Does Presence of a Mid-Ocean Ridge Enhance Biomass and Biodiversity?
In contrast to generally sparse biological communities in open-ocean settings, seamounts and ridges are perceived as areas of elevated productivity and biodiversity capable of supporting commercial fisheries. We investigated the origin of this apparent biological enhancement over a segment of the North Mid-Atlantic Ridge (MAR) using sonar, corers, trawls, traps, and a remotely operated vehicle to survey habitat, biomass, and biodiversity. Satellite remote sensing provided information on flow patterns, thermal fronts, and primary production, while sediment traps measured export flux during 2007–2010. The MAR, 3,704,404 km2 in area, accounts for 44.7% lower bathyal habitat (800–3500 m depth) in the North Atlantic and is dominated by fine soft sediment substrate (95% of area) on a series of flat terraces with intervening slopes either side of the ridge axis contributing to habitat heterogeneity. The MAR fauna comprises mainly species known from continental margins with no evidence of greater biodiversity. Primary production and export flux over the MAR were not enhanced compared with a nearby reference station over the Porcupine Abyssal Plain. Biomasses of benthic macrofauna and megafauna were similar to global averages at the same depths totalling an estimated 258.9 kt C over the entire lower bathyal north MAR. A hypothetical flat plain at 3500 m depth in place of the MAR would contain 85.6 kt C, implying an increase of 173.3 kt C attributable to the presence of the Ridge. This is approximately equal to 167 kt C of estimated pelagic biomass displaced by the volume of the MAR. There is no enhancement of biological productivity over the MAR; oceanic bathypelagic species are replaced by benthic fauna otherwise unable to survive in the mid ocean. We propose that globally sea floor elevation has no effect on deep sea biomass; pelagic plus benthic biomass is constant within a given surface productivity regime.
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Citation: Priede IG, Bergstad OA, Miller PI, Vecchione M, Gebruk A, et al. (2013) Does Presence of a Mid-Ocean Ridge Enhance Biomass and Biodiversity? PLoS ONE 8(5): e61550. doi:10.1371/journal.pone.0061550
Editor: John Murray Roberts, Heriot-Watt University, United Kingdom
Received: December 3, 2012; Accepted: March 11, 2013; Published: May 2, 2013
Copyright: © 2013 Priede et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: The research was funded by the UK Natural Environment Research Council (http://www.nerc.ac.uk/) consortium grant number NE/C512961/1 for a project entitled: ECOMAR; Ecosystem of the Mid Atlantic Ridge at the Sub-Polar Front and Charlie Gibbs Fracture Zone. Funding was also provided by the Census of Marine Life (http://www.coml.org/). Other than selecting the project for funding and approving the resources required, the funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing interests: The authors have declared that no competing interests exist.
The Mid-Atlantic Ridge (MAR) was described a century ago as the most striking feature of the Atlantic Ocean dividing the ocean into eastern and western deep basins . By the 1950s sonar surveys  had revealed the structure of the MAR with a tectonically active central rift valley bounded by elevated flanks on either side, sloping down to the abyssal plains . This forms part of the global mid-ocean ridge system occupying 33% of the total ocean floor that plays a major role in plate tectonics as the site of formation of new earth’s crust . Whilst the geological function of the mid-ocean ridge system is well known, its biological significance remains uncertain. Abundant chemosynthetically-supported life is found around hydrothermal vents that occur along ridge axes . However despite their ubiquity, locally high biomass and productivity , vent fields are small and sparsely distributed  so can only make a minor contribution to mid-ocean biological productivity. Downward export of organic carbon from photosynthesis in surface layers of the ocean is the dominant source of secondary biological productivity over mid-ocean ridge systems.
Mid-ocean shallows such as ridges and seamounts have attracted attention as areas of high fisheries productivity , ,  and biodiversity , . Generally, the most biologically productive regions of the oceans are coastal shallow seas with high incident solar radiation and rapid recycling of nutrients from the sea floor augmented by terrestrial inputs. In the open ocean, nutrient concentrations in surface layers are restricted and a significant fraction of surface primary production is exported downward into the ocean interior, gradually attenuated with depth, supporting deep-sea life throughout the water column and on the abyssal sea floor. Export production can vary with time and is reported to be 50–80% of the primary production during episodic blooms or in high productivity areas, but much lower (5–10%) outside of these periods due to recycling and re-mineralisation of organic matter in the photic zone . Benthic biomass decreases with increasing depth and distance from the continents so that at abyssal depths it is<1% of the values in coastal waters . Thus in mid ocean in the absence of a ridge – relatively low surface productivity would support a sparse abyssal fauna at >4 km depth. The presence of a mid-ocean ridge with a truncated water columns disrupts this general pattern potentially creating regions of high biomass that may arise from topographic influences on water circulation  upwelling nutrient-rich deep water as well as concentrating biomass over summits creating mid ocean regions of high productivity. Sea surface temperature fronts that are typically areas of elevated primary production , may account for enhanced production if associated with ridge topography. Elevation of the sea floor is likely to provide additional habitat for slope-dwelling bathyal fauna. Such species cannot otherwise survive in mid ocean owing to their adaptation to restricted species-specific depth ranges. Biodiversity maxima tend to occur at mid-slope depths around the ocean margins , if this trend were reflected at similar depths in mid ocean this would greatly enhance biodiversity there. Mid-ocean shallows may thus provide stepping stones for trans-oceanic dispersal of bathyal species. Conversely the ridge may act as a barrier to movement of abyssal species between the two halves of the ocean. Finally, there may be sufficient isolation of bathyal fauna on the ridge to allow development of endemic species confined to the ridge system, further enhancing oceanic biodiversity.
There is concern that deep water biogenic habitats such as corals and sponge fields on mid-ocean ridges are vulnerable to damage from fishing and other anthropogenic activity. Despite great scientific uncertainty, high seas Marine Protected Areas (MPAs) have been established over large areas of the MAR  to conserve these habitats. Globally the mid-ocean ridge system is recognised as a large scale ecosystem with extensive areas of lower bathyal habitat defined as depths of 800–3500 m . Although seamounts of similar depths have been proposed to be hotspots of biodiversity and biological productivity these paradigms have been questioned . The present study is concerned with elucidating the potential multiple effects of the presence of a mid-ocean ridge system on oceanic biology.
The present study has been conducted over an extensive segment of northern Mid-Atlantic Ridge (MAR), focussing on a region between the Azores and Iceland (Figure 1A) around the Charlie-Gibbs Fracture Zone (CGFZ). The CGFZ is a major discontinuity in the MAR at 53°N (Figure 1B) coinciding with the location of the sub-polar front (SPF) which delineates the boundary between Subarctic Intermediate Water (9–10°C) at the surface to the north and North Atlantic Central water (15–16°C, summer temperatures) to the south .