Submission PDF Urban point sources of nutrients were the leading cause for the historical spread of hypoxia across European lakes Jean-Philippe Jenny 1,2,3 , Alexandre Normandeau 4 , Pierre Francus 1,2 , Zofia Ecaterina Taranu 5,6 , Irene Gregory-Eaves 5,7 , François Lapointe 1,2 , Josue Jautzy 8 , Antti E. K. Ojala 9 , Jean-Marcel Dorioz 10 , Arndt Schimmelmann 11 , Bernd Zolitschka 12 1 Centre - Eau Terre Environnement, INRS, G1K 9A9 Québec (Qc), Canada 2 GEOTOP Research Center, Montréal (Qc), H3C 3P8, Canada 3 Max Planck Institute for Biogeochemistry, 10, 07745 Jena, Germany 4 Geological Survey of Canada, Bedford Institute of Oceanography, Dartmouth, NS B2Y 4A2, Canada-Atlantic 5 Department of Biology, McGill University, Montréal (Qc), Canada H3A 1B1 6 Departement des sciences biologiques, Université de Montréal, Montréal (Qc), Canada H2V 2S9 7 Groupe de Recherche Interuniversitaire en Limnology (GRIL), McGill University, Montréal (Qc), Canada H3A 1B1 8 Department of Earth Sciences, University of Ottawa, ON, Ottawa K1N6N5 9 Geological Survey of Finland, P.O. Box 96, 02151 Espoo, Finland 10 INRA UMR 42 CARRTEL, Université de Savoie, 74203 Thonon-les-bains Cedex, France 11 Department of Geological Sciences, Indiana University, Bloomington, IN 47405, USA 12GEOPOLAR, Institute of Geography, University of Bremen, D-28359 Bremen, Germany Submitted to Proceedings of the National Academy of Sciences of the United States of America Enhanced phosphorus export from land into streams and lakes is a primary factor driving the expansion of deep-water hypoxia in lakes during the Anthropocene. However, the interplay of regional scale environmental stressors and the lack of long-term instrumen- tal data often impede analyses attempting to associate changes in land cover with downstream aquatic responses. Herein we performed a synthesis of data that link paleolimnological recon- structions of lake bottom-water oxygenation to changes in land cover/use and climate over the last 300 years in order to evaluate whether the spread of hypoxia in European lakes was primarily associated with enhanced phosphorus exports from either grow- ing urbanization, intensified agriculture or climatic change. We showed that hypoxia started spreading in European lakes around CE 1850 and was greatly accelerated after CE 1900. Socio-economic changes in Europe beginning in CE 1850 resulted in widespread urbanization as well as a larger and more intensively cultivated surface area. However, our analysis of temporal trends demon- strated that the onset and intensification of lacustrine hypoxia were more strongly related to the growth of urban areas than to changes in agricultural areas and the application of fertilizers. These results suggest that anthropogenically-triggered hypoxia in European lakes were primarily caused by enhanced phosphorus discharges from urban point sources. To date, there have been no signs of sustained recovery of bottom water oxygenation in lakes following the enactment of European water legislation in the 1970s to 1980s, and the subsequent decrease in domestic phosphorus consumption. Anthropocene | lake hypoxia | land cover/uses | meta-analysis | varved sediment Introduction Changes in land cover and land use have been identified as impor- tant drivers of phosphorus (P) transfers from terrestrial to aquatic systems, resulting in significant impacts on water resources (1–3). In post-World War II Europe, changes in land cover, land use and P utilization caused widespread eutrophication of freshwaters (3). Elevated rates of P release from point sources to surface water bodies increased in step with population increases, with the novel use of P in domestic detergents and with enhanced connectivity of households to sewage systems that generated concentrated effluents (4). The intensification of agriculture and drastic increased use of fertilizers from industrial and manure sources resulted in elevated P concentrations in runoff from diffuse sources (4). These trends have now metastasized from Europe and North America to most nations, which explains the almost global development of eutrophication problems in surface waters (1). Much of our understanding regarding the interactions among changes in land cover/use, climate and lake eutrophication comes from detailed studies of individual lakes (5), modeling exercises (1), and/or regional-scale syntheses of instrumental data (6,7); these studies are largely based on relatively short time series (8). Depending on the multitudinous local differences in catchment and lake morphology, river transport capacity, climate, geology and regional trajectories in socioeconomic development, the re- sponses of lakes to surrounding land changes can differ greatly in intensity, modalities and kinetics (9–12). Multiple sites need to be investigated in order to quantify a regional trend as well as evaluate local to regional heterogeneities. Only a few studies have interpreted the long-term trajectories of lakes (based on >100- year lake records) in terms of eutrophication on a regional scale by analyzing trends in nutrient and dissolved CO 2 concentrations (13, 14), carbon burial rates (15), cyanobacterial dominance (16) and hypoxia development (17). However, none of these studies considered the temporal dynamics of land cover and use, and only a few studies (16, 17) considered modern land cover. Our current lack of knowledge of the effects arising from cumulative Significance Using a compilation of data arising from over 1,500 European watersheds, we have identified the relative role of different drivers in initiating hypolimnetic hypoxia, a critical indicator of lake health. In particular, our regional synthesis of lami- nated lake sediments indicated a significant acceleration in the spread of lacustrine hypoxia in the 1900s, which occurred well before the general use of commercial fertilizers in the mid-20 th century and the onset of supraregional climate warming in the 1970s. The spread of hypoxia was best explained by urban expansion and the associated intensification of anthropogenic point sources of phosphorus, whereby changes in life style increased the discharge of nutrients from treated and raw sewage, and ultimately led to enhanced lacustrine biological productivity. Reserved for Publication Footnotes 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 www.pnas.org --- --- PNAS Issue Date Volume Issue Number 1--?? 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136