The inﬂuence of atmospheric deposition of pollutants on cross–domain causal relationships for three tropical freshwater lakes in India Jitendra Pandey* Department of Botany, Banaras Hindu University, Varanasi, India Abstract This long-term study on three freshwater lakes in India was designed to investigate the following: (i) atmospheric deposi- tion of major nutrient elements; (ii) microbial biomass (C mic ) and activity at the land–water interface and their relation to lake ecosystem functioning, as inﬂuenced by catchment inputs; and (iii) the modifying inﬂuence of atmospheric deposi- tion on trans-surface causal relationships. These study results illustrated signiﬁcant differences (P < 0.001) in the atmo- spheric deposition of nitrate (NO  3 ), ammonia (NH þ 4 ), phosphate (PO 3 4 ), sulfate (SO 2 4 ), calcium (Ca 2+ ), and magnesium (Mg 2+ ), with consistently rising inputs over time. The interface of Lake Baghdara, characterized by woodland catchment, was found to be rich in phenolics and supported low C mic and activity. Alkaline phosphatase in humus declined by 13.60– 25.30% over time, although the C mic , SIR, and FDAase at the land–water interface increased signiﬁcantly in response to atmospheric nutrient deposition. Although it increased primary productivity, nutrient deposition exhibited a signiﬁcant (P < 0.001) modifying inﬂuence on the C mic and lake productivity relationships, with a time lag of 3 years. A substantial increase in dissolved organic carbon (DOC), especially in Lake Baghdara, indicated that the modifying inﬂuence of atmo- spheric deposition on microbial biomass and activity at the land–water interface could signiﬁcantly alter the carbon cap- ture and storage in surface waters and consequently future carbon balance and climate change processes. This study demonstrated that, if present trends of atmospheric deposition of pollutant elements continue, they can modify the micro- bial processes at the land–water interface and productivity of lakes and will alter the cross–domain causal relationships of freshwater tropical lakes over the long term. Key words atmospheric deposition, climate change, lake, land–water interface, microbial biomass, productivity. INTRODUCTION Wide acceptance of the causative role of nutrients in water quality deterioration has led the control of external nutrient loads as a widely used approach in lake ecosys- tem restoration. Recent studies emphasize the need to address the current status of eutrophication and biodiver- sity loss for successful implementation of environmental management programs (Fredrick et al. 2010). Because the nutrient enrichment of surface water bodies has been considered a local phenomenon centred around point sources and urban areas, minimizing the surface–nutrient inputs has been used as an effective tool to combat eutro- phication. Recent studies, however, have indicated that the solute chemistry of surface waters also depends on the chemical constituents imported through weathering, surface discharge and runoff, as well as pollutant aerosols added into the water system via atmospheric deposition (Sickman et al. 2003; Bergstrom et al. 2005; Pandey & Pandey 2009). More importantly, during recent years, anthropogenic activities have dramatically increased the atmospheric deposition of pollutant aerosols in many parts of the world including India (Galloway & Cowling 2002; Fenn et al. 2007; Pandey & Pandey 2009). Air-driven deposition can contaminate surface water- bodies, including those located far from the emission sources. This may result in restoration strategies based on reducing surface nutrient loads being failures. Although the impact of long-range atmospheric deposition *Corresponding author. Email: jiten_pandey@rediffmail.com Accepted for publication 24 January 2011. Ó 2011 The Authors Doi: 10.1111/j.1440-1770.2011.00456.x Journal compilation Ó 2011 Blackwell Publishing Asia Pty Ltd Lakes & Reservoirs: Research and Management 2011 16: 113–121