Comparison of phosphorus and nitrogen in sediments, the water column, corals and algae at Davies Reef, Great Barrier Reef

Water, sediment, algae and coral samples were collected between July 1980 and March 1981, over a wide range of sites at Davies Reef in the central Great Barrier Reef for phosphorus and nitrogen analyses.Water column samples were collected in 4 liter Niskin bottles. Subsamples for phosphorus (as soluble reactive P), nitrate, nitrite and ammonium analyses were filtered through a 20 µm plankton net into 100 ml acid-washed polythene bottles and stored at -20°C until analysis using a Technicon AutoAnalyzer II system. Samples for arsenic analysis were collected in bottles containing As-free HCl and stored at 3°C until analysis using a Varian model AA6 atomic absorption spectrophotometer.Sediment samples from the deeper waters outside the reef were collected with a van Veen grab. Inside the reef, surface sediment samples, to a maximum depth of 10 cm, were collected in labelled plastic bags by SCUBA divers. Deep cores, to 5 m, were taken from the large lagoon in a polyvinyl chloride casing tipped with a stainless steel corer and driven hydraulically. Samples of young, calcified Halimeda tips as well as young growing coral tips were collected from various locations and either immediately bleached or frozen in dry ice for transport to the laboratory.Sediment, coral and Halimeda samples were dried and ground to pass through a 50 µm sieve. Duplicate 0.5 g subsamples were analyzed for total phosphorus, after digestion, using a standard colorimetric method. For arsenic determination, 0.5 g of sediment was dissolved in 10 ml of Merck 318 HCl, heated at 80°C for 5 minutes and diluted to 50 ml. 5 ml aliquots were then analyzed as described for water analyses.Redox potential of the sediments at a large number of sites was measured in situ using a Pt/SCE combination electrode connected to a millivoltmeter sealed in a waterproof housing. Samples of sediment water were collected using aquarium stones, buried beneath the sediment surface and connected via Tygon tubing to 50-ml polythene syringes. Water samples were also obtained by extracting freshly collected small sediment cores, which were collected by inserting 50 ml syringes, with one end removed, into the sediment and manipulating the plunger to draw sediment into the tube. The syringes were excavated and sealed before final removal from the sediment. Extraction with 10 ml of seawater was carried out within 1 hour. The sediment and seawater mixture was shaken for 1 minute, centrifuged, and the cleared water decanted and stored frozen until analysis. All sediment water samples were analyzed for inorganic nitrogen and phosphorus by the methods described for water analyses.Algal samples were collected from the reef front and crest. The types of algae collected were: calcifying algae (Halimeda sp. dominant), other macroalgae, blue-green algal mats and turf algae. The samples were sorted at the reef and frozen in sealed containers until analysis. Duplicate 3 mg subsamples of finely ground algal material were analyzed for total C and N in a Perkin-Elmer model 240 elemental analyzer.
 This research was undertaken to determine whether any significant differences in the phosphorus and nitrogen content of water or sediments could be detected over a range of varying conditions such as water depth, water residence times, types and abundances of benthic organisms, and composition of sediment.

Organization: Australian Ocean Data Network