Plankton metabolic balance at two North Atlantic seamounts

We have studied the epipelagic (0–100 m) metabolic balance between gross and net community production (Pg, Pn) and community respiration (Rd) 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 Pg, due to higher Rd, 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, Rd and Chl was evident around the two seamounts. Sedlo showed higher Rd, Chl and Pt during winter, but higher Pn in summer. Seine presented higher Pt, Chl and Pn during spring, but higher Rd in summer. On average, the metabolic balance was heterotrophic (Rd>Pg) during all the sampling periods and at most stations of the two seamounts. Both Sedlo and Seine, showed higher Rd 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.