Content, composition, and transfer of polyunsaturated fatty acids in an Arctic lake food web

Abstract

Freshwater fish production depends on the production and use of polyunsaturated fatty acids (n-3 and n-6 PUFA) from lower trophic levels. Here, we aimed to identify the main trophic pathways that support PUFA content in different fish species (mean 39.7 mg/g dry weight) used in the subsistence fishery of the Inuit community in Greiner Lake near Cambridge Bay (Nunavut, Canada). We used stable isotope and taxon-specific PUFA stocks, to show that the lake food web was divided into distinctive pelagic and littoral benthic food webs and that different fish species obtained their PUFA from different sources within those food webs. The most concentrated fish in n-3 PUFA was Arctic char (Salvelinus alpinus) that obtained nutritionally valuable PUFA compounds by feeding on pelagic zooplankton rich in the essential fatty acids EPA and DHA and on littoral prey with lower PUFA content. The pelagic consumer, lake whitefish (Coregonus clupeaformis), that fed on mysids and zooplankton was also rich in n-3 PUFA. The least concentrated in n-3 PUFA was lake trout (Salvelinus namaycush) that obtained PUFA from low n-3 PUFA sticklebacks (Pungitius pungitius) and macroinvertebrates and from n-3 PUFA-rich littoral mysids. The benthic PUFA were entirely made of n-6 fatty acids and no n-3 PUFA were detected. We further quantified that from the mean daily phytoplankton production of 319 mg CÁm À2 Ád À1 , 2.9% was assimilated by zooplankton (9.4 mg CÁm À2 Ád À1 ) and thereby made available to pelagic fish. The food webs to which fish belonged were supported by PUFA produced in the pelagic and benthic zones but likely complemented by inputs from the watershed. The description of the main PUFA pathways of the Greiner Lake food webs explains for the first time the trophic interactions and underlying mechanisms responsible for the health of the fish community in a high-Arctic lake.

Sentinel responses of Arctic freshwater systems to climate: linkages, evidence, and a roadmap for future research

“…A gradual loss of benthic algae has been well documented when lakes become more eutrophic (Vadeboncouer et al 2003). Concurrently, food webs that were dominantly benthic driven have been documented to change to dominantly pelagic as a response to increasing trophic status (Korhola et al 2002;Lehnherr 2018) with consequences on macroinvertebrate (Kivilä et al 2019) and fish communities (Lehnherr 2018) that are supported by habitat and resource variability in Arctic lakes (Grosbois et al 2022). Increasing temperature and productivity appear to shift subarctic lake communities toward increasing dominance of cyanobacteria and cyprinid fish, associated with decreasing quality in terms of essential fatty acids content in phytoplankton, zooplankton and deep-water macroinvertebrates (Keva et al 2021).…”

Integrating hydrological connectivity and zooplankton composition in Arctic ponds and lakes

“…The lakes had abundant M. relicta and Gammarus sp. populations (Grosbois et al, 2022) that contribute to the production measures made with the crustacean moulting enzyme chitobiase, as was done here.…”

“…These organisms are capable of cryodormancy and exhibit a vast diversity of traits, ecological strategies, and ways of recycling materials and energy (Litchman et al, 2013). In deeper lakes, zooplankton are a key intermediate trophic level in aquatic food webs and play an important role in the transfer of energy from primary producers to fish (Beaver et al, 2019; Grosbois et al, 2022; Kiorboe, 2008). Perhaps because they lack fish and provide no obvious ecosystem services to humans (Fergus et al, 2017), the food webs of small and hydrologically isolated, but overwhelmingly numerous aquatic ecosystems, have received less scientific attention (but see Hobbie, 1980; Rautio et al, 2011).…”

Freshwater Microbial Eukaryotic Core Communities, Open-Water and Under-Ice Specialists in Southern Victoria Island Lakes (Ekaluktutiak, NU, Canada)

“…Overall, mixotrophic taxa, whether obligate or optional, appeared to be favored, at least during the times of year we were able to sample. The high zooplankton populations in the lakes at both times of year (Grosbois et al, 2022) would also benefit since, at least in North American boreal lakes, zooplankton show a preference for mixotrophic species (Hansson et al, 2019).…”

“…The thousands of lakes of southern Victoria Island, Nunavut, are an integral component of local indigenous communities that depend upon them for valuable ecosystem services, providing potable water, food security, and cultural grounding (White et al, 2007). This cultural and ecological intertwining is reflected in the local name, Ekaluktutiak, for the main hamlet (Cambridge Bay) and the surrounding region of southern Victoria Island, which means good fishing place and refers to the abundance of both lake trout (Salvelinus namaycush) and Arctic char (Salvelinus alpinus) (Grosbois et al, 2022). The aquatic macrofauna in the lakes (zooplankton and emergent insects) that support the higher food webs are in turn largely dependent on small single-celled microbial eukaryotes (phytoplankton and other protists) that live in the lakes and ponds.…”