Perturbation of persistent foraminiferal assemblages by elevated land runoff into the Great Barrier Reef

A total of 11 sediment cores were collected from inshore fringing reefs adjacent to 7 islands in the Whitsunday Islands Group, Queensland. The cores were collected from three zones along a distinct and persistent water quality gradient: inner nearshore reefs with low coral cover and high macroalgal abundance (Long Island (2 cores) and Lindeman Island (2 cores)); intermediate reefs (Dent Island (1 core), Double Cone Island (1 core) and Daydream Island (1 core)); and reefs at outer inshore islands with low algal cover and high coral cover (Edward Island (2 cores) and Deloraine Island (2 cores)).Cores were collected from back reef areas at a depth of 6-8 m below lowest astronomical tide and from areas of similar substrate type, adjacent to the reef slope. Undisturbed samples of the top 2 cm of surface sediments were also collected at each location. Cores were driven into the sediment using a core driver and a movable steel collar around the aluminium core barrel, which had an external diameter of 100 mm. In the laboratory, the cores were cut into 10 cm sections. The sediment from each section was well mixed, sieved, dried, mixed again, subsampled and all foraminifera removed. Subsampling was repeated until at least 200, >80% intact foraminifera were collected, which were then identified to genus or species level. The dry weight of sediment and foraminifera were determined to calculate densities.Sediment samples were dried and ground for determination of total carbon (carbonate carbon + organic carbon) and nitrogen content. The upper sediment core sections were dated using 210Pb and 137Cs measurements, andforaminifera from a total of 17 deeper sections were dated using 14C accelerator mass spectrometer analysis. This research was undertaken to investigate changes in foraminiferal assemblages over time. By collecting cores from reefs along a water quality gradient associated with distance from river mouths, dating core sections and identifying foraminifera within the sections, it was possible to test whether changes in foraminiferal assemblages were more likely to be caused by global stressors or by increased runoff since European settlement.