This study simulated a 9-months warming scenario on the common seagrass Zostera marina from winter into summer (December 2015 - August 2016) in the Western Baltic Sea (Kiel Fijord), using outdoor mesocosms. Two treatments were applied: Ambient temperature regime (Ambient) and Ambient + 3.6C (Heat) over the entire course of the experiment. Temperature regimes were compared to the 22-year temperature average in the area. This dataset shows concentration of dissolved oxygen per treatment (Ambient, Heat) as the mean and confidence interval (CI) across all benthcosms per treatment (n=6).