Relative Effects of Eutrophication and Warming on Freshwater Ecosystems Across Ecological Levels

Abstract

Global change strongly alters biodiversity worldwide, but our ability to predict their consequences for ecosystem functioning remains limited. This lack of knowledge may be attributed to the limited empirical evidence of the simultaneous ecological impacts of global change across multiple ecological levels. Here, we conducted a full factorial experiment to measure the isolated and combined effects of two global change drivers to which freshwater ecosystems are severely exposed: (i) nutrient addition, which mainly affects the lower trophic levels and modifies ecosystems from the bottom of the food chain, and (ii) warming, which primarily affects large organisms at the top of the food chain. We quantified the effects of the two global change drivers at four different ecological levels: community composition (zooplankton and benthic invertebrates), size structure, trophic architecture, and ecosystem functioning. We found that the impacts of treatments varied significantly across different ecological levels. Specifically, community composition was predominantly affected by warming, whereas nutrient addition played a more important role than warming in ecosystem functioning (e.g., primary production and atmospheric CO₂ uptake). More importantly, we found that food webs (described using size spectrum and stable isotope structure) represent an integrative ecological level for capturing the effects of the two global change drivers tested, integrating changes in both community structure and ecosystem functioning. These results provide valuable insights into the responses of aquatic ecosystems to global change and reveal the importance of considering multiple ecological levels to improve our understanding of the processes driving the responses of ecosystems to global change.

Keywords: aquatic invertebrates; climate change; eutrophication; food webs; freshwater ecosystems; mesocosms experiment.

FIGURE 1

Mesocosm platform used in this experiment (red line) (a) and treatment design (b) colored by treatments: Control (C) in blue, nutrient (N) in green, warming (W) in red, and nutrient + warming (NW) in yellow. Total phosphorus (c) and nitrogen (d) concentrations in the water column at the end of the experiment and mean daily water temperature during the experiment (e). Lozenges represent the mean values for each treatment, and different letters indicate significant differences (pairwise t‐test, p < 0.05).

FIGURE 2

Effects of treatments at each ecological level. (a) Community composition with the first two non‐metric multidimensional scaling (NMDS) axes of invertebrate consumers (stress = 0.176). Points represent individual mesocosms, colored by treatments, with ellipses indicating the 95% CI for each treatment. (b) Community size spectrum. Points represent abundances per size class in each mesocosm. Solid line represents the mean size spectrum per treatment (95% CI). (c) Trophic architecture. Each point represents the feeding preference of a consumer, defined by its resource origin (autochthonous/allochthonous) and mean trophic position (±SD). Point size corresponds to the mean abundance of each taxon per mesocosm, with ellipses indicating the 95% CI for each treatment. (d) Ecosystem functioning. The two principal component analysis axes in response to treatments. Each point represents a mesocosm, with ellipses indicating the 95% CI for each treatment. Aes, Aeshnidae; Anc, Ancylidae; Ase, Aselidae; Asp, Asplanchnidae; Bae, Baetidae; Bos, Bosminidae; BPP, benthic primary production; Bra, Brachionidae; Cal, Calanoidae; Cap, Capniidae; CH₄ em, CH₄ emission; Cha, Chaoboridae; Chi, Chironomidae; Cln, Cladocera nauplii; CO₂ em, CO₂ emission; Coe, Coenagrionidae; Cop, Copepoda nauplii; Cul, Culicidae; Cyc, Cyclopidae; Dap, Daphniidae; Dec, decomposition rate; DOC, dissolved organic carbon; Elm, Elmidae; Hex, Hexarthridae; Lep, Leptocentridae; Lum, Lumbricidae; Lym, Lymneaidae; Ner, Neretidae; Not, Notonectidae; Ost, Ostracoda; Phy, Physidae; Plaa, Planaridae; Plab, Planorbidae; PPP, pelagic primary production.

FIGURE 3

Effects of treatments on community composition (a–c), size spectrum parameters (d–f), trophic architecture (g–i), and ecosystem functioning (j, k). Colors represent different treatments with control (C) in blue, nutrient (N) in green, warming (W) in red, and nutrient + warming (NW) in yellow. For standard ellipse area (i), dots represent the estimated size, and boxes represent the 50%, 75%, and 95% confidence intervals. In other cases, lozenges represent the mean value per treatment, and significant differences (p < 0.05) are given by letters.

FIGURE 4

Treatment effects across ecological levels. Total effects represent the weighted mean effect sizes for each ecological level. Significant effect sizes (no overlap of confidence interval with 0) are displayed in bold. FCL, food chain length; SEA, standard ellipse area; SS, size spectrum.