Deep-sea benthic communities and oxygen fluxes in the Arctic Fram Strait controlled by sea-ice cover and water depth

Abstract

Abstract. Arctic Ocean surface sea-ice conditions are linked with the deep sea benthic oxygen fluxes via a cascade of interdependencies across ecosystem components such as primary production, food supply, activity of the benthic community, and their functions. Additionally, each ecosystem component is influenced by abiotic factors such as light availability, temperature, water depth, and grain size structure. In this study, we investigated the coupling between surface sea-ice conditions and deep-sea benthic remineralization processes through a cascade of interdependencies in the Fram Strait. We measured sea-ice concentrations, a variety of different sediment characteristics, benthic community parameters, and oxygen fluxes at 12 stations of the LTER HAUSGARTEN observatory, Fram Strait, at water depths of 275–2500 m. Our investigations reveal that the Fram Strait is bisected into two long-lasting and stable regions: (i) a permanently and highly sea-ice-covered area and (ii) a seasonally and low sea-ice-covered area. Within the Fram Strait ecosystem, sea-ice concentration and water depth are two independent abiotic factors, controlling the deep-sea benthos. Sea-ice concentration correlated with the available food and water depth with the oxygen flux. In addition, both abiotic factors sea-ice concentration and water depth correlate with the macrofauna biomass. However, at water depths > 1500 m the influence of the surface sea-ice cover is minimal with water depth becoming more dominant. Benthic remineralization across the Fram Strait on average is ∼ 1 mmol C m−2 d−1. Our data indicate that the portion of newly produced carbon that is remineralized by the benthos is 5 % in the seasonally low sea-ice-covered eastern part of Fram Strait but can be 14 % in the permanently high sea-ice-covered western part of Fram Strait. Here, by comparing a permanently sea-ice-covered area with a seasonally sea-ice-covered area, we discuss a potential scenario for the deep-sea benthic ecosystem in the future Arctic Ocean, in which an increased surface primary production may lead to increasing benthic remineralization at water depths < 1500 m.

Molecular Composition of Dissolved Organic Matter in Sediment Porewater of the Arctic Deep-Sea Observatory HAUSGARTEN (Fram Strait)

“…Porewater (0-1, 1-5, and 5-10 cm; n = 124) from oxygenated sediments (Donis et al, 2016;Hoffmann et al, 2018) and overlying bottom water samples (n = 52) were collected at the deep-sea LTER HAUSGARTEN. The LTER HAUSGARTEN is located in the Fram Strait between northern Greenland and the Svalbard archipelago (Figure 1), and is the only deep water connection for the exchange of intermediate and deep water masses between the Arctic Ocean and the North Atlantic (Fahrbach et al, 2001).…”

What Feeds the Benthos in the Arctic Basins? Assembling a Carbon Budget for the Deep Arctic Ocean

“…Standing stocks of benthic bacteria in the top 10 cm of sediments in the Arctic deep-sea are within the range of 0.5-2 g C m −2 (Boetius and Damm, 1998). These bacteria respond to input of organic matter (e.g., algal aggregates or chitin) within days to weeks, by rapidly increasing their activity and oxygen demand as well as by changing the bacterial community composition depending on the organic matter source (Hoffmann et al, 2018). Benthic bacteria may, thereby, consume more phytoplanktonbased detrital carbon than benthic macrofauna in some deep-sea areas (Glud, 2008;Sweetman et al, 2019).…”

“…m −2 (≥1000 m) with a mean density around 500,000 ind. m −2 below 3000 m (Bluhm et al, 2011 and literature cited therein;Soltwedel, 2000;Vanreusel et al, 2000;Schewe, 2001;Soltwedel et al, 2009a,b;Hoffmann et al, 2018). The few meiofauna biomass estimates available report <0.5-22 g C m −2 (nematodes only, Vanreusel et al, 2000) and ∼0.2-0.4 g C m −2 (Pfannkuche and Thiel, 1987).…”

“…The median of lowest and highest values across studies was 1 g C m −2 yr −1 and 10 g C m −2 yr −1 , respectively, and highest values were generally found near Atlantic and Pacific inflow areas. In addition, benthic carbon demand tends to decrease with water depth, and increase with increasing surface production (reviewed in Bourgeois et al, 2017;Hoffmann et al, 2018). We presume that food supply variation must be assumed to be the major driver of the variability in the carbon demand, even though methodological differences cannot be excluded either.…”

Carbon and nitrogen turnover in the Arctic deep sea: in situ benthic community response to diatom and coccolithophorid phytodetritus

“…Sediment median grain size was measured by laser diffraction using a Malvern Mastersizer 2000G, hydro version 5.40. Pigments were extracted with 90 % acetone and measured with a TURNER fluorimeter (Holm- Hansen et al, 1965;Yentsch and Menzel, 1963). TOC and TN were measured with an elemental analyser after acidification of the sediments to remove inorganic carbon in aliquots of approx.…”