Variability of nutrients in a lagoonal patch reef at Davies Reef, Great Barrier Reef

Two field trips were undertaken to examine variability in nutrient concentrations in cavities on the vertical face of a patch reef in the lagoon at Davies Reef. In November 1980, a specifically designed sampling apparatus was used to sample water at four distances within three closed caves, and two cavities connected to the reef flat by chimneys. The caves and cavities were located around the perimeter of the patch reef at approximately 6m depth. Samples were extracted four times between 0800 and 2200 hours on the 6th November 1980 and analysed for phosphate, nitrate and ammonia within an hour, using an autoanalyser onboard the research vessel.

Residence time of water was investigated in one cave and one chimney by the injection of Rhodomine B dye through the sampling apparatus and then sampling water over a period of 6 hours. Samples were analysed using a Turner fluorometer. The absorption of Rhodomine by cavity sediments was measured separately.

During December 1981, a second field trip was undertaken to investigate the observed changes in spatial patterns of nutrients over time. Samples were extracted from only a single cave at two sampling points, one at the entrance and one as deep as possible into the cave. Samples were drawn with a peristaltic pump through PTFE tubing to a small vessel on the surface and then transferred to the research vessel for analysis. Measurements of phosphate, nitrate, nitrite and ammonia were obtained on the hour for 48 hours. To obtain data on far field tidal currents, which force circulation within the permeable reef, a Q15 current meter was suspended 20 m out from the patch reef in the middle of the water column. Light readings were taken each hour at mid depth into the cave using a quantum light meter attached to the sampling apparatus.

Background nutrient concentrations at 2 m depth were determined during both field trips using a Niskin bottle deployed upstream of the patch reef.