Measurement and analysis of reef flat community metabolism at Lizard Island, Great Barrier Reef

Brief description

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. This research was undertaken to determine diel rates of benthic photosynthesis, respiration, calcification and solution of reef rock, using changes in pH and oxygen concentration to estimate these parameters.

Lineage

Maintenance and Update Frequency: notPlanned

Statement: Statement: Calibration of instruments:The light and temperature sensors had been previously calibrated against standard instruments at the Australian Institute of Marine Science. The oxygen and pH electrodes were calibrated at the Lizard Island Research Station before each set of measurements were made. The pH electrode was calibrated against Radiometer high precision pH buffers S1500 (pH = 6.865 at 25°C) and S1510 (pH = 7.410 at 25°C). The oxygen electrode was calibrated against seawater saturated with air and seawater from which oxygen had been removed by reaction with sodium dithionite. Calibrations were made at 28°C, which was close to the temperature of seawater crossing the transect.Water samples:Samples were Millepore filtered (0.45 µm) immediately after they were collected and stored in 300 ml BOD bottles in an insulated container. After they were returned to the laboratory, they were stored in the dark at 4°C. Total alkalinities were measured over two days and the instrument was calibrated at the beginning of each day. To reduce errors in the measurement of small changes in total alkalinity along a transect, samples from a single run were always measured consecutively and with the same instrument calibration. Drift in the calibration over time was accommodated by measuring a seawater standard at the beginning and end of the measurement period, and after every batch of 6 samples (ie., standard measured 1st, 8th, 15th... ).Total alkalinity was determined from pH after addition of precise amounts of acid to precise amounts of seawater (Culberson et al., 1970). Determinations were made using an automated system in which 3 ml of 0.005N HCl and 5 ml of seawater were pumped into a mixing chamber by an automated pipette. The solution was then forced through a capillary tube by air pressure into a chamber containing a pH electrode. The pH was averaged over a period of 20 s after the solution had been gently bubbled with CO2-free air for 2 minutes. Ten replicate measurements were made on each seawater sample. These 10 measurements were then checked to see if any of them fell more than 2 standard deviations from the mean. Any data point falling outside of this range was discarded, a further measurement taken to replace it and the statistical tests were repeated. Data collected by the buoy were adjusted by calibration data for the sensors. Oxygen and pH data were corrected for differences between the calibration temperature and the temperature of the seawater at the time when the sensors were interrogated (see Barnes and Devereux, 1984). Oxygen readings were also corrected for air-sea exchange using the gas exchange coefficient (Marsh and Smith, 1978; Barnes, 1983). The gas exchange coefficient was calculated from the average wind speed measured while the buoy floated across the transect (see Barnes and Devereux, 1984).References:Barnes DJ (1983) Profiling coral reef productivity and calcification using pH and oxygen electrodes. J Exp Mar Biol Ecol 66 pp. 149-161.Barnes DJ and Devereux MJ (1984) Productivity and calcification on a coral reef: a survey using pH and oxygen electrode techniques. J Exp Mar Biol Ecol 79 pp. 213-231.Culberson C, Pytkowicz RM and Hawley JE (1970) Seawater alkalinity determination by the pH method. J mar Res 28 pp. 15-21.Marsh Jr JA and Smith SV (1978) Productivity measurements of coral reefs in flowing waters. In Coral reefs: research methods, edited by Stoddart DR and Johannes RE. UNESCO, Paris, pp. 361-378.

Notes

Credit
Chisholm, John RM, Dr (Principal Investigator)