Seasonal changes in water quality and Sargassum biomass in southwest Australia

Sargassum C. Agardh is one of the most diverse genera of marine macroalgae, and commonly inhabits shallow tropical and sub-tropical waters. This study aimed at investigating the effect of seasonality and the associated water-quality changes on the distribution, canopy cover, mean thallus length and biomass of Sargassum beds around Point Peron, Shoalwater Islands Marine Park, southwest Australia. Samples of Sargassum and seawater were collected every 3 mo from the summer of 2012 to the summer of 2014, from 4 different reef zones. A combination of in situ observations and WorldView-2 satellite remote-sensing images were used to map the spatial distribution of Sargassum beds and other associated benthic habitats. The results demonstrated strong seasonal variation in the physicochemical water parameters, canopy cover, mean thallus length, and biomass of Sargassum, which were significantly (p < 0.05) influenced by nutrient concentrations (PO43-, NO3-, NH4+) and rainfall. However, no significant variation in any studied parameter was observed among the 4 reef zones. The highest Sargassum biomass peaks occurred between late spring and early summer (from September to January). The results provide essential information to guide effective conservation and management, as well as sustainable utilisation of this renewable coastal marine resource.