Food Web Structure in the Canadian Sub-Arctic: Implications for Harp Seals

The Arctic Ocean is undergoing unprecedented rates of environmental change, with multiple concurrent stressors, and is warming twice as fast as the global average (Hoegh-Guldberg and Bruno, 2010). This is leading to changes in Arctic communities and food webs, such as the northward expansion of boreal fish species (Fosshein et al. 2015). Overall, Arctic primary production has increased by 30% from 1998-2012 (Arrigo and van Dijken 2015) with regional changes in phytoplankton community size structure and diversity at the base of the food web (Li et al. 2009). These changes, both at the base of the food web and at higher trophic levels, will have implications for food web structure and function, and will ultimately impact higher predators such as seals and polar bears. Understanding how complete food webs are being reshaped over different spatial and temporal scales in response to these stressors is crucial to addressing the impacts of future change on biodiversity and ecosystem services (Eamer et al. 2013).
The Northwest Atlantic harp seal (Pagophilus groenlandicus) is an excellent indicator species of food web functioning (Laidre et al. 2008). They have a wide distribution across the Atlantic and adjacent Arctic, long range migrations and flexible foraging patterns. This population summers in the Canadian Arctic and west Greenland, and migrates each autumn to the Gulf of St. Lawrence, southern Labrador and northern Newfoundland where they give birth on drifting pack ice in Feb/March. Northwest Atlantic harp seals feed on a variety of prey items including forage fish (e.g. capelin, Arctic cod) and invertebrates such as the amphipod Thermisto libellula (Lawson et al. 1998, Hammill and Stenson 2000). Fisheries and Oceans Canada have been monitoring Northwest Atlantic food webs and harp seal populations for many years. Data obtained as part of these monitoring programmes can be used to unravel the effects of environmental change on Arctic ecosystems.
The overarching aims of this PhD are: 1) to examine if there are temporal shifts in Arctic shelf sea food webs in the context of environmental change, and 2) to determine how Northwest Atlantic harp seals may be affected by temporal changes and fluctuating environmental conditions. In order to investigate these topics, this PhD will examine a large database (>5000 data points) of stable carbon and nitrogen isotope values of fish and invertebrates, collated by the Department for Fisheries and Oceans (Canada). Since 2012, samples were collected in spring and autumn from the Newfoundland and Labrador shelves and Flemish cap. During this PhD 15N-amino acids in consumer tissues will be analysed to constrain the base of the food web and determine the trophic position of each species, see Lorrain et al. (2015). The stable isotope data obtained will be examined in conjunction with environmental parameters (e.g. ice cover and phytoplankton biomass) to elucidate the processes driving isotopic signatures in food webs. The coupling of Northwest Atlantic harp seals to the Newfoundland/Labrador Shelf food web, will be assessed using stable isotope and harp seal stomach content data.

References
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