Stable C and N isotopic composition of cold-water corals from the Newfoundland and Labrador continental slope: Examination of trophic, depth and spatial effects

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Abstract

With the aim of understanding of the trophic ecology of cold-water corals, this paper explores the tissue δ13C and δ15N values of 11 ‘coral’ species (8 alcyonacean, 1 antipatharian, 1 pennatulacean, 1 scleractinian) collected along the Newfoundland and Labrador continental slope. Isotopic results delimit species along continua of trophic level and food lability. With an isotopic signature similar to macrozooplankton, Paragorgia arborea occupies the lowest trophic level and most likely feeds on fresh phytodetritus. Primnoa resedaeformis occupies a slightly higher trophic level, likely supplementing its diet with microzooplankton. Bathypathes arctica, Pennatulacea and other alcyonaceans (Acanella arbuscula, Acanthogorgia armata, Anthomastus grandiflorus, Duva florida, Keratoisis ornata, Paramuricea sp.) had higher δ13C and δ15N values, suggesting these species feed at higher trophic levels and on a greater proportion of more degraded POM. Flabellum alabastrum had an isotopic signature similar to that of snow crab, indicating a primarily carnivorous diet. Isotopic composition did not vary significantly over a depth gradient of 50–1400 m. Coral δ13C increased slightly (<1‰) from the Hudson Strait to the southern Grand Banks, but δ15N did not. By modulating the availability and quality of suspended foods, substrate likely exerts a primary influence on the feeding habits of cold-water corals.

Introduction

There has been increasing interest in cold-water coral ecosystems over the past decade. Cold-water corals may live for decades to hundreds or even thousands of years (Druffel et al., 1995; Roark et al., 2006; Sherwood et al., 2006) and create structurally complex habitat for invertebrates and fish (Henry and Roberts, 2007; Husebø et al., 2002; Krieger and Wing, 2002; Costello et al., 2005; Edinger et al., 2007). Despite the great deal of recent interest in cold-water corals, relatively little is known about their trophic ecology (Roberts et al., 2006).

Previous research on the trophic ecology of cold-water corals has focused mainly on the scleractinian Lophelia pertusa reefs of the northeast Atlantic. Aquarium and field observations show that L. pertusa may capture live zooplankton up to 2 cm in length (Mortensen et al., 2001; Freiwald et al., 2002). Fatty acid and stable nitrogen isotope signatures suggest cold-water scleractinia may also feed on detrital particulate organic matter (POM; Duineveld et al., 2004, Duineveld et al., 2007; Kiriakoulakis et al., 2005). Suspension feeding cnidarians at the Porcupine Abyssal Plain occupy a wide trophic niche, feeding on resuspended material and swimmers to compensate for seasonal shortages in fresh phytodetritus (Iken et al., 2001). Shallower-water octocorals consume a wide range of prey items, from bacteria to zooplankton and detrital POM in proportion to availability (Fabricius et al., 1995; Ribes et al., 1999; Coma et al., 2001; Orejas et al., 2003; Tsounis et al., 2006).

Here, we use stable isotopes to examine the trophic ecology of cold-water ‘corals’ (Alcyonacea, Pennatulacea, Antipatharia, Scleractinia) collected off Newfoundland and Labrador, in the northwest Atlantic Ocean (Gass and Willison, 2005; Mortensen and Buhl-Mortensen, 2005a; Wareham and Edinger, 2007). We present new data on tissue carbon (δ13C) and nitrogen (δ15N) stable isotopic composition of 11 species collected over a depth range of 50–1400 m. The δ13C and δ15N values of organisms are used to interpret food sources and trophic levels, respectively (Fry, 1988; Hobson et al., 1995; Vander-Zanden and Rasmussen, 2001). This is particularly useful when traditional methods of diet analysis, such as gut contents or incubations (Ribes et al., 1999), are impracticable.

A second objective of this study is to explore geographic patterns in δ13C and δ15N of coral tissues. Isotopic composition of primary consumers provides time-integrated data on processes operating near the base of the food web, such as eutrophication (Heikoop et al., 2000; Ward-Paige et al., 2005; Vander-Zanden et al., 2005) and nutrient dynamics across oceanographic gradients (Dunton et al., 1989; Schell et al., 1998; Sherwood et al., 2005a). A previous study from Newfoundland and Labrador reported higher δ15N in fish and invertebrates inhabiting the inner reaches of the continental shelf than in fish and invertebrates living near the shelf break (Sherwood and Rose, 2005). Here, we assess the strength of similar isotopic gradients along the main axis of the outer Labrador Current, from its origin near the Hudson Strait to the southern Grand Banks.

Section snippets

Sampling

Since 2003, coral specimens caught as trawl by-catch during stock assessment surveys and fisheries operations have been collected and stored at the Northwest Atlantic Fisheries Center in St. John's, Newfoundland (Wareham and Edinger, 2007). The dataset consisted of 169 coral tissue samples, representing 11 species, three regions and a depth range of 47–1433 m (Table 1). The regions represented were Hudson Strait, Labrador Slope, and southern Grand Banks (Fig. 1). The Hudson Strait and Labrador

Results

Within the pooled dataset, the effect of species was highly significant on δ13C (one-way ANOVA; F10,142=10.46, p<0.001), δ13C′ (F10,138=20.18, p<0.001) and δ15N (F10,154=16.81, p<0.001). Fig. 2 shows the distribution of coral species in δ15N vs. δ13C (uncorrected for lipid effects) space compared with previously published values for other components of the Newfoundland and Labrador marine foodweb. As a proxy for resuspended POM, we used sedimentary organic matter (SOM) data from the Labrador

Inter-specific variation in stable isotopic composition

Stable isotope values of δ13C and δ15N varied significantly among species, but showed weak and inconsistent patterns of variation in relation to depth or latitude. We hypothesise that differences in feeding habits account for the strong inter-specific variation, based on the wide range of habitats and colony morphologies, and the need to reduce inter-specific competition in food-limited, deep-sea environments (Iken et al., 2001). The distribution of data in Fig. 2, Fig. 3 provide a convenient

Acknowledgements

We gratefully acknowledge the fisheries observers and the officers and crews of the Canadian Coast Guard Ships Teleost and Wilfred Templeman for providing samples. We also thank Kent Gilkinson for logistical support, Alison Pye for assisting with stable isotope analysis, Graham Sherwood for sharing isotope data on Newfoundland and Labrador fish and invertebrates and for valuable discussions. The manuscript was improved by comments from two anonymous reviewers. This work was supported by the

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