Article pubs.acs.org/est
Effects of Acidic Deposition on in-Lake Phosphorus Availability: A Lesson from Lakes Recovering from Acidification Jiří Kopácě k,*,†,‡ Josef Hejzlar,† Jiří Kaňa,† Stephen A. Norton,§ and Evžen Stuchlík∥ Institute of Hydrobiology, Biology Centre CAS, Č eské Budějovice 370 05, Czech Republic Faculty of Science, University of South Bohemia, Č eské Budějovice 370 05, Czech Republic § University of Maine, Orono, Maine 04469, United States ∥ Faculty of Science, Charles University, Prague 116 36, Czech Republic † ‡
S Supporting Information *
ABSTRACT: Lake water concentrations of phosphorus (P) recently increased in some mountain areas due to elevated atmospheric input of P rich dust. We show that increasing P concentrations also occur during stable atmospheric P inputs in central European alpine lakes recovering from atmospheric acidification. The elevated P availability in the lakes results from (1) increasing terrestrial export of P accompanying elevated leaching of dissolved organic carbon and decreasing phosphate-adsorption ability of soils due to their increasing pH, and (2) decreasing in-lake P immobilization by aluminum (Al) hydroxide due to decreasing leaching of ionic Al from the recovering soils. The P availability in the recovering lakes is modified by the extent of soil acidification, soil composition, and proportion of till and meadow soils in the catchment. These mechanisms explain several conflicting observations of the acid rain effects on surface water P concentrations.
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experimental acidification of lakes7,8 was commonly based on artificial addition of strong acids directly to waters, whereas atmospheric acidification of the whole catchment-lake systems also affects soils and elevates aluminum (Al) leaching.9 The inlake experiments thus could integrate neither terrestrial processes affecting P leaching, nor in-lake interactions between P and Al cycles and, consequently, led to the implicit conclusion that atmospheric acidification negligibly affects the original in-lake P concentrations. (3) Contrasting observations of acidification effects on terrestrial P export. Roy et al.10 and Reinhardt et al.11 showed increasing terrestrial export during acidic high discharge events whereas Broberg and Person12 and Jansson et al.13 reported decreased P leaching in acidifying areas. Both these seemingly contradictory responses were attributed to P interactions with Al hydroxide in soils. Controversial conclusions also resulted from the observed acidification effects on oligotrophication and eutrophication of lakes. Although Brodin14 considered oligotrophication as a typical feature of acidified lakes, other authors reported both acidification-induced oligotrophication and eutrophication.15,16 (4) Diverse mechanisms explaining Al effects on P availability in acidified lakes. Laboratory experiments predicted the oligotrophication of freshwaters within pH range of 5−6 due
INTRODUCTION Since the 1970s, when atmospheric acidification was recognized as a widespread phenomenon in many European and North American areas, tremendous progress has occurred in the documentation, understanding, and predictions of acid rain effects on element cycling in waters and soils, food web structures, and whole ecosystem functioning.1−3 The effect of atmospheric acidification on the phosphorus (P) cycle, however, remains less well understood, despite P being the crucial limiting nutrient for algal production of freshwater lakes.4 A reduction in P loading to lakes from external or internal sources, as well as its lowered availability to plankton can reduce productivity and cause lake oligotrophication.5 In contrast, elevated P concentrations and availability usually increase productivity of P limited lakes and cause their eutrophication. Major reasons for incomplete understanding of the P cycling in atmospherically acidifying environments are as follows: (1) Lack of background and long-term P concentration data during lake acidification. The P concentrations are typically naturally low (and close to detection limits, 1−2 μg L−1, with commonly used analytical methods) in acid sensitive (and thus naturally oligotrophic or mesotrophic) remote lakes.6 In contrast to numerous surface waters in more densely populated landscapes, remote lakes were not endangered by anthropogenic eutrophication and were not normally monitored for P concentrations prior to the beginning of acidification. (2) Complexity of the acidification process, affecting both terrestrial and aquatic processes. Long-term © XXXX American Chemical Society
Received: December 3, 2014 Revised: February 7, 2015 Accepted: February 8, 2015
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DOI: 10.1021/es5058743 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Article
Environmental Science & Technology to coprecipitation of phosphate with Al.5,17 Empirical data on P cycling, however, suggested that in-lake phosphate adsorption on Al and ferric (Fe) hydroxides (formed by hydrolysis of Al and Fe ions originating either from acidified soils or from photochemical cleaving of organically bound metals) was probably the dominant internal P immobilizing process in surface waters.10,11,18−21 Similar conditions for the disruption of in-lake P cycling by adding Al salts are used in restoration techniques for P inactivation in eutrophic lakes.22−24 The optimum pH range of this P immobilization is ∼5.5−7, similar to removal of dissolved organic carbon (DOC) from drinking water.25 Another Al−P interaction occurs as lake water pH decreases to
