Seamounts and organic matter—Is there an effect? The case of Sedlo and Seine seamounts, Part 2. Composition of suspended particulate organic matter

Abstract

The suspended particulate organic matter (sPOM) around two isolated NE Atlantic seamounts, Seine (33°46′N 14°21′W; summit at ∼170 m) and Sedlo (40°19′N 26°40′W; summit at ∼780 m), 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 (40–90 m), 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 (∼200 m) 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.

Introduction

Seamounts are considered to be “hotspots” of marine life, as there are often large zooplankton and fish stocks associated with them (Dower and Mackas, 1996; Genin, 2004 and references therein). Diversity is high (e.g., de Forges et al., 2000; Worm et al., 2003) and there are thought to be many “endemic” seamount species (de Forges et al., 2000). Less commonly, seamounts or seamount chains may exhibit reduction of prey biomass (Genin et al., 1988) or of primary productivity indicators such as chlorophyll a (Venrick, 1991) when compared to the neighbouring open ocean. Nevertheless, they seem to exert an influence on their immediate environment, and this is often referred as the “seamount effect”. Although it has long been recognised that interaction of certain physical, biological and biogeochemical processes may be largely responsible, the precise mechanisms are poorly understood.

The most common view invokes enhanced primary productivity over seamounts initiated by local upwelling and entrapment of nutrients by Taylor columns (Taylor, 1923; Uda and Ishino, 1958; Hogg, 1973; Huppert, 1975; Huppert and Bryan, 1976; Boehlert and Genin, 1987; Dower et al., 1992; Nycander and Lacasce, 2004) although direct and consistent evidence for this seems difficult to establish (reviewed by Genin, 2004). Temporal variability is not easy to resolve in the timescale of most studies (van Bröckel and Meyerhöfer, 1999; Genin, 2004) and unsuitable sampling approaches (Smith et al., 1989) or larger oceanographic phenomena, such as the presence of oxygen-minimum zones (OMZs), which may ‘mask’ seamount effects (Wishner et al., 1995), make the Taylor column hypothesis difficult to verify.

Increased fluxes of suspended organic material at seamounts due to amplified bottom flows over abrupt topographies (Eriksen, 1982, Eriksen, 1991; Mohn and Beckmann, 2002a, Mohn and Beckmann, 2002b) could also sustain high local densities of benthic organisms, zooplankton and fish (Genin et al., 1986, Genin et al., 1992; Dower and Mackas, 1996; van Bröckel and Meyerhöfer, 1999; Genin, 2004). The large-scale entrapment of water by topographically rectified currents also could help retain larvae around seamounts, further enhancing benthic recruitment (Mullineaux, 1994; Mullineaux and Mills, 1997; Beckmann and Mohn, 2002) and could increase the downward flow of high-quality particulate organic matter (POM) to benthic communities over the centre of the seamount (Brink, 1995; Mullineaux and Mills, 1997). It is quite possible that more than one process can be operative in the vicinity of seamounts and these can be temporally variable (Venrick, 1991; Odate and Furuya, 1998).

As part of a wider study, we investigated the composition and quantity of suspended POM (sPOM) in the waters around the Seine and Sedlo seamounts in the northeastern Atlantic Ocean during four cruises between March 2003 and July 2004. We hypothesised that sPOM is likely to be an important food source for local biology and that its distribution and provenance would reflect any seamount effect. We carried out elemental (C, N), molecular (chlorophyll a and lipids), stable isotopic (δ¹⁵N) and scanning electron microscopic (SEM) analyses on filtered sPOM collected by in situ pumps (SAPS, Challenger Oceanic) at selected depths around the two distinct seamounts over 2 years in different seasons. Our findings show that there is a seamount-driven effect that results in transfer of labile sPOM to deep waters although the observations suggest that this is a transient and intermittent phenomenon.