Potential contribution of surface-dwelling Sargassum algae to deep-sea ecosystems in the southern North Atlantic

Abstract

Deep-sea ecosystems, limited by their inability to use primary production as a source of carbon, rely on other sources to maintain life. Sedimentation of organic carbon into the deep sea has been previously studied, however, the high biomass of sedimented Sargassum algae discovered during the VEMA Transit expedition in 2014/2015 to the southern North Atlantic, and its potential as a regular carbon input, has been an underestimated phenomenon. To determine the potential for this carbon flux, a literature survey of previous studies that estimated the abundance of surface water Sargassum was conducted. We compared these estimates with quantitative analyses of sedimented Sargassum appearing on photos taken with an autonomous underwater vehicle (AUV) directly above the abyssal sediment during the expedition. Organismal communities associated to Sargassum fluitans from surface waters were investigated and Sargassum samples collected from surface waters and the deep sea were biochemically analyzed (fatty acids, stable isotopes, C:N ratios) to determine degradation potential and the trophic significance within deep-sea communities. The estimated Sargassum biomass (fresh weight) in the deep sea (0.07-3.75 g/m 2) was several times higher than that estimated from surface waters in the North Atlantic (0.024-0.84 g/m 2). Biochemical analysis showed degradation of Sargassum occurring during sedimentation or in the deep sea, however, fatty acid and stable isotope analysis did not indicate direct trophic interactions between the algae and benthic organisms. Thus, it is assumed that components of the deep-sea microbial food web form an important link between the macroalgae and larger benthic organisms. Evaluation of the epifauna showed a diverse nano-micro-, meio, and macrofauna on surface Sargassum and maybe transported across the Atlantic, but we had no evidence for a vertical exchange of fauna components. The large-scale sedimentation of Sargassum forms an important trophic link between surface and benthic production and has to be further considered in the future as a regular carbon input to the deep-sea floor in the North Atlantic.

Remote Sensing of Sargassum Biomass, Nutrients, and Pigments

“…In fact, connection of Sargassum to the deep-sea communities has been confirmed in field surveys where sinking Sargassum was observed on the ocean floor (Johnson & Richardson, 1977;Rowe & Staresinic, 1979). Considering the enormous blooms in the CS and CWA in recent years, massive carbon sedimentation may have already provided significant carbon input, thus potentially affecting the deep-sea fauna distribution patterns (Baker et al, 2017). Considering the enormous blooms in the CS and CWA in recent years, massive carbon sedimentation may have already provided significant carbon input, thus potentially affecting the deep-sea fauna distribution patterns (Baker et al, 2017).…”

“…These observations suggest that macroalgae may play an important role in carbon transport to the deep-sea fauna (Krause-Jensen & Duarte, 2016). Considering the enormous blooms in the CS and CWA in recent years, massive carbon sedimentation may have already provided significant carbon input, thus potentially affecting the deep-sea fauna distribution patterns (Baker et al, 2017). Additional support for carbon sedimentation comes from sediment core studies near the Deepwater Horizon Wellhead MC252 following the April 2010 oil blowout in the GOM.…”

“…It serves as a critical habitat and refuge to various marine organisms (Council, 2002;Doyle & Franks, 2015;Hu et al, 2016;Lapointe et al, 2014;Rooker et al, 2006;Witherington et al, 2012), and Sargassum sinking can potentially contribute to the carbon input to the deep-sea communities (Baker et al, 2017;Johnson & Richardson, 1977;Krause-Jensen & Duarte, 2016;Rowe & Staresinic, 1979). It serves as a critical habitat and refuge to various marine organisms (Council, 2002;Doyle & Franks, 2015;Hu et al, 2016;Lapointe et al, 2014;Rooker et al, 2006;Witherington et al, 2012), and Sargassum sinking can potentially contribute to the carbon input to the deep-sea communities (Baker et al, 2017;Johnson & Richardson, 1977;Krause-Jensen & Duarte, 2016;Rowe & Staresinic, 1979).…”

“…Pelagic Sargassum is a unique type of brown macroalgae that is mainly found in the Atlantic Ocean. It serves as a critical habitat and refuge to various marine organisms (Council, 2002;Doyle & Franks, 2015;Hu et al, 2016;Lapointe et al, 2014;Rooker et al, 2006;Witherington et al, 2012), and Sargassum sinking can potentially contribute to the carbon input to the deep-sea communities (Baker et al, 2017;Johnson & Richardson, 1977;Krause-Jensen & Duarte, 2016;Rowe & Staresinic, 1979). On the other hand, massive Sargassum beaching events can cause various environmental and economic problems in coastal areas of the Gulf of Mexico (GOM), Caribbean Sea (CS), and West Africa during bloom seasons (Franks et al, 2011;Hu et al, 2016;Schell et al, 2015;Webster & Linton, 2013).…”

“…However, due to a lack of field or laboratory measurements, nearly all remote sensing studies have focused on the areal density or relative amount (Gower et al, 2013;Gower & King, 2011;. On the other hand, knowledge of Sargassum biomass and its pigment compositions and nutrient contents, especially their distributions and temporal changes, is critical in quantifying its roles in biogeochemical cycling and ocean ecology (Baker et al, 2017;Lapointe, 1995;Lapointe et al, 2014;Rooker et al, 2006). Therefore, the objective here is to fill this knowledge gap by (1) developing a model to estimate Sargassum biomass density from reflectance; (2) determining Sargassum nutrient and pigment compositions and concentrations through field and laboratory measurements; and (3) mapping distributions of Sargassum biomass, nutrients, and pigments in the study region.…”