Investigating the Relationships between Tropical Cyclones, Sediment Loading, and Algal Blooms over the Great Barrier Reef

The ecosystem services provided by Australia's Great Barrier Reef contribute approximately AUD$6.9 billion and 66,000 jobs annually to the Australian economy. Pressures on the reef include global scale changes such as ocean warming, ocean acidification, and sea level rise, and localized disturbances such as tropical cyclones. Tropical cyclones are a complex agent of change in the region. Wind and wave energy provide the kinetic energy to break coral assemblages. High volumes of rainfall decrease the salinity of the water column, resulting in salinity-induced stress and potentially coral bleaching and die-off. Heavy rainfall and storm surges can lead to extensive flooding over coastal land areas. Upon retreating, floodwaters transport freshwater, fertilisers, herbicides, sediment, dissolved organic matter, and other nutrients into the surrounding coastal waters. Decreased coastal water quality has significant negative consequences to reef species and ecosystems, and subsequent algal blooms inhibit the passage of photosynthetically active radiation to the reef and outcompete the benthic reef organisms for resources. These impacts inhibit reef recovery after a tropical cyclone, particularly in areas already stressed by warming temperatures and acidification. This study examines the relationship between tropical cyclones and algal blooms over the Great Barrier Reef as an important element in diagnosing the complex impacts on reef systems under climate change.

Gridded MODIS 8-day average chlorophyll a data collected between January and May from 2004 and 2014 is used to infer algal activity in surface waters. Our results demonstrate that algal blooms are closely associated with both tropical cyclone events and intensity, particularly with regard to total rainfall. Concentrations are most significantly correlated with a 7-day time lag of tropical cyclone activity, and increase with event intensity. Effects are localized to individual watersheds, but the impacts of large events can span broad areas of the reef. The analysis highlights geographically varying vulnerability to extreme chlorophyll a concentrations and intense algal blooms. These findings are important for relief efforts and targeting management resources in coastal lands to reduce sediment and nutrient loading into the reef lagoon following flooding events. Further, through the development of a generalized function of the relationship between coastal rainfall intensity and maximum wind speed and the chlorophyll a concentration in the surface waters for each watershed, expected changes in impacts on the reef under tropical cyclone events in future climate change can be predicted and characterized.