Heavy rainfall has been occurring in northern Queensland since December causing widespread flooding of coastal rivers (Burdekin, Haughton, Bohle, Herbert, Tully, O’Connell and others) as well as inland catchments. In some places all-time records were broken, especially around Townsville, and the flows in the Herbert and Burdekin were both far above average (more rain may occur as well).
The river discharge events are being tracked by satellite imagery in collaboration with Arnold Dekker’s group, CSIRO, Canberra and Lachlan McKinna in Michelle Devlin’s flood plume project at JCU. The plumes are noticeable as sediment rich in the early stages (January – image1) and extending out to near Dunk Island but colour rich (chlorophyll and coloured dissolved organic matter) in the latter stages (February – image 2) extending completely across the main reef and into the Coral Sea.
The plumes are being sampled via the GBRMPA – Reef and Rainforest Research Centre, Marine Monitoring Program run by the Catchment to Reef Group, ACTFR, JCU (Michelle Devlin coordinator) and AIMS (Britta Schaffelke). Sampling from both fixed installations and vessel surveys have been going since December.
Incidentally to the plume monitoring, reports from many scientists working on the reef in the area between Mackay and Cooktown have been coming in of coral ‘bleaching’ and mortality, ‘fresh’ water layers, turbid water layers, green water and stratified water. Corals in poor condition have been reported by Katharina Fabricius (Dunk Island and surrounds), Sheriden Morris (Frankland group), Angus Thompson (Pandora, Palms, Whitsundays), Michelle Devlin, Jane Waterhouse and David Haynes (Dunk and surrounds), Britta Schaffelke (Franklands, High, Fitzroy, Pandora and others), Ray Berkelmans (Magnetic Island), Stephen Lewis and Brett Baker (Burdekin plume).
Images of white/dead coral from Franklands can be seen in image 3 and white bommies from surface near Dunk Island and the Family Group in image 4. Ongoing monitoring is being coordinated by David Wachenfeld and his team at GBRMPA.
Coral mortality and ‘bleaching’ is widespread on inner-shelf reefs in the above region. I put ‘bleaching’ in commas as this event is probably not mostly normal bleaching i.e. expulsion of zooxanthellae, but rather actual death of the coral organism. This is obviously somewhat speculative but consistent with observations of coral mortality in low salinity water by van Woesik and others after similar events in 1991 in the Keppel Islands.
The coral mortality is no doubt associated with the long period (more than 8 weeks) of low salinity flood water but other factors such as elevated suspended sediment, nutrients and pesticides may also be important. Water temperatures were also above average in the period before the floods and an element of combined stress may also be important. Disentangling the separate and combined effects of the multiple stresses and their role in the coral mortality will be a major challenge.