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Abstract: The suspended particulate organic matter (sPOM) around two isolated NE Atlantic seamounts, Seine (33°46′N 14°21′W; summit at ∼170m) and Sedlo (40°19′N 26°40′W; summit at ∼780m), was studied over a period of 2 years during four 2–4-week oceanographic surveys. Elemental (C and N), chlorophyll a and lipid biomarker concentrations and N stable isotopic values were variable close to the surface (40–90m), although some chlorophyll a enrichment above the summits was discerned sporadically. Results from near-surface waters showed a generally “fresh”, mainly phytoplankton signature in sPOM with some seasonality, which was more pronounced around Sedlo. sPOM concentrations and composition changed with depth, apparently controlled by seasonality and proximity to the seamounts. A few metres above the Seine summit, the suspended particulate organic carbon (sPOC) concentrations and labile polyunsaturated fatty acids (% of lipids) were higher than elsewhere at similar depths (∼200m) in summer 2004. In the same season at Sedlo, polyunsaturated fatty acids were also relatively more abundant (up to 43% of total lipids) around the topographic feature throughout the water column, indicating supply of more labile sPOM, perhaps by advection, downwelling or passive sinking of locally produced phytoplankton and/or in situ production. The high-quality sPOM that seems to be present around the seamounts could provide an important food source to the biological community. [Copyright &y& Elsevier]

Abstract: The temporal and spatial variability in the water-column distribution of dissolved organic carbon (DOC), and the particulate organic matter, collected both with bottles (POM-b) and in situ pumps (POM-p), was studied around Sedlo and Seine, two seamounts located in subtropical waters of the northeast Atlantic Ocean. Seine and Sedlo presented high POM-b concentrations, compared with reported background concentrations from the Azores-Madeira region, although our results show large regional, temporal and intra-seamount variability around the two seamounts. On the contrary, the POM-p pool, which contributed only 25–56% of the POC-b at surface, represented a fairly constant fraction of the suspended POM. Seine was characterized by higher POC-b concentrations than Sedlo, presumably as result of a combined mechanism of advection from the far field, and retention/accumulation of organic carbon, favoured by the secondary circulation around the seamount. Passive accumulation of POM-b was also evident at the southern flank of Sedlo during one the surveys, coinciding with a change in the local circulation around the seamount caused by the intrusion of a Mediterranean water eddy (Meddy). Average PON concentrations at the two seamounts were comparable in magnitude, and hence the C/N ratios were higher in Seine, adding support to the hypothesis of advection of allochtonous highly refractory organic matter into Seine. The contribution of DOC to the apparent oxygen utilization (AOU) in the upper 1000m was significantly higher in Seine (25%) than in Sedlo (11%). This points to a proportionally more important role of dissolved material to the overall carbon remineralization rates in the water column of Seine. [Copyright &y& Elsevier]

Abstract: We have studied the epipelagic (0–100m) metabolic balance between gross and net community production (P g, P n) and community respiration (R d) around two seamounts (Seine: 34°N, 14°W; Sedlo: 40°N, 27°W) located in the subtropical northeast Atlantic. We looked for local effects causing seamounts to increase community production and/or community respiration with respect to the surrounding open ocean. Comparatively, Seine presented similar average living plankton biomass—chlorophyll a (Chl) and particulate proteins (Pt)—but higher P g, due to higher R d, presumably the result of organic matter loading from the NW Africa upwelling system, as supported by field results and satellite imagery. Nevertheless, the large temporal and spatial variability at each seamount make the average differences non-significant. Temporal variability in P, R d and Chl was evident around the two seamounts. Sedlo showed higher R d, Chl and Pt during winter, but higher P n in summer. Seine presented higher Pt, Chl and P n during spring, but higher R d in summer. On average, the metabolic balance was heterotrophic (R d>P g) during all the sampling periods and at most stations of the two seamounts. Both Sedlo and Seine, showed higher R d with respect to average values reported for the global ocean. A clear seamount effect on phytoplankton was only observed in Seine during spring, when Chl and Pt were enhanced at the summit of the seamount. Our results suggest that, rather than increasing primary production significantly, the two seamounts could act as trapping mechanisms for organic matter, favoured by the development of Taylor Columns on the top of the seamounts. Nevertheless these effects seem to be of a lower magnitude than changes caused by temporal or regional variability, questioning the role of these seamounts as hot-spots of productivity in the oceans. [Copyright &y& Elsevier]