Hydrographic, nutrient and phytoplankton biomass surveys in Palm Passage, central Great Barrier Reef (1983)

Full description

A transect of nine oceanographic stations, extending from the GBR lagoon to the outer slope, was surveyed through Palm Passage (central Great Barrier Reef) in 1983, with an emphasis on the summer upwelling period. In total, 128 temperature and salinity profiles were obtained. Water samples were collected at five stations on the shelf and the outer slope (total = 100 stations) for analysis of inorganic nutrients and phytoplankton pigments. The frequency of sampling was most intense during the late summer (January - March), with five cruises of six days duration at one to three week intervals. During these cruises, the transect was occupied very second day. For the remainder of the year, single transects were occupied at intervals of approximately two months.

Lineage

Maintenance and Update Frequency: notPlanned

Statement: Statement: 1. Salinity, temperature and density:A remote recording CTD, manufactured at AIMS, was used to obtain water column conductivity and temperature profiles. The profiler was initially calibrated in the laboratory with a quartz thermometer and precision resistance loop. Empirical temperature and salinity calibrations at sea were made by simultaneous Niskin bottle/CTD casts at Station 5, using Rigosha reversing thermometers (accurate to 0.02°C) and laboratory salinity determinations using a Plessey Model 6230N salinometer (accurate to 0.003 ppt) standardized with IAPSO seawater. When compared to bottle data by linear regressions, the CTD data was found to have standard errors for temperature and salinity of 0.17°C and 0.033 ppt, respectively. The CTD data were adjusted by the following equations to be equivalent to bottle values.Thermometer temperature =0.982 (CTD Temperature) + 0.478°C.Bottle salinity =0.963 (CTD Salinity) + 1.69 ppt.A standardized sampling procedure was followed at all stations. The CTD profiler was initially held just below the water surface to equilibrate the sensors to ambient conditions, then lowered through the water column at approximately 1m/sec to within 2-3 metres of the bottom. It was then returned to the surface at a similar rate. The profiler was programmed to record data at 4 Hz. After each profile, the data was transferred to a shipboard micro-computer for storage and preliminary analysis. Only data from the down casts were analysed further.Nominal salinity and temperature values were calculated for each CTD sample. Potential temperature was calculated according to the method described in "Bryden, H.L. (1973) New polynominals for thermal expansion, adiabatic temperature gradient, and potential temperature of seawater. Deep-Sea Research 20:401-408". Rough salinity and temperature profiles were smoothed with a 3-point running mean and potential density calculated according to the method described in "Cox, R.A., M.J. McCartney and F. Calkin (1970) The specific gravity/salinity/temperature relationships in natural seawater. Deep-Sea Research 17: 679-689". Salinity, potential temperature and potential density values were interpolated to even-metre depths. Values for the nominal depths of water bottle samples were then selected by inspection.A Sippican XBT was carried as a back-up temperature profiler. XBT drops were used in conjunction with reversing thermometers on transects 18 and 20, following malfunctions of the CTD system. Temperatures reported for Station I of Transects 18 and 20 are from XBT data. Temperature data for the other stations on transects 18 and 20 were obtained with Rigosho reversing thermometers on Niskin bottles.2. Niskin bottle samples:Water samples were collected with 5 litre Niskin bottles. At stations on the shelf (1-4), samples were collected at 10 metre intervals to the bottom with an additional bottle approximately 2-3 m above the bottom usually included. At station 5, samples were collected at 25 metre intervals to 150 m and thereafter at 50 metre intervals to the bottom (307 m). Sub-samples were immediately drawn out for size fractionation, phytoplankton pigment and nutrient determinations.3. Nutrients:Water samples for nutrient determinations were prefiltered through 20 µm nylon plankton netting to exclude macro-zooplankton. Two nutrient sub-samples were routinely taken: one for on-board determination of ammonia and one which was frozen and stored for subsequent analyses of the other major nutrients at AIMS. Acid-washed labware was used throughout to minimize sample contamination. On-board ammonia determinations were done immediately by the manual, spectrophotometric method (see "Solorzano, L. (1969) Determination of ammonia in natural waters by the phenolhypochorite method. Limnology and Oceanography 14: 799"), with suggestions on sample handling and reagent preparation given by J.J. McCarthy (pers. comm.). Analyses of the frozen seawater samples for the remaining nutrients (nitrate, nitrite, phosphate and silicate) were made within two weeks of collection, using a multi-channel autoanalyzer adapted for low nutrient levels in tropical waters (see "Ryle, V., H.R. Mueller and P. Gentien (1981) Automated analysis of nutrients in tropical seawater. Australian Institute of Marine Science Technical Bulletin, Oceanography Series OS-81-2"). Detection limits for nutrients were:Ammonium - 0.03 µg at/l, nitrate - 0.05 µg at/l, nitrite - 0.02 µg at/l, phosphate - 0.02 µg at/l and silicate - 0.4 µg at/l (see "Ryle, V., H.R. Mueller and P. Gentien 1981 Automated analysis of nutrients in tropical seawater. Australian Institute of Marine Science Technical Bulletin, Oceanography Series OS-81-2". It has been previously demonstrated that short-term storage has relatively minor effect on levels of nutrients other than ammonium (see "Ryle, V. and H. Mueller 1981 Filtration, storage and preservation of seawater samples for nutrient analysis. Australian Institute of Marine Science Anal. Serv. Lab. Rept. No. 10").4. Phytoplankton pigments:Subsamples of seawater were fractionated into three size classes for phytoplankton pigment determinations: Total (unfractionated), Duplicate 100 ml subsamples of each size fraction were filtered under low vacuum pressure 5. Weather DataAll wind speed and direction data were obtained from a remote weather station (see "Colman, R., K. Carr and E. Gill 1982 A versatile long-range telemetry system particularly suited to meteorological and environmental sensing at remote locations. Australian Institute of Marine Science Technical Bulletin, Instrumental Facilities IF-82-1") located at Rib Reef (18° 28.8'S, 146° 53.2'E), about 3 nautical miles south of station 2. Data were recorded at half hour intervals and telemetered daily to AIMS for storage and analysis.

Notes

Credit
Furnas, Miles J, Dr (Principal Investigator)