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Ocean Depths: the Mesopelagic and Implications for Global Warming

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Ocean Depths: the Mesopelagic and Implications for Global Warming Current Biology Dispatches Ocean Depths: The Mesopelagic and Implications for Global Warming Mark J. Costello1,* and Sean Breyer2 1Institute of Marine Science, University of Auckland, Auckland, 1142, New Zealand 2ESRI, Redlands, CA 92373, USA *Correspondence: [email protected] http://dx.doi.org/10.1016/j.cub.2016.11.042 The mesopelagic or ‘twilight zone’ of the oceans occurs too deep for photosynthesis, but is a major part of the world’s carbon cycle. Depth boundaries for the mesopelagic have now been shown on a global scale using the distribution of pelagic animals detected by compiling echo-soundings from ships around the world, and been used to predict the effect of global warming on regional fish production. Depth Zonation Analyses were at 5 m depth intervals to However, it remains to be clearly shown The classical concepts for depth zonation 1,000 m deep, and a spatial resolution of whether the abyssal zone is ecologically [1] in the ocean begin at the seashore 300 km2. distinct from the bathyal. (Table 1). Distinct communities are visible The environment changes less as we The data shown in Figure 1 are global on the rocky seashore, and reflect the go deeper (Figure 1), so we expect the averages, and local exceptions will occur, adaptations of their animals and plants vertical extent of ecological zones to particularly in more enclosed waters such to exposure to air and wave action, increase with depth. While the rocky as the Mediterranean and Black Seas [6]. as well as the effects of grazing and seashore may have distinct habitats only The seabed-resident fauna (benthos) will predation [2]. Below the low-tide mark, tens of centimetres deep, the epipelagic be influenced by these variables but plants characterize both the ‘infralittoral’ and infralittoral are tens of metres and the additionally by the seabed substratum seabed and the open-water ‘epipelagic’ mesopelagic hundreds of metres deep. and currents. Topographic variation zones (Table 1). In these sunlit (photic) Below 1,000 m there is minimal variation includes about 70,000 seamounts — zones, photosynthesis uses nitrate and in environmental parameters: it is dark, subsurface mountains over 1,000 m high produces oxygen, so these indicators of with the temperature at about 2 ± 1oC (as from seabed, some of which include biological activity are lowest and highest, in polar seas), nitrate 32 ± 1 mmol l-1 and chemosynthetic hydrothermal vents — on respectively, in the epipelagic (Figure 1). oxygen 4 mg l-1 (Figure 1). Thus, there an otherwise almost flat (<2% slope) Light penetration depends on the density may be just one ecological depth zone sediment-cloaked seabed [7]. It is thus of sediment and plankton in the water, so below the mesopelagic, a true ‘deep-sea’ possible that the benthos may occupy the actual depth of the photic zone will zone combining what are sometimes distinct areas within the pelagic zones. extend deeper further from land [3]. called bathyal, abyssal, and hadal zones. Deeper still, light in the twilight — or Alternatively, there may be an abyssal Deep Scattering Layers mesopelagic — zone is insufficient for zone below 2,000 m where there is even Marine mapping uses the echoes of photosynthesis, but animals can use less environmental variation (Figure 1). acoustic signals to detect not only seabed this zone for feeding, avoiding predators, and other interactions. Thus, respiration dominates, resulting in low oxygen, and Table 1. Features of the concepts of ecological depth zones in the oceans. such mid-water ‘oxygen-minimum zones’ Depth Environment Pelagos Benthos Ecosystem function (Figure 1) harbor distinct fauna [4]. Below Above low tide Tidal air exposure Epipelagic Photosynthesis the mesopelagic there is no light and less (phytoplankton) food than in surface waters, so biological <200 m Photic zone Infralittoral (seaweeds) activity is low, and oxygen less depleted (Figure 1). However, while conceptually <1,000 m Twilight zone Mesopelagic Circalittoral Respiration high defined, the actual depth boundaries of <1% light High nutrients Lower oxygen these zones have not been defined by unsupervised data analysis; new work ? <2,000 m Aphotic zone Bathypelagic Deep-sea Respiration low <4 oC High nutrients published by Proud et al. [5] in this issue of ? Abyssopelagic Current Biology now provides such data. Deepest Hadopelagic The study represented over half of the The changes in the air and light exposure of the environment with depth are reflected in changes in world ocean. However, it lacked data dominance of plants in both the pelagos (the water-column biota, including phytoplankton) and from major regions, including the mid to benthos (the seabed biota including seaweeds and seagrass). These biota then determine the south Atlantic, the south-east and north ecosystem function, including concentrations of oxygen, carbon dioxide and available nutrients. Pacific, and northern Indian Ocean. The sublittoral is the infra- and circalittoral combined. R36 Current Biology 27, R19–R41, January 9, 2017 ª 2017 Elsevier Ltd. Current Biology Dispatches Temperature (°C) Nitrate (μmol l–1) Oxygen (mg l–1) Changes in ocean temperature vary -4 4 12 20 28 0 102030405004812 geographically, so the authors predicted 0 that fish production will decrease in some -1,000 areas, including the Atlantic, while increasing in others. -2,000 Species-distribution models predict -3,000 Depth (m) that ocean warming will result in most species increasing their geographic -4,000 ranges by moving away from the equator, -5,000 thereby increasing species richness at Current Biology higher latitudes (see for example [10,11]). Proud et al. [5] further predict increased Figure 1. Environmental changes in relation to ocean depth. Depth profiles of sea temperature, nitrate and oxygen based on data from the World Ocean Atlas, which productivity. Together, we can expect can now be explored as Ecological Marine Units [6]. White vertical lines (left to right) are the minimum, climate warming to result in increased mean, and maximum values for 100 m depth intervals across the world ocean. Yellow horizontal biodiversity in terms of species richness lines indicate boundaries between epipelagic and mesopelagic. The dashed line suggests a potential and biomass in the epipelagic and abyssopelagic boundary. mesopelagic zones away from the equator. In contrast, species may suffer topography, but also the presence of fish, layers (SSL) occurred within the top 100 m temperature stress in shallow equatorial crustaceans and other materials in mid- than in the mesopelagic, and lower SSL waters. Recent coral bleaching may be a water [8]. In the ocean, these acoustic between 200–400 m; a region the signal of this stress. Indeed, a dip in signals detect a ‘deep scattering layer’ authors called the ‘migrant zone’. These species richness has been noticed across (DSL) comprised of animals that migrate SSL and DSL provide the first global- a wide range of benthic and pelagic, vertically in the water column. During the scale remote sensing of biological data vertebrate and invertebrate, taxa between night they rise to feed in shallower waters to distinguish the epipelagic and 0o and À15o latitude [12]. and by day they shelter from predators in mesopelagic zones. The mesopelagic is a significant zone deeper waters [4]. Proud et al. [5] provide in terms of ocean volume (Figure 2). As the first global synthesis of DSL data from Significance for Climate Change and it lacks plants, it is a net consumer of around the world. They found the DSL to Fisheries oxygen and producer of carbon dioxide. range from ten to hundreds of metres Although defining the mesopelagic may Whereas carbon dioxide is absorbed from deep, and to extend horizontally for tens largely confirm previous studies, Proud the atmosphere at the ocean surface, and to thousands of kilometres. The DSL was et al. [5] took their findings a step further is then assimilated by plant growth, the generally centered around 525 ± 100 m by considering the effect of climate mesopelagic fauna intercepts about 90% below the surface during the day with a warming on mesopelagic fauna. They of organic carbon before it can sink to secondary DSL around 800 m. Due to predicted remote sensing of higher deep-sea sediments, and then releases vertical migration, both of these DSL biomass accumulation because of carbon dioxide back into the water [4]. would become shallower at night. Thus increases in temperature-driven Indeed, the mesopelagic may contribute the acoustic data define the epipelagic metabolism, growth, and trophic as much as 30% of ocean carbon to be above 200 m, and mesopelagic efficiency through the food-web, dioxide production [4]. However, faecal between 200–1,000 m, confirming assuming sufficient nutrients and food for pellets from mesopelagic fauna may previous site-specific observations [4]. fish. There is empirical support for such increase carbon sedimentation rates. Much higher-density sound-scattering predictions. Field data from off the south- Understanding the biology of the east coast of Australia showed increased mesopelagic is therefore essential for growth of fish with modal depth ranges of predicting global carbon dynamics and Volume (millions km3) the effects of climate change. 010203020–200 m in response to climate warming 0 [9]. However, the growth of fish with Mesopelagic modal depth ranges of 1,100 m and REFERENCES -1,000 1,300 m decreased and correlated with 1. Costello, M.J. (2009). Distinguishing marine -2,000 cooling of deep waters. Thus ocean warming may benefit fisheries production habitat classification concepts for ecological data management. Mar. Ecol. Progr. Ser. 397, Depth (m) -3,000 in the mesopelagic but not in deeper 253–268. waters. -4,000 2. Costello, M.J., and Emblow, C. (2005). A In the Proud et al. study, the depth classification of inshore marine biotopes. In -5,000 differentiation between the epipelagic and The Intertidal Ecosystem: the Value of Ireland’s Current Biology mesopelagic was important because Shores, J.G.
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