Exploring The Suitability Of Ecosystem Metabolomes To Assess Imprints Of Brownification And Nutrient Enrichment On Lakes

Loadings of colored dissolved organic matter (cDOM) and nutrients affect lake ecosystem functioning in opposite ways, rendering assessments of combined effects challenging. We used the "ecosystem metabolome" as a conceptual framework to overcome this problem by characterizing the chemically diverse pool of DOM in lakes. The underlying rationale is that the diversity of dissolved metabolites bears the legacy of allochthonous inputs, autochthonous primary production, and a wealth of organic matter transformations resulting from microbial or photodegradation. Our objective was to assess whether high-resolution mass-spectrometric analyses can unlock that information on DOM origin and transformation pathways as well as environmental drivers imprinting the lake ecosystem metabolome. We performed a large-scale enclosure experiment to assess the influences of brownification and nutrient enrichment on the composition and diversity of DOM, and a complementary bottle incubation to isolate the effect of photodegradation. For validation, we assessed whether the same patterns emerge from published observational data from 109 Swedish lakes. Ultra-high-resolution mass spectrometry distinguished similar to 3000 metabolites in solid-phase extracts of lake water. Network analysis revealed five metabolite clusters that could be related to different source processes based on molecular weight, position in van Krevelen diagrams and assignment to molecular categories (peptides, lipids, etc.). Emergent DOM properties such as molecular diversity provided insights into the processes generating each of the five DOM clusters. Overall, our data suggest that the thousands of molecular formulas comprising ecosystem metabolomes of lakes arise from few major processes and reflect imprints of environmental drivers such as brownification and nutrient enrichment.Plain Language Summary The brownification and eutrophication of lakes are common phenomena driven by colored dissolved organic matter (cDOM) and nutrient loading. These stressors can act simultaneously to affect the pool of DOM, a central entity in lake ecosystems consisting of thousands of organic compounds. We reasoned that the high chemical diversity of DOM can inform about the origins of the compounds, the transformation processes generating them, and the environmental context. We explored this idea by adding nutrients and colored organic compounds to a large experimental facility deployed in a lake. To assess the importance of cDOM photodegradation, we conducted a complementary bottle experiment. A high-resolution analytical technique identified almost 3,000 distinct compounds grouping into only five clusters. We assigned each cluster to a specific origin based on molecular formulas, and assessed the major processes leading to a particular group of compounds. Application of the approach to a published data set on the chemical composition of DOM from 109 lakes in Sweden indicated that our classification was transferable to natural lake ecosystems. Taken together, our results indicate that the diverse pool of dissolved organic compounds in lakes provides valuable information to pinpoint important processes as well as to identify the influence of environmental stressors.