Sediment community responses to marine vs. terrigenous organic matter in a submarine canyon

Abstract

Abstract. The Whittard Canyon is a branching submarine canyon on the Celtic continental margin, which may act as a conduit for sediment and organic matter (OM) transport from the European continental slope to the abyssal sea floor. In situ stable-isotope labelling experiments were conducted in the eastern and western branches of the Whittard Canyon, testing short-term (3-7 days) responses of sediment communities to deposition of nitrogen-rich marine (Thalassiosira weissflogii) and nitrogen-poor terrigenous (Triticum aestivum) phytodetritus. 13 C and 15 N labels were traced into faunal biomass and bulk sediments, and the 13 C label traced into bacterial polar lipid fatty acids (PLFAs). Isotopic labels penetrated to 5 cm sediment depth, with no differences between stations or experimental treatments (substrate or time). Macrofaunal assemblage structure differed between the eastern and western canyon branches. Following deposition of marine phytodetritus, no changes in macrofaunal feeding activity were observed between the eastern and western branches, with little change between 3 and 7 days. Macrofaunal C and N uptake was substantially lower following deposition of terrigenous phytodetritus with feeding activity governed by a strong N demand. Bacterial C uptake was greatest in the western branch of the Whittard Canyon, but feeding activity decreased between 3 and 7 days. Bacterial processing of marine and terrigenous OM were similar to the macrofauna in surficial (0-1 cm) sediments. However, in deeper sediments bacteria utilised greater proportions of terrigenous OM. Bacterial biomass decreased following phytodetritus deposition and was negatively correlated to macrofaunal feeding activity. Consequently, this study suggests that macrofaunal-bacterial interactions influence benthic C cycling in the Whittard Canyon, resulting in differential fates for marine and terrigenous OM.

The Whittard Canyon – A case study of submarine canyon processes

“…However, the ranking of the second and third most abundant taxa varied between branches and were respectively Sipunculida (12.5%) and Bivalvia (8.4 %) in the western branch; Isopoda (16.2 %) and Tanaidacea (7.3 %) in the eastern middle branch; Isopoda (10.7 %) and Bivalvia (8.6 %) in the eastern branch. Hunter et al (2013) also noted a difference in macrofaunal composition between canyon branches at 3500 m. In the eastern branch macrofaunal-sized nematodes (> 50 %) and polychaetes (cirratulids and spionids) contributed most to the assemblage, whereas in the western branch crustaceans (tanaids and harpacticoid copepods) and polychaetes dominated. The differences between the two studies may reflect the sampling gear and sampling processing techniques used.…”

“…(2015b) used a megacorer and a 300 μm sieve, whereas Hunter et al (2013) used ROV push cores and a 250 μm sieve. A megacorer will collect a larger sediment sample, while a larger sieve would retain fewer animals.…”

“…A megacorer will collect a larger sediment sample, while a larger sieve would retain fewer animals. Furthermore, Hunter et al (2013) included nematodes in their macrofaunal analysis, but Gunton et al (2015b) only included macrofauna sensu stricto. Polychaete family assemblage composition also varied throughout the canyon.…”

“…Polychaete family assemblage composition also varied throughout the canyon. Hunter et al (2013) reported that the western branch had a high proportion of Amphinomidae, whereas Cirratulidae and Spionidae contributed most to the assemblage in the eastern branch. Gunton et al (2015b) reported high numbers of Amphinomidae (all Paramphinome jeffreysii) in the western, central and eastern branches.…”

Macrofaunal abundance and community composition at lower bathyal depths in different branches of the Whittard Canyon and on the adjacent slope (3500 m; NE Atlantic)

“…Previous studies have shown that this feature hosts increased foraminiferal (Duros et al, 2011) and nematode (Ingels et al, 2011) abundances compared with the open slope. Hunter et al (2013) reported high macrofaunal abundances inside the canyon. Duineveld et al (2001) reported a significantly higher macro-and mega-faunal biomass inside the canyon compared with the slope, but no significant difference in macrofaunal abundance.…”