Global Change Reshapes Biosynthesis of LC‐PUFA in Northern Lake Food Webs

Global change is altering the physico-chemical characteristics of northern lakes through warming, shifting nutrient regimes, and increasing dissolved organic carbon (DOC). These changes are reshaping the trophic structure and nutritional quality of freshwater food webs. To investigate how such environmental gradients influence the production and trophic transfer of long-chain polyunsaturated fatty acids (LC-PUFA), we quantified LC-PUFA biosynthesis in 12 boreal lakes along a trophic gradient. Bayesian models were used to relate biosynthesis to phytoplankton biomass and community composition, while compound-specific stable hydrogen isotopes and fads2 gene expression were applied to distinguish dietary acquisition from endogenous n-3 LC-PUFA synthesis in zooplankton and planktivorous fish. We further extrapolated LC-PUFA biosynthesis across 643 boreal and subarctic lakes covering a~7°C temperature gradient and a 1–160 μg L−1 phosphorus range using long-term phytoplankton monitoring data. Phytoplankton LC-PUFA biosynthesis ranged from 1 to 400 ng g C−1 h−1, of which the edible (< 35 μm) fraction for zooplankton contributed only 45%–74%. Stable isotope and gene-expression analysis indicated that biosynthesis rates exceeding ~100 ng g C−1 h−1 in the edible fraction were required to reduce consumer reliance on endogenous bioconversion to below 10%. Across the 643 lakes, LC-PUFA production exhibited a unimodal relationship with phosphorus, peaking at intermediate nutrient levels; however, only 166 lakes produced sufficient LC-PUFA to meet inferred consumer requirements. Nutrient enrichment, warming, and browning acted synergistically to restructure phytoplankton community composition and constrain LC-PUFA availability. Eutrophication and browning promoted LC-PUFA-poor taxa, whereas warming altered biomass distribution and reduced LC-PUFA production within the edible phytoplankton size fraction. Together, these changes intensified a nutritional bottleneck at the base of aquatic food webs. Our findings demonstrate that global change profoundly modifies phytoplankton community structure, LC-PUFA production, and trophic transfer efficiency, which may generate cascading effects on the functioning and productivity of northern freshwater ecosystems.