Calcium (Ca) is an essential element for almost all living organisms. Here, we examined global variation and controls of freshwater Ca concentrations, using 440 599 water samples from 43 184 inland water sites in 57 countries. We found that the global median Ca concentration was 4.0 mg L-1 with 20.7% of the water samples showing Ca concentrations ≤ 1.5 mg L-1, a threshold considered critical for the survival of many Ca-demanding organisms. Spatially, freshwater Ca concentrations were strongly and proportionally linked to carbonate alkalinity, with the highest Ca and carbonate alkalinity in waters with a pH around 8.0 and decreasing in concentrations towards lower pH. However, on a temporal scale, by analyzing decadal trends in >200 water bodies since the 1980s, we observed a frequent decoupling between carbonate alkalinity and Ca concentrations, which we attributed mainly to the influence of anthropogenic acid deposition. As acid deposition has been ameliorated, in many freshwaters carbonate alkalinity concentrations have increased or remained constant, while Ca concentrations have rapidly declined towards or even below pre-industrial conditions as a consequence of recovery from anthropogenic acidification. Thus, a paradoxical outcome of the successful remediation of acid deposition is a globally widespread freshwater Ca concentration decline towards critically low levels for many aquatic organisms.

Biological Station Lake Neusiedl 7142 Illmitz Austria

Centre for Limnology Institute of Agricultural and Environmental Sciences Estonian University of Life Sciences 51117 Rannu Tartu County Estonia

CNR Water Research Institute L go Tonolli 50 1 28922 Verbania Pallanza Italy

Department of Aquatic Ecology and Environmental Biology Institute for Water and Wetland Research Radboud University 6525AJ Nijmegen The Netherlands

Department of Biological Sciences Rensselaer Polytechnic Institute Troy NY 12180 USA

Department of Biosciences Centre for Biogeochemistry in the Anthropocene University of Oslo Box 1066 Blindern 0316 Norway

Department of Ecology and Genetics Limnology Uppsala University Norbyvägen 18D 752 36 Uppsala Sweden

Department of Limnology and Biological Oceanography University of Vienna Althanstrasse 14 1090 Vienna Austria

Department of Sustainable Agro ecosystems and Bioresources Research and Innovation Centre Fondazione Edmund Mach Via E Mach 1 38010 San Michele all'Adige Italy

Dorset Environmental Science Centre Dorset ON P0A 1E0 Canada

Helmholtz Centre for Environmental Research Department of Lake Research and Department of Aquatic Ecosystem Analysis Magdeburg Germany

IISD Experimental Lakes Area Inc 111 Lombard Avenue Suite 325 Winnipeg R3B 0T5 Canada

INRA CARRTEL 75 bis avenue de Corzent 74203 Thonon les Bains cx France

Institute for Geology Center for Earth System Research and Sustainability University of Hamburg Bundesstraße 55 20146 Hamburg Germany

Institute of Agricultural and Environmental Sciences Estonian University of Life Sciences Kreutzwaldi 5 51014 Tartu Estonia

Institute of Biology University of Latvia Miera Str 3 Salaspils LV 2169 Latvia

Institute of Hydrobiology Biology Centre CAS Na Sádkách 7 370 05 České Budějovice Czech Republic

Irstea RiverLy 5 Rue de la Doua 69625 Villeurbanne cedex France

Kellogg Biological Station and Department of Integrative Biology Michigan State University Hickory Corners MI 49060 and Cary Institute of Ecosystem Studies Millbrook NY 12545 USA

National Institute of Water and Atmospheric Research Hamilton New Zealand

Norwegian Institute for Water Research Gaustadalléen 23 NO 0349 Oslo Norway

Surface Waters Research and Management Eawag Swiss Federal Institute of Aquatic Science and Technology Seestrasse 79 6047 Kastanienbaum Switzerland

U S Environmental Protection Agency Clean Air Markets Division Washington DC 20460 USA

U S Geological Survey New York Water Science Center Troy NY 12180 USA

Vermont Department of Environmental Services 1 National Life Drive Montpelier Vermont USA

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