2.3.10 Water column

In this section, the term ‘water column’ refers to the pelagic zone, which encompasses the open ocean area between the coast and the outer boundary of the Marine Park. With a total volume of 7200 cubic kilometres, the water column serves as a crucial conduit linking all the habitats in the Region. It supports numerous species, including plankton and microbes, invertebrates, pelagic fish, sharks and rays, and cetaceans (whales and dolphins). The water column is an important medium for energy and nutrient cycling and facilitates the dispersal of numerous larval organisms via ocean currents. Condition of the water column as a habitat considers various physical, chemical, and biological factors (Chapter 3) that interact in complex ways. The assessment of condition (and trend in condition) of the Reef’s pelagic habitats relies on expert knowledge from experimental, observational, and modelling science.248

Climate change is causing changes to various aspects of the water column habitat, including both physical oceanographic phenomena and environmental conditions 249 (Section 6.3.1). Plankton and microbes (Section 2.4.6) are crucial component of the water column and play a role in maintaining Reef function.250 They are sensitive indicators of environmental change, and reorganisation of these communities is occurring consistent with expectations under climate change 251 (Box 2.5). Changes in ocean currents and circulation patterns can alter nutrient availability and the transport of larvae,249 thereby affecting the distribution of fish and other marine species.252 One key driver of change is increasing ocean temperatures, which can affect the distribution and behaviour of marine species, including fish and plankton.109 Warming waters also restrict the mixing of ocean layers, affecting nutrient and oxygen transfer through the water column.253These combined changes influence primary productivity, the abundance and species composition of plankton and microbes, and the timing and extent of phytoplankton blooms.251 Although understanding of climate change impacts on water column assemblages is improving, key knowledge gaps remain, and systematic monitoring remains limited across the Region.251

Climate change is affecting water column conditions on large and  small scales

In many inshore areas, water column condition is affected by delivery, resuspension, and recycling of excess nutrients and fine sediments from current and historical land-based runoff, which affects plankton and microbial communities (Sections 2.4.6, 3.4 and 6.5). 

Important knowledge gaps remain regarding the condition (and trend in condition) of the water column. Increasing temperatures and changing ocean chemistry are directly affecting this habitat. Understanding of complex climate change impacts on diverse water column assemblages is improving but remains limited across the Region. Excess nutrients and fine sediments from land-based run-off also affect water column condition in many inshore areas. 

An underwater photograph of the surface of the ocean and water column with rays of light breaking through the surface and visible particles floating on the water surface.
Water column at Konomie Island (formerly known a North Keppel Island) © Ryan Ramasamy 2023
  • 109. Cooley, S.R., Schoeman, D.S., Bopp, L., Boyd, P., Donner, S., et al. 2022, Oceans and coastal ecosystems and their services, in Climate Change 2022: Impacts, Adaptation and Vulnerability. Contribution of Working Group II to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change Cambridge University Press, Cambridge, UK and New York, USA, pp. 379-550.
  • 248. Robson, B.J., Skerratt, J., Baird, M.E., Davies, C., Herzfeld, M., et al. 2020, Enhanced assessment of the eReefs biogeochemical model for the Great Barrier Reef using the Concept/State/Process/System model evaluation framework, Environmental Modelling & Software 129: 104707.
  • 249. Bashevkin, S.M., Dibble, C.D., Dunn, R.P., Hollarsmith, J.A., Ng, G., et al. 2020, Larval dispersal in a changing ocean with an emphasis on upwelling regions, Ecosphere 11(1): e03015.
  • 250. Baird, M., Dutkiewicz, S., Hickman, A., Mongin, M., Soja-Wozniak, M., Skerratt, J., Wild-Allen, K. 2022, Modeling phytoplankton processes in multiple functional types, in Advances in Phytoplankton Ecology, ed. R.E. L. Clementson A. Willis, Elsevier, Amsterdam, pp. 245-264.
  • 251. Richardson et al. 2023, Changes in plankton on the Great Barrier Reef, Unpublished report for the Reef Authority.
  • 252. Benthuysen, J.A., Emslie, M.J., Currey-Randall, L.M., Cheal, A.J. and Heupel, M.R. 2022, Oceanographic influences on reef fish assemblages along the Great Barrier Reef, Progress in Oceanography 208: 102901.
  • 253. Li, G., Cheng, L., Zhu, J., Trenberth, K.E., Mann, M.E., et al. 2020, Increasing ocean stratification over the past half-century, Nature Climate Change 10(12): 1116-1123.