LETTER Atmospheric nitrogen deposition is associated with elevated phosphorus limitation of lake zooplankton James J. Elser, 1 * Angela L. Peace, 1 Marcia Kyle, 1 Marcin Wojewodzic, 2 Michelle L. McCrackin, 1 Tom Andersen 2 and Dag O. Hessen 2 1 School of Life Sciences, Arizona State University, Tempe, AZ 85287, USA 2 Department of Biology, University of Oslo, PO Box 1027, Blindern, Oslo N-0316, Norway *Correspondence: E-mail: j.elser@asu.edu Abstract Here, we present data that for the first time suggests that the effects of atmospheric nitrogen (N) deposition on nutrient limitation extend into the food web. We used a novel and sensitive assay for an enzyme that is over-expressed in animals growing under dietary phosphorus (P) deficiency (alkaline phosphatase activity, APA) to assess the nutritional status of major crustacean zooplankton taxa in lakes across a gradient of atmospheric N deposition in Norway. Lakes receiving high N deposition had suspended organic matter (seston) with significantly elevated carbon:P and N:P ratios, indicative of amplified phytoplankton P limitation. This P limitation appeared to be transferred up the food chain, as the cosmopolitan seston-feeding zooplankton taxa Daphnia and Holopedium had significantly increased APA. These results indicate that N deposition can impair the efficiency of trophic interactions by accentuating stoichiometric food quality constraints in lake food webs. Keywords Nitrogen deposition, phosphorus limitation, stoichiometry, zooplankton. Ecology Letters (2010) 13: 1256–1261 INTRODUCTION Atmospheric nitrogen (N) deposition to ecosystems has increased markedly in recent decades largely due to fossil fuel combustion and agricultural intensification (Galloway et al. 2008). The effects of such deposition on both terrestrial and aquatic ecosystems have received consider- able attention but primarily with respect to effects on biogeochemical cycling and on the production and nutrient limitation status of autotrophs (plants, phytoplankton; Aber et al. 1998; Baron et al. 2000; Fenn et al. 2003; Elser et al. 2009a). In ecosystems, primary producers assimilate inorganic forms of chemical elements [such as carbon (C), N, and phosphorus (P)] during primary production. This produc- tion is then consumed by herbivores, forming the base for support of higher trophic levels including, eventually, humans. However, the efficiency of trophic transfer from primary production to secondary production varies tremen- dously for a variety of reasons, including the existence of strong imbalance in chemical composition between primary producers and consumers (Sterner & Elser 2002; Cebrian et al. 2009). In particular, it is now well-established that nutrient (N, P) limitation of primary producers leads to elevated C:nutrient ratios in primary producer biomass, resulting in greatly decreased energetic (or C) growth efficiency of herbivores due to limitation of their growth by low N or P in the diet (Sterner & Elser 2002). Indeed, several experiments have demonstrated direct limitation of consumer growth by dietary P content in the laboratory (Urabe et al. 1997) and the field (Elser et al. 2001). The existence of these stoichiometric food quality constraints raises the possibility that processes such as atmospheric N deposition that accentuate primary producer P limitation and increase producer C:P and N:P ratios should also induce direct P limitation of herbivores. To test this possibility, in summer 2009, we sampled 20 lakes in southern Norway at either ends of a strong gradient of atmospheric N deposition. These lakes are a subset of those included in a study in 2007 showing that elevated N deposition is associated with significant increases in water column NO 3 and total N (TN) concentrations, TN:total P ratios, and seston C:P and N:P ratios along with increased frequency and severity of phytoplankton P limitation (Elser et al. 2009a). We assessed these chemical parameters again, along with additional sampling and analysis of zooplankton alkaline phosphatase activity (APA) to test for consumer P limitation. Alkaline phosphatase is an enzyme that cleaves phosphomonester bonds and is well known in microbes and algae to be induced by the onset of P limitation. Ecology Letters, (2010) 13: 1256–1261 doi: 10.1111/j.1461-0248.2010.01519.x Ó 2010 Blackwell Publishing Ltd/CNRS