Water sampling was carried out at the channel mouth of Coral Creek, Missionary Bay, Hinchinbrook Island to determine the transport and exchange of carbon and nitrogen in particulate organic carbon (POC), the component of particulate organic matter consisting of finely divided materials which range in size from 10 µm to 2 mm. Samples were taken during 13 tidal cycles over a 2-year period. Beginning at low tide, water samples were collected from three depths (surface, mid-depth (4-5 m) and at 0.5 m above the creek bed) at half-hour intervals during the complete 12 hour tidal cycle. Initially, samples were taken at three different points across the channel and later only from a single central station, which was adequate to monitor exchanges.Analysis of POC was carried out using the wet digestion method. On two runs, concurrent samples were filtered through 25-mm GFC filters and analysed with a Perkin Elmer analyser to obtain the C : N ratio of the material and to verify the wet digestion method for POC analysis. Concurrent sampling for ATP and chlorophyll analyses was also carried out on two tidal cycle sampling runs, to obtain information on the character of the particulate matter.During each sampling run, regular measurements of tidal depth at the mouth of the channel were made. The volume of water entering or leaving the basin during any time period was calculated from the tide height measurements and the survey data. The average POC concentration during the same time period was interpolated from the graphs of the average POC (average of the three depths) versus time and the amount of POC passing through the mouth of the creek during that time period was calculated. Summation of the individual fluxes for the entire cycle gives the net import or export of POC for that tidal cycle. To check the accuracy of the calculated volume-time data, a bank of Salvonius-type current meters, with automated data logging, were placed in a 3 X 3 matrix across the channel mouth. These meters measured the current flow every 5 min during a continuous series of tidal cycles for 4 days and the average current was determined as a function of time.