No compilation of interesting facts about the Abyssal Zone can be considered complete without the inclusion of topics like abyssal gigantism, chemosynthesis, and bioluminescence.
![]() Interesting Facts About the Abyssal Zone That’ll Startle You
We detected a consistent warming signal in the abyssal ocean around the globe. The strongest warming is occurring in the Southern Ocean, around Antarctica, at a rate of approximately 0.03 degrees. Abyssal Zone is conveniently defined as the area where water drops to 4 degrees Celsius. At the ocean’s average depth of 4000m, the average temperature is 2.
Forget universe, we are yet to explore the ocean!
That 95 percent of the ocean remains unexplored makes sense when we realize that we know so little about the life underwater.
Until mid-1800s, the term ‘deep sea’ was used for the area ‘beyond the reach of fisherman’. Things have changed considerably over the period. Today―though there is a lack of consensus―it refers to the area below the depth of 1800 m (5900 ft). Then again, until recently, we had no idea as to what was in store for us beyond a certain depth. It was this lack of information that prompted the British naturalist, Edward Forbes to claim that the possibility of life below 540 m (less than 2,000 ft) was as good as none.
It was only after we pulled off some remarkable deep sea expeditions that we realized that the dark depths of the ocean are much more fascinating than what we thought. For instance, we realized that there existed species which could withstand the extreme pressure in the deepest part of the world’s ocean, only when Swiss oceanographer, Jacques Piccard and Lt. Don Walsh of the United States Navy explored the Challenger Deep in 1959.
Over the course of time, more of such facts came to light, and researchers divided the deep sea into five pelagic layers. Of these, the largest is the Abyssal Zone, which roughly begins at a depth of 4,000 m (13,000 ft). Despite the extreme conditions prevailing in this zone, it has quite a few interesting and some unusual things to its credit.
Interesting Facts About the Abyssopelagic Zone
The Abyssal Zone, also known as the Abyssopelagic Zone or simply, the Abyss―derived from the Greek word, meaning bottomless―is the part of the ocean which is typically characterized by uniform darkness, low temperature (around 3 degrees celsius), and unique fauna.
It lies between the Bathyal Zone and Hadal Zone. It roughly starts at a depth of 4,000 m (13,000 ft) and ends at 6,000 m (19,500), which is where the Hadal Zone, named after the Greek god, Hades, begins. The latter is primarily made up of deep trenches, like the Mariana Trench of the Challenger Deep fame.
✦ The Abyssal Zone covers an area of over 115,000,000 mi, which is roughly about 83 percent of the total area of the ocean and 60 percent of the Earth’s surface.
✦ Like we said earlier, it begins at a depth of around 4000 m. At this depth, it doesn’t receive sunlight or precipitation. Nor is it affected by the climate above the water surface.
✦ In the absence of photosynthesis, the oxygen content of this zone depends on the oxygen that gets dissolved into the water in polar areas and makes it to the other parts of the ocean.
✦ Its nutrient salt concentration is uniform―at times, higher than what it is in shallow water; courtesy, the dead biological matter that settles in this zone.
✦ As the pressure in the Abyssal Zone can reach 11,000 psi at times, the ability to withstand tremendous amount of pressure is a necessity. The animals inhabiting this zone seem to excel in this context.
✦ As there is no sunlight to support the process of photosynthesis, the Abyssal Zone is devoid of plants. Instead, tiny microbes, known as cormophytes, act as producers in this zone by resorting to the process of chemosynthesis and oxidizing hydrogen sulfide coming from vents in the ocean floor.
✦ Some examples of abyssal zone animals are giant squid (Architeuthis dux), colossal squid (Mesonychoteuthis hamiltoni), black swallower (Chiasmodon niger), humpback anglerfish (Melanocetus johnsonii), deep-sea glass squid (Teuthowenia pellucida), ogrefish (Anoplogaster cornuta), hagfish (Eptatretus cirrhatus), hydrothermal vent crab (Bythograea thermydron), giant tubeworm (Tevnia jerichonana), etc.
✦ As food is scarce in the Abyssal Zone, most animals resort to various physical and behavioral adaptations to survive. The humpback anglerfish (Melanocetus johnsonii), for instance, has a large mouth with long teeth, which helps it grab its prey the moment it is in close range.
✦ Other physical adaptations include gray or black color, which acts as a camouflage in the deep sea environment, and bioluminescence, i.e., the ability to produce light by physiological processes, which helps the species lure their prey.
✦ Most species found in this zone are ambush predators, who wait for their prey to come to them, instead of expending energy chasing them. For these species, bioluminescence definitely comes as a blessing in disguise.
✦ In what is referred to as abyssal gigantism, species found in these dark depths of the ocean are known to be larger than the related species in shallow water. At 12 – 14 m (39 – 46 ft), the colossal squid (Mesonychoteuthis hamiltoni), for instance, is much larger than any shallow-water squid species.
✦ Tubeworms are best suited for the conditions prevailing in the Abyssal Zone. They don’t have mouth or digestive system. So they absorb inorganic compounds through their skin and depend on bacteria in their body for food.
✦ This zone is characterized by the presence of hydrothermal vents, also known as black smokers that release toxic chemicals. Interestingly, the area around these vents is known for its rich biodiversity. The organisms found here resort to chemosynthesis for energy.
The Abyssal zone is among the least-studied zones of the ocean, but that doesn’t mean it’s free from human intervention. That’s simply not possible with all that pollution we are causing. The problem perhaps, is that we don’t realize the extent of damage we can cause to this zone, or we have already caused for that matter. What we need to realize, is the fact that the ocean is a single unit and anything we drop in it will simply keep moving within and affecting lifeforms that come in contact with it. After all, who would have thought that the rubber ducks that were lost in the Pacific would go around the continent and reach the east coast?
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AbstractThe deep sea encompasses the largest ecosystems on Earth. Although poorly known, deep seafloor ecosystems provide services that are vitally important to the entire ocean and biosphere. Rising atmospheric greenhouse gases are bringing about significant changes in the environmental properties of the ocean realm in terms of water column oxygenation, temperature, pH and food supply, with concomitant impacts on deep-sea ecosystems. Projections suggest that abyssal (3000–6000 m) ocean temperatures could increase by 1°C over the next 84 years, while abyssal seafloor habitats under areas of deep-water formation may experience reductions in water column oxygen concentrations by as much as 0.03 mL L –1 by 2100.
Bathyal depths (200–3000 m) worldwide will undergo the most significant reductions in pH in all oceans by the year 2100 (0.29 to 0.37 pH units). O 2 concentrations will also decline in the bathyal NE Pacific and Southern Oceans, with losses up to 3.7% or more, especially at intermediate depths. Another important environmental parameter, the flux of particulate organic matter to the seafloor, is likely to decline significantly in most oceans, most notably in the abyssal and bathyal Indian Ocean where it is predicted to decrease by 40–55% by the end of the century. Unfortunately, how these major changes will affect deep-seafloor ecosystems is, in some cases, very poorly understood.
In this paper, we provide a detailed overview of the impacts of these changing environmental parameters on deep-seafloor ecosystems that will most likely be seen by 2100 in continental margin, abyssal and polar settings. We also consider how these changes may combine with other anthropogenic stressors (e.g., fishing, mineral mining, oil and gas extraction) to further impact deep-seafloor ecosystems and discuss the possible societal implications. Knowledge Domain: Earth & Environmental Science Ecology Ocean Science. IntroductionThe oceans are a major sink for CO 2 produced by the burning of fossil fuels ( ) as well as for the heat produced by the greenhouse effect ( ). Oceans thus help to buffer multiple aspects of global climate change and their effects on marine and terrestrial ecosystems ( ). Deep-sea ecological processes and characteristics, such as nutrient cycling, carbon sequestration, productivity, habitat provision, and trophic support, underlie the healthy functioning of ocean ecosystems and provide valuable ecosystem services to mankind ( ).
For example, nutrients produced during the re-mineralization of organic matter at the deep seafloor are ultimately used by phytoplankton to produce organic matter that fuels secondary production. At the same time, organic-matter degradation and re-mineralisation contribute to carbon biogeochemical cycling in the ocean, and help to buffer the ocean against pH changes and the effects of ocean acidification (;; ).
The health and sustainable functioning of the planet are therefore highly dependent on the deep sea (defined here as 200 m), which accounts for more than 95% of the volume of the Earth’s oceans.Atmospheric CO 2 concentrations have risen from 280 ppm during pre-industrial times to 407 ppm today as a result of the burning of fossil fuels, deforestation and the removal of other habitats that sequester carbon. Continued use of fossil fuels into the 21st century is predicted to lead to atmospheric CO 2 levels 900 ppm by 2100 (under Representative Concentration Pathway (RCP) 8.5; ), though the precise level is highly dependent on the emission scenario ( ). These rising atmospheric greenhouse gas concentrations have led to an increase in global average temperatures of 0.2°C decade –1, much of which has been absorbed by the oceans, whilst the oceanic uptake of atmospheric CO 2 has led to major changes in surface ocean pH (,;;;; ).The deep sea has experienced dramatic changes in physical and chemical variables in the geological past. For example, major expansion and strengthening of oxygen minimum zones (OMZs; O 2.
MethodsTo identify the present and future state of deep-sea ecosystems, we used a combination of expert opinion, current literature, and the output of the IPCC (Intergovernmental Panel on Climate Change) Fifth Assessment Report (AR5) models. Characterization of present and future seafloor environmental conditionsAbyssal (water depth 3000–6000 m) and polar habitats can be characterized as cold, slightly alkaline, and well-oxygenated systems (Table; Figure ). They are also quite dynamic environments with environmental conditions (e.g., temperature, POC flux) fluctuating over intra- and interannual timescales. At bathyal depths (200–3000 m) on continental margins, spatial gradients in temperature, pH, O 2 and food supply can be much steeper (Table; Figure ). One of the major differences between abyssal and bathyal regions is in terms of food supply to the seafloor, with abyssal regions being characterized by extreme food limitation over many thousands of kilometers (Table; Figure ). TemperatureCurrently, temperatures at the abyssal seafloor at low to mid latitudes in the Atlantic, Indian and Pacific Oceans range between about 0.01 and 4°C (Table; Figure ) ( ).
Bathyal temperatures typically range from 2 to 8°C (Figure ), though exceptionally warm seafloor temperatures can be found at upper bathyal depths, and in smaller enclosed seas such as the deep Red Sea ( 20°C; ), and bathyal and abyssal Mediterranean Sea (12–14°C). Sub-zero seafloor temperatures tend to occur only at high latitudes (Table; Figure ).Earth-system-model analyses suggest that some abyssal ecosystems are presently warming at rates of 0.01 to 0.1°C decade –1 (e.g., the Southern Ocean; ). Approximately 19% of the ocean’s heat uptake has gone into the deep ocean 2000 m (Talley et al., 2015). Over the next 84 years, the highest temperature changes are likely to occur at the abyssal seafloor in the North Atlantic, Southern and Arctic Oceans ( ) (Table; Figure ). Bathyal depths are also likely to experience increasing temperatures ( ); Mora et al. ( ) modeled increases of 3.6, 4.4 and 3.7°C in the Pacific, Atlantic, and Arctic Oceans (e.g., Barents and Kara Sea), with lower temperature increases in the Indian and Southern Oceans (e.g., at sites of deep-water formation in the Weddell Sea) (Table; Figure ) by 2100.
These predictions of temperature change are in alignment with evidence that the deep Greenland Sea has warmed by 0.7°C since the 1950s ( 0.01°C yr –1; ). Bathyal waters off Antarctica are also warming by 0.005–0.01°C yr –1 (; ).
OxygenationPresently, much of the Atlantic Ocean is well oxygenated (Figure ) relative to the North Indian and Pacific Oceans, where bottom water O 2 concentrations are lower because of the biological removal of O 2 as thermohaline circulation moves deep waters across ocean basins from the North and South Atlantic towards the North Pacific, in isolation from the surface ocean. Warming of the oceans will enhance thermal stratification and density gradients, which will reduce vertical mixing. Combined with a reduction in O 2 solubility in warmer water, increased thermal stratification is predicted to create widespread ocean de-oxygenation (; ), with the greatest effect in intermediate waters (100–1000 m;; ). Already, distinct deep-water masses in the Southern Ocean ( ), eastern North Atlantic (e.g., Sub-polar Mode Water, the Intermediate Water and the Mediterranean Outflow Water; ), and in the West Pacific (North Pacific Subtropical mode water; ) display long-term decreases in O 2 concentration. Bathyal seafloor habitats in the North Pacific, North Atlantic, Arctic and Southern Oceans could experience a reduction in bottom-water oxygenation by 0.03–0.05 mL L –1 by the year 2100 (Table; Figure ), which represents a reduction in water column O 2 levels by 0.7%–3.7% (Table; Figure ). Significant OMZs are presently found along the continental margins of the East Pacific, Southeast Atlantic, West Pacific and North Indian Oceans (; ). Ecosystems within and on the fringes of OMZs could be particularly affected by the O 2 and warming changes predicted for bathyal environments (Table; Figures,;; ).
Though not resolved at the grid resolution shown in Figure, tropical and subtropical bathyal waters between 200 and 700 m, including those bathyal margins of all major eastern boundaries, have already lost considerable amounts of O 2 over the last half-century, and many OMZs have expanded in volume (,;; ). Figure 3Relative environmental changes at the deep seafloor in the year 2100. Relative change (%) in dissolved oxygen (mL L –1) and seafloor POC flux (mg C m –2 d –1) conditions that could be seen at the deep ( 200 m) seafloor by 2100 relative to present-day conditions. DOI:In the abyssal realm, seafloor habitats under areas of deep-water formation (e.g., those in the North Atlantic and Southern Oceans) could experience a maximum decline in O 2 concentration of 0.03 mL L –1 by 2100 (i.e., a 0.5% drop from current levels; Tables,; Figures, ). Greenhouse warming may also exert an effect on abyssal O 2 levels (as well as pH and temperature) by changing thermohaline circulation (; ). Reduced Atlantic Ocean overturning circulation will initially lead to lower O 2 levels at the deep seafloor, and may alter the intensity of Pacific and Indian Ocean OMZs ( ). However, over longer terms, deep-water oxygenation may also increase even if Atlantic meridional overturning circulation becomes weaker, as deep convection in the Weddell Sea and Antarctic Bottom Water becomes enhanced ( ).
PHThe North Atlantic Ocean is currently the most alkaline in terms of seafloor pH, while the pH of the deep North Pacific Ocean is lower (Figure ). This spatial gradient in pH reflects the age and isolation of the water masses, which accumulate CO 2 released by biological respiration as they move through the ocean basins. There is also a contribution of excess atmospheric CO 2 absorption introduced to deep-water masses from dense, cold CO 2-rich surface waters at downwelling sites (e.g., North Atlantic), which then move through the oceans via meridional overturning circulation. Presently, studies in the Pacific Ocean have revealed that intermediate waters down to 500 m depth have experienced a decline in pH of 0.06 units between 1991 and 2006, with the greatest changes occurring around 25°N ( ). Model predictions from the North Atlantic have revealed that over 17% of the seafloor area below 500 m depth will experience pH reductions exceeding 0.2 units by 2100 because of subduction of high-CO 2 waters by thermohaline circulation ( ).
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These major pH reductions are projected to occur over important deep-sea features, such as seamounts and canyons ( ). Bathyal seafloor habitats in other areas of the world’s oceans will also experience significant reductions in pH by the year 2100 (e.g., a decrease of 0.29 to 0.37 pH units) as a result of the entrainment of CO 2-rich seawater to the seafloor at sites of bottom-water formation (Table; Figure ). Since it is estimated to take 1000 years for water entrained at deep-water formation sites to circulate throughout the abyssal ocean basins, vast areas of the abyssal Indian and North Pacific Oceans will experience a lesser shift in pH by 2100 (maximum decrease in pH = 0.06; Table; Figure ).
POC flux or food supplyWith the exception of certain continental margin habitats (e.g., OMZs, some canyons and seamounts) and chemosynthetic ecosystems, most of the deep sea, and particularly the abyss, is characterized by severe food limitation (POC flux of 1–2 g C m –2 yr –1; ) (Table; Figure ). Currently, regions with the highest POC export lie at high latitudes, although transfer efficiency (the proportion of POC exported that arrives at the seafloor) is lowest here compared to lower latitudes. At lower latitudes, extensive mineralization takes place in the upper water column leading to less export from the euphotic zone, but transfer efficiency is higher, as most of the exported carbon tends to be refractory ( ). Enhanced warming of the upper ocean is predicted to enhance stratification, reducing nutrient input to the upper euphotic zone and causing a shift in phytoplankton assemblages from large, fast-sinking diatoms (with low surface area:volume SA:V ratios) to slow-sinking picoplankton (with high SA:V ratios; ). This shift is likely to reduce export flux to the seafloor, as well as transfer efficiency (;,; ). Furthermore, freshening of Arctic regions by sea-ice meltwater and episodic input of large river runoff have been shown to reduce phytoplankton size and, by inference, export flux, a trend that has been projected to continue into the future (, ).
The areas likely to be impacted by significant declines in POC flux owing to enhanced water column stratification lie in the North and South Pacific, North and South Atlantic, and North and South Indian Oceans (Tables,; Figures, ). The abyssal and bathyal regions of the Indian Ocean are predicted to experience declines in POC flux by as much as 40% and 55%, respectively (Tables,; Figures, ) by 2100. For abyssal seafloor habitats, these changes could be severe, as they are some of the most food-limited regions on the planet (;;, ). In contrast, the polar oceans are areas where POC flux is likely to rise.
Vast abyssal and bathyal areas of the Arctic and Southern Oceans are predicted to experience a POC flux increase of upto 60 and 53%, respectively (Tables,; Figures, ) as a result of longer ice-free periods ( ), though accounting for future shifts in phytoplankton size and their nutrient supply could modify this expectation (; ). Localized increases in POC are also predicted for upwelling regions, such as coastal Chile and the west coast of the United States (Figures, ) ( ). These regions have already seen an increase in primary production and frequency of high POC flux to the deep seafloor because of increased wind stress and nutrient upwelling. The continental marginsThe continental margins of the world’s ocean are among the most heterogeneous and diverse of the ocean’s environments ( ).
Hydrographic, topographic and biotic influences create a multitude of seabed habitats and biomass/biodiversity hotspots, and can influence biodiversity–ecosystem functioning relationships ( ). These include broad expanses of organic-rich sediment, low O 2 zones and OMZs, seamounts, banks, ridges, fjords, canyons, basins, coral and sponge reefs, organic falls, and areas of methane seepage. This heterogeneity supports the biodiversity responsible for a whole host of ecosystem functions and services (; ). Because of the shallower depth of continental margin habitats and closer connections with land compared to abyssal habitats, continental margin ecosystems are likely to experience a greater degree of change in all environmental parameters compared to the abyssal seafloor (Tables, ).While few continental margin systems have been investigated directly in terms of the consequences of climate change, their strong gradients and regional variations have allowed significant understanding of the effects of temperature, pH, O 2 and POC flux on deep-sea benthic ecosystems.
Warming of surface waters along continental margins, and increased thermal stratification and reduced nutrient supply to the surface are likely to reduce both productivity and phytoplankton type and size (;, ), yielding reduced phytodetrital flux to the seabed (;; ). Model outputs suggest that bathyal areas particularly prone to declining POC flux lie in the Norwegian and Caribbean Seas, NW and NE Atlantic, the eastern tropical Pacific, and bathyal Indian and Southern Oceans, which could experience as much as a 55% decline in POC flux by 2100 (Tables,; Figures, ). Elevated seafloor temperatures at northerly latitudes (Figure ) will lead to warming boundary currents and may trigger massive release of methane from gas hydrates buried on margins (; ) especially in the Arctic, with simultaneous effects on global climate, aerobic methane oxidation, water column de-oxygenation and ocean acidification (; ).
Along canyon-cut margins (e.g., the western Mediterranean), warming may additionally reduce density-driven cascading events, leading to decreased organic matter transport to the seafloor ( ), though this very process is also likely to reduce physical disturbance at the seafloor. Greenhouse warming will also increase temperature differentials between land and oceans, and intensify wind-driven upwelling in eastern boundary currents, stimulating photosynthetic production at the surface (; Bakun et al., 2015; ). However, this new production will ultimately start to decay as it sinks and increase biogeochemical drawdown of O 2. Upwelling will also bring low-O 2, high-CO 2 water onto the shelf and upper slope (;;;; ). Increased levels of precipitation on land will also alter terrestrial inputs, including sediments and organic debris, nutrients, and contaminants (; ) that may smother seafloor sediments, and alter the trophic ecology of deep-sea habitats situated close to land (, ).Margin habitats are noted for dense, high biomass aggregations of structure-forming species, such as cold-water coral (CWC) reefs and coral ‘garden’ habitats (;;;; ). CWC structures provide shelter from predation for a variety of fauna and, as such, can act as nursery grounds for commercially important species (,; ).
The habitat complexity of these biogenic reefs also leads to high levels of biodiversity on the reefs ( ). By altering internal currents, CWCs can also act as ecosystem engineers boosting organic matter deposition at the seafloor (; ). CWCs and other calcifying taxa (e.g., bivalves and echinoderms) may be susceptible to ocean acidification leading to brittle structures, enhanced susceptibility to predation and a loss of habitat as a result of lowered aragonite and calcite saturation states (Figures, ).To date, studies of aragonitic, scleractinian CWC responses to ocean acidification have frequently examined short-term acclimation, with effects on coral bio-mineralization, growth, and skeletal strength only becoming evident in experiments run for periods of a year or more (e.g., ). Intriguingly, aragonitic CWC species are found close to and even below the aragonite saturation horizon (; ), raising the question of whether species adapted to lower saturation states may have inherent adaptations to future lower pH ocean conditions. However, with many of the known CWC reefs projected to be bathed in under-saturated water by the end of the century (; ) the accumulated biogenic reef structures will degrade over time, even if living corals persist ( ). This degradation could have implications for habitat provision with consequent effects on fish populations and fisheries production.
Likely major impact zones include CWC reefs found in the northern Atlantic and Arctic Oceans, the Southern Ocean, and around New Zealand (; ) where deep-water pH could decrease by approximately 0.3–0.4 pH units by 2100 relative to current day values. Reduced food supply owing to lower POC fluxes could exacerbate these impacts because the metabolic cost of increased rates of calcification become greater as pH declines ( ). Calcareous reef habitats in the northern Atlantic could therefore be especially hard-hit (Figure ).The expansion of low O 2 zones will affect many aspects of deep-sea ecosystem structure and function ( ). Biodiversity declines as O 2 levels decline, which can be manifested in multiple ways. Many species of octocorals (including gorgonians and pennatulaceans) provide habitat for a diverse array of associated invertebrates, but octocorals often decrease in abundance as O 2 levels decline (e.g.,;; ). Reductions in octocoral abundance from de-oxygenation could significantly impact hard substratum availability, habitat complexity and benthic biodiversity (; ). Sediment-burrowing fauna will probably be increasingly excluded as water column O 2 levels decline leading to a reduction in the mixed layer depth, and altered bioturbation rates and C-sequestration in sediments (;;;; Figure ).
This cascade of effects is highly likely at depths of 500–1000 m in parts of the Eastern Pacific, where OMZ expansion is projected to exceed thresholds for biodiversity (; Figures, ). Figure 4Predicted effects of climate change on deep-sea benthic ecosystems.
Concept depictions showing how changes in temperature (A), oxygen (B), pH (C), and POC flux (D) may alter specific ecosystem properties of deep-sea benthic ecosystems. DOI:Intolerant pelagic, demersal and benthic fish and invertebrate species that are mobile will experience habitat compression into shallower depths (;; ), or adapt by migrating horizontally along continental slopes into higher-O 2 environments.
Hypoxia expansion over methane seeps may inhibit oxidizing symbionts that support dense mussel and tubeworm aggregations. These chemosynthetic aggregations typically enhance production and biodiversity on margins (; ), and can provide critical nursery habitats ( ). In contrast, hypoxia-tolerant taxa (e.g., squid and jellyfish) may expand their population sizes and distributions with consequences for food-web structure and pelagic-benthic coupling (;; ).Single stressors like warming will limit tolerance windows for other stressors such as low O 2 or low pH (; ). Reductions in food supply and warming together with expansion of low O 2 and pH zones, will increase the vulnerability of key habitats (e.g., CWC reefs) to anthropogenic disturbance (e.g., benthic trawling) and retard recovery of these fragile habitats from physical damage. Heavily-fished areas off the northern coast of Norway, which are also home to abundant CWC reefs ( ), could be especially sensitive, as they are predicted to experience an increase in temperature of 2–3°C and pH changes of –0.3 to –0.35 units, while also being subject to declining O 2 (0.03 mL L –1: Figures, ).Benthic organisms inhabiting sediments along continental margins are responsible for most nitrogen cycling, while over 50% of carbon burial in the ocean occurs in continental margin sediments.
In eastern current boundary systems (e.g., off the coast of Namibia), where O 2 is already at sub-oxic levels, these regulating services (e.g., nitrogen removal, carbon sequestration) are currently, and will continue to be highly sensitive to small changes in oxygenation ( ). For example, expansion of low O 2 waters could easily shift carbon-processing pathways by favoring chemosynthesis and by increasing the relative importance of bacteria, protozoa (e.g., foraminifera) and metazoan meiofauna in biogeochemical cycling relative to larger taxa (;;; ), which would impact energy flow to upper trophic levels (Figure ). The abyssal zoneMajor changes in the upper ocean resulting from global warming are likely to include increased sea-surface temperatures and thermal stratification, and reduced nutrient upwelling over vast areas of the open ocean (;;; ). Ocean acidification is also predicted to reduce microbial production of nitrate from ammonium ( ), which could have major consequences for oceanic primary production because a significant fraction of the nitrate used by phytoplankton is generated by nitrification at the ocean surface ( ).
Major consequences of such changes over regional scales will probably include (1) reductions in primary production combined with (2) shifts from diatom-dominated (low SA:V ratio) phytoplankton assemblages with high POC-export efficiencies to picoplankton communities (high SA:V ratio) characterized by low export efficiencies (;; ). In addition, reductions in calcification from lowered pH in surface waters could reduce phytoplankton sinking rates through loss of ballast ( ), though this effect will depend on the ratio of the fraction of ballasted vs. Un-ballasted fractions of the sinking POC. Our model outputs suggest that seafloor POC flux will decline in most oceanic areas with exceptions off Peru, the northern coast of Chile, and the Southern and Arctic Oceans (Tables,; Figures, ). The continued reduction in the extent of sea ice in the Arctic is expected to lead to increased photosynthetic primary production and POC flux there ( ), which could benefit fauna whose energetic demands increase as a result of ocean acidification (e.g., calcifying taxa). Reductions in seafloor POC flux will be most drastic, on a percentage basis, in the oceanic gyres and equatorial upwelling zones, with the northern and southern Pacific Ocean and southern Indian Ocean gyres experiencing as much as a 32–40% decline in POC flux (Tables,; Figures, ). Recent studies have suggested that the NE Atlantic Ocean could also undergo similar reductions in POC flux ( ).
Because the quantity and quality of POC flux is an important ecological forcing factor in the abyss, abyssal ecosystems will be highly sensitive to such changes (,; ). For example, 3-fold reductions in POC flux (e.g., from 1.5 to 0.5 g C m –2 yr –1), which might occur in the equatorial Pacific ( ), are predicted to halve benthic microbial and nematode biomass (Figure ). These POC flux changes could also lead to a 5-fold decline in macrofaunal biomass, and cause dramatic reductions in the sediment mixed-layer depth, benthic respiration, and bioturbation intensity (;; Figure ).
Such a decrease in POC flux would also mean a decline in the diversity of nematodes and macrofauna, which are thought to be key functional components of abyssal seafloor ecosystems. This decline in diversity is particularly likely as these groups of organisms tend to rely heavily on detrital matter sinking to the seafloor for their energy requirements (;; ).Holothurians are often the prominent abyssal benthic megafauna ( ) and play an important role in organic carbon processing and bioturbation. Holothurian population dynamics have been linked to POC flux ( ) and are considered indicators of climate change impacts on abyssal ecosystems ( ). Holothurian community dynamics have been examined in detail at long-term time-series sites in relation to shifts in surface phytodetritus input linked to the North Atlantic Oscillation (Porcupine Abyssal Plain, NE Atlantic; ) and to the Northern Oscillation Index and Bakun Upwelling Index (Station M, NE Pacific; ). At both sites, variation in these climate indices were correlated with increased pulses of POC, which resulted in significant increases in abundance of holothurian species, particularly those species able to rapidly use phytodetrital material and successfully reproduce and recruit (;,; ).
These studies confirm the predictions of basic macro-ecology and spatial-gradient studies that climate change fluctuations can cause temporal changes in food inputs leading to changes in overall macro- and megafaunal biomass and community structure in terms of size distributions and dominance (Ruhl et al., 2008, ). Thus, it is a reasonable expectation that macro- and megafaunal communities will shift in relation to future climatically linked changes in POC flux to the abyssal seafloor (Figure ). Episodic pulses provide food supply to sustain benthic communities over periods of deficit (, ); the predicted reduction in POC input over large abyssal areas will likely increase these deficits with a significant impact on faunal communities and their role in ecosystem functioning (Figure ).POC flux to the seafloor, and its degree of seasonality, also strongly influence benthic foraminiferal abundance, diversity and assemblage composition (;;; ). There are well-established decreases in the relative abundance of calcareous taxa (as well as calcite saturation) compared to agglutinated taxa, and in labile organic matter flux, with increasing water depth.
Thus, one consequence of reduced POC flux may be enhanced abundances of agglutinated relative to calcareous foraminifera (; ). Shoaling of the carbonate compensation depth (CCD) over time as a result of increased CO 2 levels would also have important consequences for the diversity of abyssal benthic foraminifera, especially in regions where the depth of the seafloor lies close to the present depth of the CCD (e.g., the eastern Clarion Clipperton Zone, Pacific Ocean). This is an area where foraminifera are overwhelmingly dominated by agglutinated taxa already (; ). A shift from calcareous to agglutinated foraminifera would likely impact deep-sea function (e.g., deep-sea carbon cycling) in addition to altering the biogeographical distribution of fauna ( ).
Agglutinated foraminifera, particularly forms such as komokiaceans, which are a dominant faunal component in the abyssal deep sea ( ), are believed to have a lower metabolic rate and to be less active in carbon processing than calcareous foraminifera ( ).All of these changes predicted for the abyssal zone are likely to fundamentally alter the structure of abyssal ecosystems, as well as the functions that they provide. Changes to microbial and faunal biomass, as well as shifts in biodiversity resulting from changes in POC flux (Figure ), and the complex interactions among benthic organisms, have the potential to feed back over long timescales to a range of intertwined functions, such as carbon cycling, which is highly dependent on benthic biomass and diversity ( ).
The polar deep seasThe Arctic Ocean and Antarctic coastal seas have particular characteristics in common, notably seasonality in solar radiation, sea-ice cover and temperature, that can modulate surface primary production. Despite these commonalities, the oceanographic and physiographic settings and the geological histories of the Arctic and Antarctic regions are very different. There are also major differences in their faunal characteristics (; ), as well as contrasting macro-ecological patterns (; ). For example, the Arctic seafloor has many more surface-burrowing species, such as echiurans, polychaetes, echinoderms and crustaceans, relative to the Antarctic ( ), but deep-sea diversity is generally much lower in the Arctic ( ).
This difference is thought to reflect more glacial disturbance and insufficient time for recolonization in the Arctic, as well as fluctuations in surface productivity and reduced circulation during glacial episodes ( ).The Arctic and many areas of the Antarctic (e.g., western Antarctic Peninsula or WAP) are predicted to undergo more surface-water warming than other parts of the Earth over the next century, which will affect surface production, sea-ice cover, and hence food availability and quality for deep-sea benthic organisms (see, for Antarctic coastal to deep-sea ecosystems). Parts of the Antarctic Peninsula, including the WAP, are already experiencing the greatest increase in mean annual atmospheric temperature on Earth (;;; ), and temperatures at the seafloor in the Southern Ocean are predicted to rise by as much as 0.7°C at abyssal depths and 1.7°C at bathyal depths by 2100 (Table ). Field and modeling studies have also revealed rapid atmospheric and surface-water warming in the Arctic Ocean during recent decades (; ). Bathyal Arctic waters are following this rapid warming trend ( ), and temperatures at both bathyal and abyssal depths could increase by as much as 0.1–3.7°C relative to present-day temperatures by 2100 (Table; Figure ). A recent study has shown that deep-sea benthic Archaea can be more sensitive to temperature shifts than their bacterial counterparts; changes in deep-water temperature may thus alter the relative importance of Archaea in benthic ecosystem processes at polar latitudes ( ). Warming at polar latitudes will also open up new habitat for invasive species (Figure ). For example, in the Arctic, the commercially important snow crab has extended its range to the north (; ), and warming appears to have led to the appearance of highly predacious, temperature-sensitive king crabs ( Lithodidae) in waters of the WAP ( ).
These invasive top predators can threaten the diversity of epi- and infaunal communities, as well as physically disturb large areas of soft sediment ( ). On the other hand, warming-induced extensions of the ranges of temperate–subpolar benthic species into polar oceans ( ) may increase benthic diversity, both in the short-term ( ) and the long-term (; ), although these invasive species are likely to displace less competitive, endemic species unable to cope with increased temperatures (Figure ).Although open ocean polar regions are likely to experience increased primary productivity and POC flux, the production and transport of organic matter to the seafloor will probably initially decline in deep-sea habitats located close to land. As polar and sub-polar regions become warmer, glacial meltwater and erosion of melting tundra ( ) will enhance water column turbidity in coastal zones (; ), reducing water column light levels and phytoplankton production, which could affect near-shore deep-sea systems.
The increased sedimentation in deep coastal areas, particularly fjords, may also smother or clog the breathing and feeding apparatus of sessile suspension-feeding fauna. Ophiuroids, capitellid polychaetes and other opportunists may be favored by increased sediment inputs. In deep Arctic fjords, high sediment fluxes already create large areas of burial disturbance, which can negatively impact trophic complexity, diversity and productivity of benthic assemblages while also inducing O 2 stress (;; ). In time, continued warming will reduce sediment fluxes into many high-latitude fjords as a result of glacial retreat onto land, potentially increasing benthic productivity and biodiversity ( ).Changing ice regimes will impact glacial and ice-sheet calving, with ramifications for physical disturbance in the deep sea.
Large icebergs can scour the sediment down to 400 m on the Antarctic shelf. This disturbance leads to scale-dependent recolonization of scoured areas and an increased input of dropstones ( ).
These processes will enhance seafloor heterogeneity and create hard substrates for sessile megafauna (;, ). Dropstones also create diverse microhabitats for meiofauna, allowing for greater trophic and functional diversity around stones (; ). In the longer term, iceberg scouring and dropstone deposition will tend to elevate diversity on regional scales through (re)colonization processes, although the immediate effect of scouring will be local elimination of many species (; ). In addition, recent evidence suggests that iceberg production followed by melting might significantly elevate local nutrient levels, driving greater primary production and POC flux to the seafloor in deeper waters ( ), though this will probably decline after initial increases under continued warming. Thus, the immediate direct impacts on seafloor communities will be relatively short-lived, but the wider effects may be longer lasting. Finally, melting of icebergs and glacial ice may lead to freshening of surface waters leading to enhanced stratification of the upper water column and the release of essential nutrients and trace metals such as iron ( ).
Together with decreased sea-ice cover these factors may act to increase primary production and POC flux. However, increased respiration (owing to increased temperatures) might result in local hypoxia, especially in isolated intra-shelf basins and fjords such as those found along the WAP.Changes in the quantity and the quality of POC flux to the seafloor will have impacts on ecosystem structure and function (Figure ). Present-day reductions in sea-ice and ice-shelf cover ( ) are leading to changes in upper-ocean pelagic dynamics (e.g., increasing surface primary production, and generating shifts from krill to salps;;,; ). Under high O 2 conditions at shallow depths, metazoans tend to outcompete bacteria in terms of organic matter processing when carbon input to the seafloor increases (; ). If the same holds true for the deep seafloor, then elevated POC fluxes caused by reduced sea-ice cover could trigger a switch from dominance of benthic organic matter processing by bacteria to dominance by metazoans with consequences for energy flow to upper-trophic levels (Figure ). Increases in the abundance, biomass and diversity of benthic communities, the depth of bioturbation, the prevalence of large, habitat-forming taxa (sponges, benthic cnidarians), and the extension of species ranges downslope into deeper water ( ) are other likely consequences of enhanced POC flux (Figure ).
However, minor increases in temperature will also increase overall metabolic rates of benthic and pelagic communities. In Antarctic regions, one of the key processes that is thought to govern the deep-water soft sediment communities is captured in the FOODBANCS hypothesis ( ), according to which concentrated summer food pulses and slow microbial enzymatic activity caused by the cold temperatures provide a long-lasting food bank that supports benthic metazoan communities ( ). With increasing temperatures, bacterial recycling will be enhanced, potentially leading to a non-linear increase in the overall metabolism of the benthic community and increased food limitation in deeper seas (Figure ). This effect could explain why some groups like meiofauna may prefer to feed on bacterial food sources in polar areas ( ).Another important stressor, ocean acidification (OA), will be enhanced at high latitudes because of the higher capacity of seawater to absorb CO 2 at low temperatures (; Table; Figure ). While some areas will experience increased POC flux resulting from a loss of sea-ice cover that could help alleviate some OA effects ( ), the additional carbon deposited will increase respiratory CO 2 production with impacts on carbonate dissolution.
Carbonate dissolution is an issue of particular concern around the Antarctic continent, where it exerts a strong influence on the distributions of benthic foraminifera and other calcareous fauna. Here, the CCD is currently ‘multi-bathic’, ranging from a few hundred meters in intra-shelf basins to several thousand meters over abyssal plains. The depression of the CCD will increase the energy required for organism calcification ( ), impacting many of the archetypal Antarctic fauna (e.g., molluscs, echinoderms, foraminifera and ostracods).
The exact nature of these effects, however, needs detailed examination because, as with all of these factors, there will be synergies that could either exacerbate or ameliorate the stress induced by another perturbation.Many of the changes outlined above could affect both Arctic and Antarctic seafloor ecosystems, although their intensity will probably vary between the poles and among areas within polar regions that are warming at different rates (e.g., WAP versus the eastern Ross Sea). Moreover, different organism groups (e.g., bacteria versus metazoans), and different life stages of the same species (e.g., larvae versus adults), may respond differently and display different degrees of sensitivity to environmental changes ( ). Funding informationWe thank the Norwegian Research Council for awarding funding to A.K. Sweetman, L.A.
Thurber and C.R. Smith to run the workshop “CLIDEEP – Workshop to explore the impacts of climate change on deep-sea pelagic and benthic ecosystems” (NFR grant No. 216598) at Friday Harbor Laboratories, University of Washington, where the foundations for this paper were laid. Sweetman D.O.B. Danovaro acknowledge funding from the European Union Seventh Framework Programme (FP7/2007–2013) under grant agreement 603418 (MIDAS), and the European Union Horizon 2020 research and innovation programme under grant agreement 689518 (MERCES). Henry and J.M. Roberts acknowledge funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 678760 (ATLAS): this output reflects only the authors’ views and the European Union cannot be held responsible for any use that may be made of the information contained therein.
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