Benthic cyanobacterial mat formation during severe coral bleaching at Lizard Island: The mediating role of water currents.

Cyanobacterial mats are increasingly recognised as a symptom of coral reef change. However, the spatial distribution of cyanobacterial mats during coral bleaching has received limited attention. We explored cyanobacterial mat distribution during a bleaching event at Lizard Island and considered hydrodynamics as a potential modifier. During bleaching cyanobacterial mats covered up to 34% of the benthos at a transect scale, while some quadrats (1 m) were covered almost entirely (97.5%). The spatial distribution of cyanobacterial mats was limited to areas with slower water currents. Coral cover declined by 44% overall, although cyanobacterial mats were not spatially coupled to the magnitude of coral loss. Overall, the marked increase in cyanobacterial mat cover was an ephemeral spike, not a sustained change, with cover returning to 0.4% within 6 months. Cyanobacterial mats clearly represent dynamic space holders on coral reefs, with a marked capacity to rapidly exploit change, if conditions are right.