Trophic structure of the macrobenthic community of Hornsund, Spitsbergen, based on the determination of stable carbon and nitrogen isotopic signatures

Abstract

Hornsund is a cold-water fjord in southwestern Spitsbergen, Svalbard Archipelago, with a resident biota that exhibit typical low-temperature Arctic features. Carbon (d 13 C) and nitrogen (d 15 N) isotopic signatures of macrobenthic fauna and its potential food sources were measured in summer 2008 to delineate the trophic structure of the bottom community and to identify its principal carbon sources. The soft-bottom fauna at a water depth of 100 m was found to rely primarily on detritus, which is supplied by sedimentation of suspended organic matter from the water column and horizontal transport of refractory macroalgae from euphotic coastal habitats. Through resuspension by bottom currents, deposited particles also contributed to the diet of benthic filter-feeders. Since benthic organisms were significantly enriched in 13 C compared to epibenthic zooplankton (mainly amphipods and decapods), the stable carbon signature provides a tool to differentiate benthic and pelagic feeding habits. The benthic food web was characterized by a conventional trophic structure with decreasing species numbers in increasing trophic levels. Primary consumers feeding on a mixture of plant matter, fecal pellets, decaying animal tissue, bacteria, and protists accounted for the greatest biomass share (62 % of the total macrobenthic biomass), followed by secondary consumers (38 %). Based on d 15 N signatures, three trophic levels were detected, corresponding to the following feeding guilds: filter-feeders and feeding generalists (mainly bivalves, crustaceans, polychaetes, and some fish), mixed detritivore-carnivores (polychaetes, priapulids, crustaceans, and ophiuroids) and obligate carnivores (ascidians). The average food chain length (4.5 trophic levels) suggests that high-quality food is readily available in this Arctic fjord ecosystem.

Food sources of macrozoobenthos in an Arctic kelp belt: trophic relationships revealed by stable isotope and fatty acid analyses

“…Macroalgae, particularly Rhodophyta, had a very wide range of carbon isotope compositions, and δ 13 C values were also very variable within species. These wide ranges of δ 13 C values observed in Kongsfjorden, which are comparable with those measured in other Arctic ecosystems (Sokołowski et al 2014, Renaud et al 2015, limit a separation of these primary producers solely based on their δ 13 C values. Differences between Phaeophyta and erect Rhodophyta could nevertheless be observed, with erect Rhodophyta much more 13 C depleted (<−35.8 ‰) than Phaeophyta (>−21.0 ‰), as already observed in Isfjorden (Renaud et al 2015).…”

“…Macroalgal δ 15 N values were markedly low compared to those measured in northeastern Greenland (Hobson et al 1995) and the Chukchi Sea (Iken et al 2010). In shallow areas, light atmospheric nitrogen can play a crucial role during the assimilation of dissolved nitrogen by primary producers under nutrient limitation (Sokołowski et al 2014). Low concentrations of nutrients can occur during summer months, as demonstrated in Kongsfjorden (van De Poll et al 2016), and hence might explain the observed relatively low δ 15 N values of the studied macroalgae.…”

“…Nevertheless, the very low amount of macroalgal FAs and the very low 20:4(n-6)/20:5(n-3) ratio of TPOM clearly demonstrate that kelp material poorly contributed to TPOM composition. Instead, the relatively high lev-els of diatom and flagellate markers highlighted that TPOM was mostly made of microalgae probably from benthic and pelagic origin, as already demonstrated by Sokołowski et al (2014) in deep soft-bottom habitats in Hornsund (located in the southern part of Spitsbergen). The similar FA profiles of TPOM and SPOM highlight the importance of the flow of organic matter from the water column to the benthic system in this polar fjord.…”

Trophic structure of shallow-water benthic communities in the sub-Antarctic Strait of Magellan

“…Bryozoans, for instance, are known to capture smaller heterotrophic organisms like microprotozoans (see, e.g. Winston 1978;Sokolowski et al 2014). Moreover, some of these animals may be able to shift from suspension feeding to deposit feeding, conditional of the environment and the availability of suspended POM (Taghon et al 1980).…”

Working on a baseline for the Kongsfjorden food web: production and properties of macroalgal particulate organic matter (POM)

“…At the Western side of the Atlantic in Nova Scotia, extensive research on kelp detritus was conducted by Scheibling (2011, 2012). Macroalgal material was considered a substantial part of the detritus supporting the macrobenthic community of the cold Arctic Hornsund (Spitsbergen) by Sokołowski et al (2014) and Renaud et al (2015) provided ample evidence for the Arctic Isfjorden (Spitsbergen).…”