Climate warming restructures an aquatic food web over 28 years

• Published in: Global change biology Vol. 26; no. 12; pp. 6852 - 6866
• Main Authors: Tanentzap, Andrew J., Morabito, Giuseppe, Volta, Pietro, Rogora, Michela, Yan, Norman D., Manca, Marina
• Format: Journal Article
• Language: English
• Published: England Blackwell Publishing Ltd 01.12.2020
• Subjects: Abundance; Animals; Biomass; Bythotrephes; causal networks; Climate; Climate change; Daphnia; Fish; Fishing; Fishing effort; Food Chain; Food chains; Food webs; Freshwater crustaceans; Global warming; Interspecific relationships; Italy; Lakes; oligotrophication; Pelagic fisheries; phenology; population dynamics; Population growth; Predation; Predators; Predatory Behavior; Prey; Primary production; Trophic levels; Trophic state; Weekly; Italy; food webs; causal networks; oligotrophication; population dynamics; phenology; climate change
• ISSN: 1354-1013; 1365-2486
• DOI: 10.1111/gcb.15347

Climate warming can restructure lake food webs if trophic levels differ in their thermal responses, but evidence for these changes and their underlying mechanisms remain scarce in nature. Here we document how warming lake temperatures by up to 2°C, rather than changes in trophic state or fishing effort, have restructured the pelagic food web of a large European lake (Lake Maggiore, Italy). Our approach exploited abundance and biomass data collected weekly to yearly across five trophic levels from 1981 to 2008. Temperature generally had stronger effects on taxa than changes in fish predation or trophic state mediated through primary productivity. Consequently, we found that, as the lake warmed, the food web shifted in numerical abundance towards predators occupying middle trophic positions. Of these taxa, the spiny water flea (Bythotrephes longimanus) most prospered. Bythotrephes strongly limited abundances of the keystone grazer Daphnia, strengthening top‐down structuring of the food web. Warmer temperatures partly restructured the food web by advancing peak Bythotrephes densities by approximately 60 days and extending periods of positive population growth by three times. Nonetheless, our results suggested that advances in the timing and size of peak Bythotrephes densities could not outpace changes in the timing and size of peak densities in their Daphnia prey. Our results provide rare evidence from nature as to how long‐term warming can favour higher trophic levels, with the potential to strengthen top‐down control of food webs. We provide rare evidence from nature of how long‐term climate warming restructures food webs and their phenology. By analysing exceptionally long time series in an iconic European lake, we found predators most benefited from warming, partly by peaking in density earlier and experiencing a longer growing season. The effects of warming exceeded those of other global changes, namely in nutrient status and overfishing, and ultimately strengthened top‐down control over the lake food web.