Ecology, 90(7), 2009, pp. 1910–1922 Ó 2009 by the Ecological Society of America Elevated pH regulates bacterial carbon cycling in lakes with high photosynthetic activity SUZANNE E. TANK, 1,4 LANCE F. W. LESACK, 1,2 AND DONALD J. MCQUEEN 3 1 Department of Biological Sciences, Simon Fraser University, 8888 University Drive, Burnaby, British Columbia V5A 1S6 Canada 2 Department of Geography, Simon Fraser University, 8888 University Drive, Burnaby, British Columbia V5A 1S6 Canada 3 125 Pirates Lane, Nanaimo, British Columbia V9R 6R1 Canada Abstract. Bacteria are critically important for carbon (C) cycling and energy flow in aquatic environments. However, studies to date have largely focused on the role of substrate quality in the regulation of this important process. As such, we know little about the role of other ecological drivers in shaping bacterially mediated C cycling. Here we examine the manner in which planktonic bacterial abundance (BA), productivity (BP), respiration (BR), and growth efficiency (BGE), and thus C cycling are affected by elevated pH, an ecological factor that occurs commonly in highly productive aquatic systems. We undertook our study in lakes of the Mackenzie Delta region of Canada. These lakes routinely experience high pH caused by rapid macrophyte photosynthesis. Two different experiment types were employed: first, a series of short-term experiments was used to assess the direct effects of elevated pH on bacteria experiencing differing pH levels in situ. Second, long-term mesocosms were used to explore the effect of elevated pH on bacteria over longer time scales and in the presence of other trophic levels. Bacterial productivity and BR slowed dramatically with elevated pH over the short term, potentially uncoupling bacterial processing of organic matter from its in-lake production and causing a switch away from biomass creation and toward C mineralization. With longer term exposure, bacterial communities adapted to the direct stress of elevated pH, but responses at higher trophic levels caused a cascade that mediated the effect of alkalization on bacteria, in a manner that could well vary among aquatic ecosystems. Our study establishes elevated pH as a key driver of bacterial C cycling and energy flow in aquatic systems with high autotrophic productivity. Key words: alkalization; bacterial growth efficiency; bacterial production; bacterial respiration; carbon cycling; community adaptation; ecological stressors; high productivity; Mackenzie Delta, western Canadian Arctic; pH; trophic cascades. INTRODUCTION Bacteria are the most important biological component of carbon (C) cycling in the biosphere (del Giorgio and Cole 1998). In freshwaters the balance between rates of bacterial productivity (BP) and respiration (BR) con- trols the degree to which dissolved organic matter (DOM) is subsequently passed to higher trophic levels or emitted from lakes as carbon dioxide (CO 2 ; Cotner and Biddanda 2002). Thus, elucidating the mechanisms that shape bacterial C processing is essential to understanding energy flow and C flux within aquatic ecosystems. While there has been much work examining the effects of bottom-up (i.e., substrate quality) pro- cesses toward this end, we know little about how other ecological drivers regulate bacterial C cycling. Elevated pH has been clearly documented in fresh- waters, often as the result of rapid photosynthesis sequentially depleting CO 2 and then HCO 3  in the water column, which can lead to pH levels in excess of 10.0 (e.g., Schindler et al. 1972, Mitchell and Prepas 1990, Hesslein et al. 1991). Such high pH coupled with high autotrophic productivity could have unexpected implications for bacterial C cycling. The release of DOM that is a byproduct of algal and macrophytic photosyn- thesis provides a high-quality substrate for bacterial growth (Findlay et al. 1992, Pe´rez and Sommaruga 2006) that can represent an important energy source to the microbial food web. The rapid use of this labile DOM by bacteria sets up a tight coupling between autotrophic DOM production and its subsequent bacterial processing (Kritzberg et al. 2005). However, when photosynthesis increases to the point at which pH also rises, resultant impacts on the bacterial community could disrupt C cycling through several different, potentially co-occurring, effects. First, a pH-mediated reduction in the rate of bacterial DOM processing could uncouple the continued production of autotrophic DOM from its concomitant use by bacteria. Second, a change in the rate of BP relative to BR might cause a switch in microbial loop function, with a shift in the balance of C available to higher trophic levels vs. that Manuscript received 29 May 2008; revised 23 September 2008; accepted 21 October 2008. Corresponding Editor: S. Findlay. 4 E-mail: suzanne_tank@sfu.ca 1910