic falls have looked at macrofaunal taxa, but there has yet to be a systematic comparison of the community-level attributes of sediment macrobenthos in various reducing ecosystems. Here we review key similarities and differences in the sediment-dwelling assemblages of each system with the goals of (1) generating a predictive framework for the exploration and study of newly identified reducing habitats, and (2) identifying taxa and communities that overlap across ecosystems. We show that deep-sea seep, vent and organic-fall sediments are highly heterogeneous. They sustain different geochemical and microbial processes that are reflected in a complex mosaic of habitats inhabited by a mixture of specialist (heterotrophic and symbiont-associated) and background fauna. Community-level comparisons reveal that vent, seep and organic-fall macrofauna are very distinct in terms of composition at the family level, although they share many dominant taxa among these highly sulphidic habitats. Stress gradients are good predictors of macrofaunal diversity at some sites, but habitat heterogeneity and facilitation often modify community structure. The biogeochemical differences across ecosystems and within habitats result in wide differences in organic utilization (i.e., food sources) and in the prevalence of chemosynthesis-derived nutrition. In the Pacific, vents, seeps and organic-falls exhibit distinct macrofaunal assemblages at broad-scales contributing to ß diversity. This has important implications for the conservation of reducing ecosystems, which face growing threats from human activities.

Accelerated oxygen loss in both coastal and open oceans is generating complex biological responses; future understanding and management will require holistic integration of currently fragmented oxygen observation and research programmes.

Under the Paris Agreement nations made pledges known as nationally determined contributions (NDCs), which indicate how national governments are evaluating climate risks and policy opportunities. We find that NDCs reveal important systematic patterns reflecting national interests and capabilities. Because the ocean plays critical roles in climate mitigation and adaptation, we created a quantitative marine focus factor (MFF) to evaluate how governments address marine issues. In contrast to the past, when oceans received minimal attention in climate negotiations, 70% of 161 NDCs we analysed include marine issues. The percentage of the population living in low-lying areas—vulnerable to rising seas—positively influences the MFF, but negotiating group (Annex 1 or small island developing states) is equally important, suggesting political motivations are crucial to NDC development. The analysis reveals gaps between scientific and government attention, including on ocean deoxygenation, which is barely mentioned. Governments display a keen interest in expanding marine research on climate priorities. The ocean is a key part of the climate system but is often neglected in individual country priorities. Analysis of Nationally Determined Contributions reveals 70% include marine issues. The level of inclusion varies dependent on country factors including vulnerability to rising seas.