Habitat heterogeneity influences cold‐seep macrofaunal communities within and among seeps along the Norwegian margin – Part 2: contribution of chemosynthesis and nutritional patterns

Abstract

The relative contribution of chemosynthesis in heterotrophic fauna at seeps is known to be influenced by depth and by habitat. Using stable isotopes of carbon and nitrogen, we investigated macro‐ and megafaunal nutritional patterns in Norwegian margin cold seeps by comparing food webs both among habitats within a seep site and between different sites. The very active Håkon Mosby mud volcano (HMMV) is characterized by geochemical gradients, microbial activity and faunal zonation from the centre to the periphery. The Storegga Slide (600–900 m depth) has pockmarks with patchy less active seeps, and also shows concentric zonation of habitats but at much smaller spatial scale. The dominant carbon source for macrofaunal nutrition in both areas was chemosynthetically fixed and the bulk of organic carbon was derived from sulphur‐oxidizing bacteria. In HMMV, food chains were clearly separated according to habitats, with significantly lighter δ13C signatures on microbial mats and adjacent sediment (−33.06 to −50.62‰) than in siboglinid fields (−19.83 to −35.03‰). Mixing model outputs revealed that the contribution of methane‐derived carbon was small in siboglinid fields (0–17%) but significant (39–61%) in the microbial mats. Moreover, the variability of macrofauna signatures within this later habitat suggests the co‐occurrence of two food chains, one based on primary production via methanotrophy and the other via sulphide oxidation. The length of the food chains also varied among habitats, with at least one more trophic level in the siboglinid fields located at the periphery of the volcano. Conversely, in Storrega pockmarks, faunal δ13C signatures did not vary among habitats but among species, although separate food chains seem to co‐occur. The small size of the seepage areas and their lower fluxes compared to HMMV allow more background species to penetrate the seep area, increasing the range of δ15N and the trophic level number. Probably due to the higher flux of photosynthetic particulate organic carbon, the overall chemosynthesis‐based carbon contribution in invertebrate nutrition was lower than that in HMMV.

Methane cold seeps as biological oases in the high‐Arctic deep sea

“…VR3. Tanaid crustaceans are commonly observed in deep‐sea macrobenthos (Bluhm et al ) and have previously been identified at deeper shelf systems around Svalbard (Włodarska‐Kowalczuk et al ; Soltwedel et al ) and cold seep environments along the Norwegian shelf (Blazewicz‐Paszkowycz and Bamber ; Decker and Olu ). Tanaidacea can exploit both chemo‐, and photo‐autotrophic energy sources and have been suggested to feed on microbial mats, (Sellanes et al ; Levin et al ), which offer alternative food resources during periods of low organic matter input.…”

“…We attribute these differences to the methane activity at the cold seep. Four of the top five infaunal taxa at VR seeps (Table ) are recognized from reduced environments including cold seeps (Dubilier et al ; Blazewicz‐Paszkowycz and Bamber ; Decker and Olu ). The most numerically dominant taxa among these is Tanaidacea that occurred en masse at Sta.…”

Methane-Carbon Flow into the Benthic Food Web at Cold Seeps – A Case Study from the Costa Rica Subduction Zone

“…Current knowledge is mostly based on measurements of the stable C isotope ratio of faunal bulk tissue [5], [9], [31], [32]. At cold seeps, both methane and its oxidation product CO 2 , are strongly depleted in 13 C [33].…”

The trophic and metabolic pathways of foraminifera in the Arabian Sea: evidence from cellular stable isotopes

“…At the time of sample collection, bacterial mats were visible in still images across the Oman margin from depths of 1000 to 3300 m, colonising both single jellyfish and jelly detritus (Billett et al, 2006). Sulfuroxidising bacteria at cold seeps are characterised by depleted δ 15 N signatures (Decker and Olu, 2011;Demopoulos et al, 2010;Levin and Michener, 2002). We do not have isotopic data for bacteria from the Oman margin, but bacteria from the Pakistan margin had δ 15 N values ranging from −0.4 to 11.9 ‰, including Thioploca sp., which had a δ 15 N value of −0.1 ‰.…”