Does Presence of a Mid-Ocean Ridge Enhance Biomass and Biodiversity?

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.

Near seafloor bioluminescence, macrozooplankton and macroparticles at the Mid-Atlantic Ridge

“…At the MAR, semidiurnal tidal currents are constrained by the topography, producing a northsouth pulsing of tidal flows at the ridge (Priede et al, 2013a). Turbulence is enhanced when currents are forced to cross ridge topography (St Laurent and Thurnherr, 2007).…”

“…Turbulence is enhanced when currents are forced to cross ridge topography (St Laurent and Thurnherr, 2007). The interaction of the complex topography of the MAR with flow variability drives spatial and temporal heterogeneity within the near seafloor body of water (Priede et al, 2013a). Increased hydrodynamic heterogeneity is likely to augment patchiness in the near seafloor distribution of organic matter and zooplankton.…”

“…The present study was conducted in the near seafloor region of the Mid-Atlantic Ridge (MAR), where sediment planes are punctuated by steep, rocky cliffs and slopes (Niedzielski et al, 2013;Priede et al, 2013a) and focused on bioluminescent zooplankton, macrozooplankton (41 cm) and macroparticles (4430 μm). The hazards of sampling near the seafloor in regions of rough terrain hinder research into the BBL of such areas (Angel, 1990).…”

Trophic interactions of fish communities at midwater depths enhance long-term carbon storage and benthic production on continental slopes

“…The role of demersal (bottom living) organisms in carbon capture is relatively understudied, partly owing to limited knowledge of trophic linkages between the mesopelagic and demersal communities [9,14]. On a global scale, the biomass of benthic organisms generally declines with depth, implying that most energy is transferred to the benthos through the passive flux of settling particles [10,15].…”

Thermal front variability along the North Atlantic Current observed using microwave and infrared satellite data

“…Abundant and diverse marine life requires enhanced nutrient availability and it was hypothesised that the presence of the MAR would increase Fig. 1 mixing, elevate nutrient concentrations and provide the ideal conditions for increased marine productivity and biodiversity (Priede et al, 2013a).…”