A Spatially Explicit Model of Kelp Forest Biomass: Mapping Vulnerability in Western Australia’s Great Southern ReefEcosystem Services

Lianna Gendall¹, Melinda Coleman^2,3,1, Tom Davis^2,3, Karen Filbee-Dexter^1,4, Thomas Wernberg^1,4

¹University of Western Australia, Australia - ²New South Wales Department of Primary Industries, Australia - ³New South Wales Fisheries, National Marine Science Centre, Australia - ⁴Institute of Marine Research, Norway

Australia’s Great Southern Reef (GSR) stretches 8,000 kilometres along Australia’s southern coastline. Its kelp forests provide habitat for thousands of unique marine species, underpin critical ecosystem services including carbon sequestration and nutrient cycling, and generate billions in economic value through fisheries and tourism. The biomass of these kelp forests can be a good indicator of their condition and is a proxy for diversity, carbon stocks, and habitat structure. Nevertheless, we have a limited understanding of the spatial distribution of kelp biomass, and how it varies with environmental conditions. To address this critical need and inform targeted management strategies and future conservation efforts, we developed a spatially explicit biomass model encompassing all major canopy-forming seaweed species along Western Australia’s GSR coastline. Our optimal model was based on Ecklonia, Sargassum, Cystophora, Scytothalia, Platythalia and Acrocarpia genera, incorporating key abiotic factors of temperature, light, depth, and current, and was constrained by a pre-existing model of kelp forest distribution representing rocky reef habitat. The results revealed biomass hotspots in cool southern parts of the GSR and lower biomass forests in warmer northern regions, potentially highlighting functional differences and the thermal vulnerability of these communities. These patterns also suggest reductions of biomass could occur in the future with rising sea temperatures. This detailed mapping of biomass provides critical insight into the function and condition of these ecosystems identifying priority areas for conservation while providing foundational data for modelling ecosystem functions. When integrated with other models, we can enhance our ability to better understand productivity, carbon export, and nutrient cycling capacity across the GSR, enabling us to better understand and inform strategies to mitigate the consequences of kelp loss under climate change.