Marine mammals and the community structure of the Estuary and Gulf of St Lawrence, Canada: evidence from stable isotope analysis

Abstract

The trophic relationships of both the benthic and pelagic communities in the Estuary and Gulf of St Lawrence regions were examined, with a special focus on the trophic position (TP) and relationship(s) among harbour, grey, hooded and harp seals and beluga whales. A multiple stable isotope and multiple tissue approach, used in conjunction with conventional dietary information, suggested that marine mammals occupied the highest trophic positions in the food webs of both communities and that they overlapped with one another to some extent trophically. Harbour seals Phoca vitulina and hooded seals Cystophora cristata occupied the highest TP, grey seals Halichoerus grypus, Gulf harp seals Phoca groenlandica, and male beluga whales Delphinapterus leucas were intermediate, and Estuary harp seals and female beluga whales were at the lowest TP. A general pattern of increasing enrichment of 13 C or 15 N with age was observed in marine mammals (as well as fishes), although yearlings showed a decreased enrichment compared to both younger and older age classes. Sex also influenced δ 15 N values. Males were more 15 N-enriched than females, with the difference between the sexes increasing with age, and being most pronounced in species that are sexually dimorphic with respect to body size. Geographical location also influenced isotope abundance. Estuary organisms were generally 13 C-enriched relative to Gulf animals. δ 13 C values were on average lower in short-term diet integrators (blood serum) than in longer-term diet integrators (red blood cells) of harbour seals captured in April to June in the Estuary, which suggests that they probably did not move outside the Lower Estuary during the winter. Grey seals captured in the Lower Estuary did, however, show evidence of having been in the Gulf region some weeks or months before capture.

Stable isotopes, beaks and predators: a new tool to study the trophic ecology of cephalopods, including giant and colossal squids

“…This is more than previously found in communities of closely related marine organisms (Hobson et al 1994;Lesage et al 2001;Nyssen et al 2002), thus suggesting a greater diversity in the feeding habits of coexisting cephalopods. The structure of the community showed both a trophic continuum and distinct trophic levels (figure 4).…”

Sources of organic matter for intertidal consumers on Ascophyllum-shores (SW Iceland): a multi-stable isotope approach

“…This indicates that the same CO 2 reservoir is exploited by macroalgae regardless of location, and that rhythms of emersion due to tides were similar (sensu Cooper and McRoy 1988;Criss 1999). The isotopic values of the present study concur with ranges given in the literature (Lesage et al 2001;Adin and Riera 2003). Therefore, even though the primary production (Ascophyllum-based) at the three Icelandic sites was the same, local diVerences in food web structure arose from varying origins of available POM and SOM.…”

Commensal vs. parasitic relationship between Carapini fish and their hosts: some further insight through δ13C and δ15N measurements

“…Strikingly, y 13 C values displayed by C. homei and C. boraborensis adults (from À 15.8x to À 18x) reflect a diet including prey depleted in 13 C, such as pelagic prey or organisms just arrived in the lagoon. Indeed, stable carbon isotope ratios have proven most useful in identifying where particular organisms feed, as y 13 C values are typically higher in species from coastal or benthic food webs than in offshore food webs (Hobson and Clark, 1992;Lesage et al, 2001). In a similar coral reef ecosystem, annelids and gastropods were similarly 13 C-enriched compared to the POM ( À 18.6x) and filter feeders ( À 12x) such as the mollusc Tricadna sp.…”