8′W and 50°20.7′N, 04°07.78′W) using an anchor grab. Sediment was collected from Cawsand, Plymouth Sound (∼15 m water depth, 50°19.8′N, 04°11.5′W) using an anchor grab. Sediment was sieved (500 μm Fulvestrant cell line mesh) in a seawater bath to remove macrofauna, allowed to settle to retain the fine fraction and homogenised by stirring, before being added to individual cores (capped PVC cores, 100 mm diameter, 200 mm tall) to a depth of 150 mm and overlain by 50 mm seawater. All cores were held in a recirculating seawater system until they were used in the exposure trials. CO2 gas was bubbled

through natural seawater (salinity ∼35) enabling the gas to dissolve rapidly into solution. Release of CO2 gas, to maintain the pH, was controlled via a solenoid valve connected to the gas cylinder and monitored using a pH controller (Aqua Digital pH-201,

accuracy ±0.1% + 0.02) which was cross checked weekly against values given by a regularly calibrated pH metre (InLab® 413SG, Mettler-Toledo). The reservoir electrodes did not require calibration learn more over the course of the study. Two 1m3 tanks, one containing the acidified sea water and one containing ambient seawater were used to acclimatise both the A. filiformis and the sediment (including meiofauna and microorganisms) prior to the experiment. Cores containing individuals of A. filiformis (n = 5 mesocosm−1, density equivalent to 640 individuals m−2) or sediment with no macrofauna were positioned randomly in the acclimatisation tanks for 96 h prior to the start of the experiment ( Fig. 1). Salinity, temperature and alkalinity in both tanks were monitored three times per week (Monday, Wednesday and Friday) throughout the duration of the experiment. Unmeasured carbonate parameters were calculated from these data using constants supplied by Lueker et al., 2000 and Millero, 2010 with CO2 calc., an application developed by the U.S. Geological Survey Florida Shelf Ecosystems Response to Climate Change Project ( Robbins et al., 2010). Following the acclimatisation period, sediment and fauna were transferred into rectangular thin-walled (5 mm) Perspex aquaria (33 × 10 × 10 cm, density equivalent to 500 individuals m−2).

Each aquarium was maintained in PJ34 HCl a temperature controlled room (10 °C) and supplied with seawater (on a flow through system from the acclimatisation tanks) at the appropriate pH level and at a rate of ∼10 ml min−1 using a peristaltic pump (Watson–Marlow 323). The faunal redistribution of sediment particles was measured non-invasively using a time lapse sediment profile imaging system (f-SPI, following Solan et al., 2004b), optically modified to preferentially visualise fluorescent dyed sediment particles (luminophores, see Maire et al., 2008) housed in a UV illuminated imaging box (32 × 87 × 62 cm with Phillips blacklight, 8 W, Schiffers et al., 2011). The camera (Canon 400D, 3900 × 2600 pixels, i.e. 10 megapixels, effective resolution = 64 × 64 μm per pixel) was set for an exposure of 4s, f = 5.