The potential for leakage of CO2 from a storage reservoir into the overlying marine sediments and into the water column and the impacts on benthic ecosystems are major challenges associated with Carbon Capture and Storage (CCS) in subseafloor reservoirs. A field-scale controlled CO2 release experiment was conducted in shallow, unconsolidated marine sediments. Changes were monitored of the chemical composition of the sediments and overlying water column before, during and up to 1 year after the 37-day long CO2 release from May 2012 to May 2013 in Ardmucknish Bay. Meiofaunal samples were collected and meiofauna higher taxa and the nematodes species (where possible) were identified by Plymouth Marine Laboratory. This dataset was collected under the program QICS (Quantifying and monitoring environmental impacts of geological carbon storage) which was funded by the Natural Environment Research Council (NERC), with support from the Scottish Government. The results are contained in an Excel file. QICS project website: www.bgs.ac.uk/qics/home.html. This data is currently under embargo until publication of the dataset in research article (estimated end of 2015).