Abundance and respiration of free and attached microorganisms were monitored during the decomposition of the seagrass Scirpus maritimus leaves in laboratory microcosms for 30 days. There was a clear succession between bacteria and heterotrophic flagellates during the course of the study. The beginning of the study (1-4 days) was characterized by higher rates of bacterial respiration, compared to the later periods. Free microorganisms were responsible for more than half of the respiration (65%) within the microcosms, suggesting that they were responsible for the mineralization of the bulk of the macrophyte detritus following its dissolution. On the other hand, estimates of activity on a per cell basis revealed that individual attached bacteria had much higher (3- to 4-fold) respiration rates than free bacteria, suggesting attached bacterial activity may play a key role in the breakdown and dissolution of particulate detritus in estuarine waters. The findings suggest different but coupled roles for attached and free bacteria in nature. (C) 2003 Elsevier Science B.V. All rights reserved.