Abstract
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.
Highlights
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 [1]
Primary production and export flux over the MAR were not enhanced compared with a nearby reference station over the Porcupine Abyssal Plain
This is in contrast to oceanic islands and sea mounts in the Southern Ocean where natural iron fertilisation of surface waters does locally increase primary production, export flux and deep-sea benthic biomass [45,46] Our acoustic surveys (Fig 5) and previous acoustic surveys across the MAR at 43u539N and 56uN show some association of pelagic fauna with local topography [47]
Summary
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 [1]. By the 1950s sonar surveys [2] 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 [3]. 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 [4]. 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
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