Is there a seamount effect on microbial community structure and biomass? The case study of Seine and Sedlo seamounts (northeast Atlantic)

Abstract

Seamounts are considered to be "hotspots" of marine life but, their role in oceans primary productivity is still under discussion. We have studied the microbial community structure and biomass of the epipelagic zone (0-150 m) at two northeast Atlantic seamounts (Seine and Sedlo) and compared those with the surrounding ocean. Results from two cruises to Sedlo and three to Seine are presented. Main results show large temporal and spatial microbial community variability on both seamounts. Both Seine and Sedlo heterotrophic community (abundance and biomass) dominate during winter and summer months, representing 75% (Sedlo, July) to 86% (Seine, November) of the total plankton biomass. In Seine, during springtime the contribution to total plankton biomass is similar (47% autotrophic and 53% heterotrophic). Both seamounts present an autotrophic community structure dominated by small cells (nano and picophytoplankton). It is also during spring that a relatively important contribution (26%) of large cells to total autotrophic biomass is found. In some cases, a "seamount effect" is observed on Seine and Sedlo microbial community structure and biomass. In Seine this is only observed during spring through enhancement of large autotrophic cells at the summit and seamount stations. In Sedlo, and despite the observed low biomasses, some clear peaks of picoplankton at the summit or at stations within the seamount area are also observed during summer. Our results suggest that the dominance of heterotrophs is presumably related to the trapping effect of organic matter by seamounts. Nevertheless, the complex circulation around both seamounts with the presence of different sources of mesoscale variability (e.g. presence of meddies, intrusion of African upwelling water) may have contributed to the different patterns of distribution, abundances and also changes observed in the microbial community.

Figure 1. Monthly-averaged (July 2004) MODIS chlorophyll a (in mg m⁻³) image showing the geographical location of Sedlo (white circle) and Seine (red circle) seamounts.

AzC: Azores Current.

Figure 2. Bathymetry (m) of Sedlo (top) and Seine (down) seamounts showing stations positions.

Stations F and X1 at Sedlo and stations H and I at Seine are considered reference “far field” stations.

Figure 3. Biomass variability between seamounts and far-field stations.

Water-column (0–150 m) integrated values (mg C m⁻²) averaged (±SE) from November 2003 and July 2004. (A) Total autotrophs versus total heterotrophs; (B) Autotrophs: microphytoplankton versus nanophytoplankton and picophytoplankton; (C) Heterotrophs: nanoplankton versus picoplankton.

Figure 4. Vertical abundance distributions of micro and nanophytoplankton at Seine and Sedlo during different sampling periods.

Values correspond to averaged data (±SE) from all stations around each seamount (excluding the far-field stations).

Figure 5. Vertical abundance distributions of eukaryotic and prokaryotic picophytoplankton at Seine and Sedlo during different sampling periods.

Values correspond to averaged data (±SE) from all stations around each seamount (excluding the far-field stations). Vertical average profiles of Chlorophyll a (Chl a; mg m⁻³) are also added in B, C, and D.

Figure 6. Vertical abundance distribution of heterotrophic planktonic organisms during different sampling periods.

Values correspond to averaged data (±SE) from all stations around each seamount (excluding the far-field stations). Vertical average profiles of microplankton proteins (Pt; mg m⁻³) are also presented.

Figure 7. Intra-seamount variability in Seine.

Variability in (A) dinoflagellates biomass (mg C m⁻³) and (B) abundance (cells ml⁻¹) during July 2004. Comparisons are made between the summit station (“A”), a seamount station at the east margin (“C”) and the northeast and southeast far-field stations “H” and “I”, respectively.