Warming and depth interact to affect carbon dioxide concentration in aquatic mesocosms

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.