Application of Computer-Aided Tomography (CT) to the Study of Estuarine Benthic Communities

Sediment cores were imaged using a Computer-Aided Tomography (CT)scanner at Massachusetts General Hospital, Boston, Massachusetts, United States. Procedures were developed, using the attenuation of X-rays, to differentiate between sediment and the water contained in macrobenthic tubes and tunnels. The effects of sediment type on the ability to discriminate tubes as small as 1.5 mm were examined. Soft sediments with mean X-ray attenuations (SXA) from 450 to 576 CT numbers were successfully scanned in cores of 15.2 cm diameter by 30 cm depth. We demonstrated the accessibility and availability of CT technology to ecological studies by negotiating a reduced research rate ($200 per core) for sediment scanning at a nearby small hospital. Additionally, we were able to transfer these image data from the local hospital environment to a personal computer, by developing specialized computer software. These steps allowed greater opportunity for data exploration, manipulation, and statistical evaluation than would be available in a medical facility. CT analysis was applied to intact sediment cores from five stations along a 31-kmpollution gradient in Narragansett Bay, Rhode Island, United States. The percentage of tube and tunnel area (PTTA) within the top 18 cm of sediment from each station was measured and ranged from 0.07% to 1.13%. PTTA increased along this gradient with distance from the pollution sources (r2= 0.81, P < 0.01). The mean X-ray attenuation for sediment(excluding animals, their tubes and tunnels, and shells) was determined at each station. It also showed a highly significant relationship along this gradient (r2= 0.98, P < 0.01) and ranged from 271 to 576 CT numbers. The measurement of PTTA may be an effective management tool to assess and monitor the effects of organic carbon loading on benthic communities in Narragansett Bay and similarly impacted estuaries.