Dynamic Phenology of Canopy-Forming Macroalgal Forests within a Tropical Seascape

Abstract Biomass dynamics of primary producers provide a basis for assessing their capacity to capture carbon and mitigate excess greenhouse gases. Here, we estimate inter-annual and seasonal biomass cycles in canopy-forming Sargassaceae at 19 sites spanning ~ 200 km of the Ningaloo World Heritage Area over 11 consecutive years. Our global average summer estimate of 1.53 kg wet wt m −2 equates to 4.1–9.4 Gg of standing macroalgal carbon (14.9–34.6 Gg CO 2 ) across 38,300 ha of all macroalgae habitats. Biomass estimates did not differ significantly among years but varied by an order of magnitude among sites. Accumulation typically occurred during the spring, followed by a loss of 42–94 Gg wet weight, or 1.4–3.3 Gg organic carbon (5.3–12.0 Gg CO 2 ) fixed in macroalgal habitats during autumn. Seasonal patterns of biomass accumulation and release were, however, spatially and temporally variable, some sites accumulating biomass in autumn and releasing it during the following spring. Spring biomass accumulation was reduced by herbivorous urchins, whilst the extent of macroalgae biomass change in the autumn was directly correlated with sea temperature and standing biomass the previous summer. These findings imply canopy-forming macroalgae provide a regular injection of biomass into the surrounding ecosystems, which support productivity of surrounding habitats and is a potential source of organic carbon for burial in carbon sinks throughout the year. Modelling that seeks to link macroalgae sources with sinks therefore requires a spatially explicit understanding of when seasonal senescence of canopy-forming macroalgae occurs, inter-site variability and the environmental conditions that govern this phenology.