Increasing concentration of polyunsaturated fatty acids in browning boreal lakes is driven by nuisance alga Gonyostomum

Abstract

Elevated concentrations of dissolved organic carbon (DOC) promote browning of lakes, which alters the physicochemical properties of water and ecosystem functioning. However, browning‐induced changes in basal production of polyunsaturated fatty acids from the n‐3 and n‐6 families (n‐3 and n‐6 PUFA) in lakes are not fully understood. The concentrations of PUFA, which are micronutrients required to maintain growth and reproduction of consumers, have been suggested to either rise or decline in seston as a response to lake browning. Elevated DOC concentrations may also promote bacterial biomass and production and thus potentially increase the concentration of bacterial fatty acids (BAFA) in seston. We analyzed phytoplankton, bacteria and heterotroph biomasses, the taxonomic composition of phytoplankton, and the concentrations and mass fractions of bioseston PUFA, BAFA, eicosapentaenoic, and docosahexaenoic acids in ten boreal lakes in eastern Finland, with DOC concentration ranging from 2.8 to 18.7 mg/L. Our results showed that the abundance of PUFA in seston depended on the responses of phytoplankton biomass and community composition to lake browning. Lake browning increased seston PUFA and BAFA concentrations (expressed as µg/L) but not the contents (expressed as µg/mg bioseston C). Although low DOC lakes had a favorable phytoplankton community (in terms of PUFA content), the phytoplankton biomass in these oligotrophic lakes was so low that the concentration of PUFA remained low compared to high DOC lakes. The increasing concentration of PUFA in bioseston along the DOC gradient was mainly due to the increasing biomass of nuisance alga Gonyostomum semen. However, Gonyostomum may be too large for small‐sized zooplankton to ingest, and thus, the trophic transfer of PUFA may be impaired. The trajectories for lake browning and the basal production of PUFA also may depend on the source of carbon and associated nutrient loading; DOC and nutrient loading from agricultural areas may promote cyanobacteria dominance and decrease PUFA availability in lakes, while DOC runoff from more acidic and nutrient‐poor peatlands may promote Gonyostomum dominance and increase seston PUFA concentration.

Lake browning impacts community structure and essential fatty acid content of littoral invertebrates in boreal lakes

“…There is evidence that the FA composition of Chironomidae follows that of their diet, but they also actively modify dietary FAs (Strandberg et al, 2020a). They contained a lot of SAFAs, especially the ones sampled from humic lakes.…”

Periphyton as a key diet source of essential fatty acids for macroinvertebrates across a nutrient and dissolved organic carbon gradient in boreal lakes

“…This capacity probably differs among species and may, hypothetically, be well-developed with species such as C. riparius, which are adapted to low-oxygenated and poorly lit environments, where the access to dietary ALA and EPA is limited. Strandberg et al (2020) and Goedkoop et al (2007) also demonstrate remarkably higher growth and viability of C. riparius when algal-based food was available. Similarly, Grieve and Lau (2018) reported higher somatic growth of Asellus aquaticus when they were fed with both leaf litter and algae, and, surprisingly, when leaf litter was predominant.…”

“…This may be the result of a larger sample size and also a narrower confidence interval among chironomids. There is experimental evidence (Goedkoop et al 2007; Strandberg et al 2020) that Chironomus riparius can synthesize ARA and EPA from precursor fatty acids and sustain viable populations in the presence of poor‐quality food. This capacity probably differs among species and may, hypothetically, be well‐developed with species such as C. riparius , which are adapted to low‐oxygenated and poorly lit environments, where the access to dietary ALA and EPA is limited.…”

“…Docosahexaenoic acid (DHA, 22 : 6n‐3), eicosapentaenoic acid (EPA, 20 : 5n‐3), and arachidonic acid (ARA, 20 : 4n‐6) are physiologically required for consumers as constituents of cell membranes (Vance and Vance 1996). Consumers have variable and often limited ability to biosynthesize these PUFA from their precursors, that is, alpha‐linolenic acid (ALA, 18 : 3n‐3) and linoleic acid (LIN, 18 : 2n‐6) (Koussoroplis et al 2014; Murray et al 2014; but see Strandberg et al 2020). Experimental studies reported that terrestrial sources are very low in omega‐3 (n‐3) and omega‐6 (n‐6) PUFA and contain only traces of EPA or DHA (Taipale et al 2014; Hiltunen et al 2019).…”

Seawater browning alters community composition and reduces nutritional quality of plankton in a subarctic marine ecosystem

“…In addition to phytoplankton, heterotrophic flagellates and ciliates may also be important seston components (Strandberg et al 2020), but they were not assessed in our study. Yet, it is unlikely that these organisms alone had provided sufficient amounts of PUFA for zooplankton even in the browner water conditions, due to the inefficient energy transfer via the bacterial trophic pathway (Hiltunen et al 2017).…”