Increasing temperature and productivity change biomass, trophic pyramids and community‐level omega‐3 fatty acid content in subarctic lake food webs

Abstract

Climate change in the Arctic is outpacing the global average and land‐use is intensifying due to exploitation of previously inaccessible or unprofitable natural resources. A comprehensive understanding of how the joint effects of changing climate and productivity modify lake food web structure, biomass, trophic pyramid shape and abundance of physiologically essential biomolecules (omega‐3 fatty acids) in the biotic community is lacking. We conducted a space‐for‐time study in 20 subarctic lakes spanning a climatic (+3.2°C and precipitation: +30%) and chemical (dissolved organic carbon: +10 mg/L, total phosphorus: +45 µg/L and total nitrogen: +1,000 µg/L) gradient to test how temperature and productivity jointly affect the structure, biomass and community fatty acid content (eicosapentaenoic acid [EPA] and docosahexaenoic acid [DHA]) of whole food webs. Increasing temperature and productivity shifted lake communities towards dominance of warmer, murky‐water‐adapted taxa, with a general increase in the biomass of primary producers, and secondary and tertiary consumers, while primary invertebrate consumers did not show equally clear trends. This process altered various trophic pyramid structures towards an hour glass shape in the warmest and most productive lakes. Increasing temperature and productivity had negative fatty acid content trends (mg EPA + DHA/g dry weight) in primary producers and primary consumers, but not in secondary nor tertiary fish consumers. The massive biomass increment of fish led to increasing areal fatty acid content (kg EPA + DHA/ha) towards increasingly warmer, more productive lakes, but there were no significant trends in other trophic levels. Increasing temperature and productivity are shifting subarctic lake communities towards systems characterized by increasing dominance of cyanobacteria and cyprinid fish, although decreasing quality in terms of EPA + DHA content was observed only in phytoplankton, zooplankton and profundal benthos.

Lowered nutritional quality of plankton caused by global environmental changes

“…Hence, there would be a lower EPA and DHA supply from these zooplankton taxa to fish that heavily depend on these PUFA for growth, reproduction, and physiological functions (Ahlgren et al, 2009; Arts & Kohler, 2009; Sargent et al, 2003). However, fish also have a strong trophic reliance on benthic prey in subarctic and boreal lakes (Karlsson et al, 2009; Lau et al, 2017), where the EPA and DHA contents of fish are not necessarily linked to those of zooplankton (Keva et al, 2021). Fish may adapt to nutritional quality changes in zooplankton by shifting to use benthic prey resources (Lau et al, 2017).…”

“…In warmer conditions, the lower PUFA contents in zooplankton could be the result of reduced diet quality and reduced PUFA demands for maintaining cell membrane fluidity, that is, homeoviscous adaptation (Hixson & Arts, 2016; Senar et al, 2019). Warming and/or shorter ice cover duration are expected to enhance phytoplankton biomass in northern lakes (Keva et al, 2021; Weyhenmeyer et al, 2013) and the resultant phytoplankton community may become less or more taxonomically diverse (Lau et al, 2020; Weyhenmeyer et al, 2013), and will potentially increase the abundance of cyanobacteria which are unable to synthesize long‐chain PUFA (Keva et al, 2021; Weyhenmeyer et al, 2013). Warming may also reduce the nutritional quality of individual phytoplankton taxa, such as diatoms and cryptophytes, which are rich in long‐chain PUFA, as their need for homeoviscous adaptation is lower in a warmer environment (Hixson & Arts, 2016).…”

“…Previous studies have implied that nutrient depletion and declines in lake DIN:TP ratio, coupled with browning, will intensify phytoplankton N‐limitation (Isles et al, 2018, 2020), reduce phytoplankton biomass and impact their mineral and biochemical compositions (Bergström et al, 2020; Deininger et al, Faithfull, & Bergström,2017), and affect the trophic support of phytoplankton for pelagic food webs (Deininger, Faithfull, Karlsson, et al, 2017). However, to what extent these changes in phytoplankton propagate to affect the nutritional quality of zooplankton, which are an important food web link to pelagic predators (e.g., fish), remains largely unknown (but see Keva et al, 2021).…”

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

“…The most concentrated apex fish species in total FA and PUFA were Arctic char and lake whitefish because of their dietary dependence on lipid-rich zooplankton, a result which demonstrates the key role of pelagic crustaceans in the transfer of total FA and PUFA. The EPA + DHA content of Arctic char in the high-Arctic Greiner Lake (4.2 µg FA/mg DW) closely compare to predatory fishes (including Arctic char) found in ultraoligotrophic sub-arctic lakes from Northern Finland (5.4 µg FA/mg DW; Keva et al 2021). Lake Greiner Lake whitefish EPA + DHA contents (3.6 µg FA/mg DW) similarly parallel those of European whitefish (Coregonus laveratus) from sub-Arctic northern Finland during reproduction (3.1 µg FA/mg DW) but were lower than annual average (7.8 µg FA/mg DW; Keva et al 2019).…”

Population niche breadth and individual trophic specialisation of fish along a climate-productivity gradient

“…However, our results identify the key importance of prey abundance (rather than prey diversity) in driving individual and population feeding responses to new lentic scenarios driven by environmental change. Thus, biomass production in different trophic levels is likely to be key in understanding future changes in aquatic food web structure, where at high temperature and productivity tend to reduce invertebrate prey availability for fish consumers (Keva et al 2021). It is reasonable to posit that feeding habits of animals, and thus individual trophic specialisation, can be limited either by low prey abundance or high consumer abundance (e.g.…”