The emergence of fatty acids—Aquatic insects as vectors along a productivity gradient

Abstract

1. Aquatic and terrestrial ecosystems differ fundamentally in the abundance of longchained polyunsaturated fatty acids (PUFAs), such as eicosapentaenoic acid (EPA; 20:5n-3), which are produced by aquatic algae, but only in low quantities by terrestrial plants. Aquatic insects, such as Chironomidae (non-biting midges) feed on algae during their larval stage, making them rich in EPA and therefore high-quality prey for insectivores after emergence. However, the magnitude of EPA subsidies from aquatic insects may be different among water bodies in response to abiotic (e.g. nutrient load) as well as biotic factors (e.g. food web structure).2. To test the predation effects of crucian carp (Carassius carassius), nutrient concentrations, and Chironomidae community composition on the fatty acid export from aquatic ecosystems, we conducted a 25-day experiment across 20 1,500-L mesocosms covering a total phosphorus (TP) gradient of 20-1,000 µg/L.3. Twice a week, we collected adult emerging Chironomidae and found differences in fatty acid composition in the two most abundant chironomid species emerging from the mesocosms. Two PUFAs, α-linolenic acid and EPA, contributed to most of the variation in Chironomidae fatty acid content across the nutrient gradient.Whereas the proportions of α-linolenic acid were positively correlated to the mesocosm TP concentration, we found a negative correlation for the proportions of Chironomidae EPA and mesocosm TP concentration. However, despite lower biomass-specific EPA content at higher TP, higher biomass of emerging Chironomidae at intermediate TP concentrations resulted in higher total export of PUFAs from water to land. 4. Predation pressure from carp decreased the biomass of emerging Chironomidae on average 8-fold. Chironomidae biomass showed a hump-shaped relationship along the TP-gradient and was strongly influenced by periphyton biomass. 5. Export rates of EPA and fatty acids in general responded in a quadratic manner along the nutrient gradient, reaching a maximum value at a TP of 400 µg/L and decreasing thereafter.

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

“…While the EPA + DHA content of primary producers and consumers did show a clear decrease as suggested by earlier studies (Creed et al, 2018;Taipale et al, 2016), the EPA + DHA areal content at a systemic level did not show corresponding decrease due to massive increase of total biomass; similar patterns have previously been observed in mesocosm studies of benthic communities (Scharnweber et al, 2020). Surprisingly, EPA + DHA areal content in secondary and tertiary consumers indeed increased towards more murky lakes, indicating that total biomass clearly overrode the decline of quality.…”

Short‐term apparent mutualism drives responses of aquatic prey to increasing productivity

“…This may be due to the monopolization of nutrients by periphyton which likely contributed to the absence of a phytoplankton response to nutrient additions (Figure 1a,b). However, the effects of apparent mutualism on the top‐down control on Chironomidae affected the flux of essential nutrients to terrestrial environments via Chironomidae emergence (Scharnweber et al., 2020). Thus, the effects of indirect prey–prey interactions within aquatic systems can also propagate to adjacent terrestrial ecosystems via cross ecosystem fluxes and affect trophic interactions therein.…”

“…The experiment consisted of 20 white‐opaque, high‐density polyethylene, cylindrical enclosures, 1.5 m deep and 1,000–1,200 L approximately, which were attached to a dock in Lake Erken (59°51′N, 18°36′E) a meso‐eutrophic lake in Central Sweden (Scharnweber et al., 2020). We deployed the mesocosms on 4 July 2017.…”

“…Detailed information describing the experimental design is available in Scharnweber et al. (2020). In short, we created two equivalent nutrient gradients with 10 mesocosms each, where we added increasing amounts of inorganic phosphorus (P; KH 2 PO 4 ) and nitrogen (N; NH 4 NO 3 ) following an N: P mass ratio of 11.3:1 (Scharnweber et al., 2020).…”

“…We assessed Chironomidae abundance by quantifying the emergence of adult stages using cone‐shaped emergence traps (mesh size: 2 mm; diameter: 61 cm) built inspired by LeSage and Harrison (1979) (see Scharnweber et al., 2020 for more information). The traps were sampled carefully with the help of a small vacuum pipe twice a week from 15 August to 19 September 2017 coinciding with the typical late‐summer peak of emerging Chironomidae from Lake Erken (Sandberg, 1969).…”