Measurements of community productivity and calcification were made in seawater flowing across a transect established on the windward reef flat at Lizard Island in the northern section of the Great Barrier Reef. The transect began 50 m behind the reef crest and ended 300 m downstream, approximately 550 m from the lagoon.
The upstream and downstream limits of the transect were marked with surface buoys attached to sea-anchors by rope and chain. The upstream buoy served also as a mooring for a small boat. Changes in the oxygen concentration, pH and temperature of seawater were monitored by an instrument package as it floated across the reef with the current. This instrument package, or "buoy", carried a galvanic oxygen sensor, a pH electrode and a thermistor. It also carried a light sensor. A person in the boat moored at the head of the transect controlled the rate of drift of the buoy by adjusting the rate of release of a line attached to the buoy. At the beginning of each run along the transect, a dye marker was placed in the water about 50 m to one side of the boat moored at the front of the transect. The buoy tended to move faster than the current because it was pushed by waves. Varying degrees of resistance were applied to the drift-control line to keep the buoy alongside the dye patch and, thus, within the same body of water. A datalogger on board the buoy was programmed to interrogate the sensors at 4-10 s intervals. Scan intervals were unusually long because of the low velocity with which water moved across the reef flat. The rope was marked at 5 m intervals and the time was noted when each mark crossed the transom of the boat. Measurements recorded by the buoy could be assigned to distance along the transect by cross-referencing times logged by the buoy with these manually-recorded times. The instrument package was floated across the reef flat transect 11 times between the 14th and 21st March, 1996.
Water velocity was determined from distance-time data recorded while paying out the drift-control line attached to the buoy. Water depths were measured with a graduated pole. Depth measurements were made alongside the buoy as it passed through 50 m points on the transect. Wind speed was measured at the start, middle and end of each transect using a hand-held anemometer.
On certain transects, 3 replicate seawater samples were taken for determination of total alkalinity. These were taken alongside the buoy at 0 m, 100 m, 200 m and 300 m points. Samples were taken on 2 runs early in the period over which measurements were made when irradiance was high (15 March) and on two runs near the end of the period after night had fallen (20 and 21 March).
Benthic communities and substrata in different parts of the transect were also recorded photographically by a diver while snorkelling.
