Poster Presentation Australian Marine Sciences Association Annual Meeting 2023

The Biogeochemistry of the Halimeda Bioherms of the Northern Great Barrier Reef (#360)

Kim Chua 1 , Helen Bostock 1 , Luke Nothdurft 2 , Dirk Erler 3 , Jody Webster 4 , Mardi McNeil 5 , Peter Hughes 6 7 , Rick Cuthbertson 6 , Damian Pretyman 6
  1. School of Earth and Environmental Sciences, University of Queensland, Brisbane, QLD, Australia
  2. School of Earth and Atmospheric Science, Queensland University of Technology, Brisbane, QLD, Australia
  3. Faculty of Science and Engineering, Southern Cross University, Lismore, NSW, Australia
  4. School of Geosciences, University of Sydney, Sydney, NSW, Australia
  5. Geoscience Australia, Canberra, ACT, Australia
  6. National Collections and Marine Infrastructure, CSIRO, Hobart, TAS, Australia
  7. IOMRC Crawley, CSIRO, Perth, WA, Australia

The Halimeda bioherms of the northern GBR form a substantial inter-reefal habitat, sitting between 20-50 m water depth and occupying more than 6000 km2. Upwelling of nutrients through inter-reef passages onto the shelf has been proposed to drive bioherm development. This study investigates the nutrient concentrations from seawater over the bioherms using live Halimeda incubation experiments to understand nutrient cycling within the system. Three bioherm areas were surveyed as part of Project HALO and research voyage IN2022_V07 between August and September 2022.

Preliminary results showed low nutrient concentrations in shelf waters over the bioherms in winter. However, the incubation experiments reflected elevated nutrients in the water overlying the Halimeda-rich sediments in dark treatments. This indicates nutrient cycling between the sediments and water column, further supported by large amounts of organic matter in the fine fraction (<63 µm) sediments. We propose that the bioherms are sustained by the upwelling of nutrient-rich subtropical waters onto the shelf in summer, while being sustained by tight nutrient recycling within the bioherms throughout the rest of the year. A deeper understanding of the factors sustaining the bioherms and its ecological relationships to the adjacent reefs, can better inform conservation policies on the northern GBR.