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 km(2) 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.

Figure 1. Analysis of lower bathyal area of the Atlantic Ocean.

(A) The North Atlantic basin showing the lower bathyal zones (depths 800–3500 m) on the MAR (red), continental margins (green) and non-contiguous seamounts (orange). Dashed rectangle – area shown in Figure 1B. (B)NE, NW, SE, SW, - MAR stations at 2500 m depth with instrumented sediment trap moorings 2007–2010 and sites of detailed surveys. Swath bathymetry survey tracks. PAP - Porcupine Abyssal Plain PSB - Porcupine Seabight.

Figure 2. Sea floor topography and flow on the flanks of the MAR.

(a) 3D projection from swath bathymetry surveys around the NW MAR station, vertical and horizontal scales the same. (b) Location of the NW mooring (base 2500 m depth) and flow at different heights above the sea floor. Red vectors - long term mean velocity. Green - ellipses of the dominant (M2) tidal constituent. (c) Diagrammatic cross section of a flank of the ridge showing the relationship between flat plains, gentle slopes and steep cliffs. The steep cliffs face towards the ridge axis.

Figure 3. Sea floor images on the MAR.

Optical ROV images of the sea floor in flat (<5°), gentle slope (5–30°) and steep (>30°) terrains on the MAR. Red or green laser spots are 10 cm apart.

Figure 4. Remote sensing imagery of distribution of fronts.

Seasonal oceanic front frequency map indicating the percentage of time a strong front was observed at each location during spring (Mar-May), derived from merged microwave and infrared SST (Sea Surface Temperature) data, 2007–2009. Blue arrows are inferred paths of branches of the North Atlantic Current (NAC) delineating the sub-polar front. Frontal features related to bathymetry are labelled: B - Rockall Bank, C - Iceland-Greenland branch of NAC, H - deeper hollow in sea bed, M - recurring meander, N - near to MAR, PAP – Porcupine Abyssal Plain, PSB – Porcupine Seabight, R – Reykjanes Ridge.

Figure 5. Spatial distribution of area backscatter.

Area backscattering coefficient (m² nautical mile⁻²), from the mesopelagic deep scattering layer. Recordings from multi-ship surveys using calibrated SIMRAD 38 kHz echosounders integrating down to 750 m depth (1000 m after 1999). Composite image of data from 1996–2009 during June–July.

Figure 6. Vertical distribution of bioluminescent organisms.

Abundance of organisms (number.m⁻³) between the MAR and the Porcupine Seabight from profiles at the triangle symbols in Figure 4. Black area is the sea floor.

Figure 7. Macrofauna biomass.

Data from sediment core samples at the four MAR stations (green triangles) compared with locations around the North Atlantic margin (open circles) and the CoML global trend line.

Figure 8. Benthic megafauna biomass.

Data from trawl samples at the NW, NE and SW MAR stations (green triangles) compared with locations around the North Atlantic margin (open circles) and the CoML global trend line.

Figure 9. Demersal fish biomass.

Data from trawl samples at the NW, NE and SW MAR stations (assuming C = 10% wet weight), (Green triangles) compared with locations around the North Atlantic margin (open circles).

Figure 10. Effect of a ridge on mid-ocean biomass.

Comparison of sections of a hypothetical ocean with the MAR truncated at the 3500 m depth horizon (upper panel) with the real ocean with a ridge present (lower panel). The thickness of the red section indicates the benthic biomass that decreases with depth. V_ridge is the volume of water, including pelagic biomass displaced by the ridge.

Figure 11. The relationship between depth and biomass.

Decrease in pelagic biomass (mg C.m⁻³) as a function of depth, back-calculated from the CoML global trend for benthic biomass (sloping line). At any given depth (indicated here by a dashed vertical line), the benthic biomass (mg C.m⁻²) is represented by the dark shaded area to the right and the total pelagic biomass above the sea floor from 800 m depth (mg C.m⁻²) by the paler shading to the left. Pelagic plus benthic biomass is hence constant.

Figure 12. Comparison of estimates of pelagic biomass.

Open circles are data from the BIOTRANS station at 47°N, 20°W east of the MAR . The grey line is based on data from 31°17′N 25°24′ W over the Madeira Plain . The solid black line is the theoretical relationship derived from integrating the CoML global trend for benthic biomass as shown in Figure 12.

Figure 13. Faunal overlap between the MAR and continental slopes.

Sørensen indices of species overlap between MAR and the Northwest (WNA, filled) and Northeast (ENA, open) Atlantic derived from species occurrence data. Numbers in parentheses are the numbers of species of each group found on the MAR.