Warming and depth interact to affect carbon dioxide concentration in aquatic mesocosms KYLA M. FLANAGAN AND EDWARD MCCAULEY Ecology and Evolutionary Biology Group, Department of Biological Sciences 2500 University Dr N.W., Calgary, AB, Canada SUMMARY 1. Climate change may significantly influence lake carbon dynamics and consequently the exchange of CO 2 with the atmosphere. Warming will accelerate multiple processes that either absorb or release CO 2 , making predicting the net effect of warming on CO 2 exchange with the atmosphere difficult. Here we experimentally test how the CO 2 flux of deep and shallow systems responds to warming. To do this, we conducted a greenhouse experiment using mesocosms of two depths, experiencing either ambient or warmed temperatures. 2. Deeper mesocosms were found to have a lower average CO 2 concentration than shallower mesocosms under ambient temperature conditions. In addition, warming interacts with mesocosm depth to affect the average CO 2 concentration; there was no effect of warming on the average CO 2 concentration of deep mesocosms, but shallow mesocosms had significantly lower average CO 2 concentrations when warmed. 3. The difference in CO 2 concentration resulting from the depth manipulation was due to varying loss rates of particulate carbon to the sediments. There was a strong negative correlation between CO 2 and sedimentation rates in the deep mesocosms suggesting that high particulate carbon loss to the sediments lowered the CO 2 concentration in the water column. There was no correlation between CO 2 and sedimentation rates observed for shallow mesocosms suggesting enhanced carbon regeneration from the sediments was maintaining higher CO 2 concentrations in the water column. 4. Relationships between CO 2 and algal concentrations indicate that the reduction in CO 2 concentrations resulting from warming is due to increased per capita algal turnover rates depleting CO 2 in the water column. Our results suggest that the carbon dynamics and CO 2 flux of shallow systems will be affected more by climate warming than deep systems and the net effect of warming is to increase CO 2 uptake. Keywords: carbon dioxide, climate change, depth, lakes, mesocosm Introduction Climate change is anticipated to affect biological rates and interactions, potentially even altering how a lake functions within the landscape (Schindler et al., 1990; Stefan, Fang & Eaton, 2001; Mckee et al., 2003; Strecker, Cobb & Vinebrooke, 2004). Carbon dioxide (CO 2 ) saturation is a measure of lake function that has recently been examined closely because it is indicative of the dominant energy pathway in a lake (Cole et al., 1994, 2002) and is important for carbon cycling at the landscape scale (Algesten et al., 2004). Despite its importance, the effect of climate change on CO 2 saturation in lakes remains poorly understood. Biological processes influence the concentration of CO 2 in a lake and its subsequent flux to the atmo- sphere via the net balance of primary production (P) and ecosystem respiration (R) (Del Giorgio & Peters, 1993, 1994 ). Lake CO 2 concentrations will be affected by warming if either P or R is disproportionately Correspondence: Kyla M. Flanagan, Ecology and Evolutionary Biology Group, 2500 University Dr N.W., Calgary, AB, Canada T2N 1N4. E-mail: kmflanag@ucalgary.ca Freshwater Biology (2008) 53, 669–680 doi:10.1111/j.1365-2427.2007.01914.x Ó 2007 The Authors, Journal compilation Ó 2007 Blackwell Publishing Ltd 669