Policies aiming to preserve vegetated coastal ecosystems (VCE; tidal marshes, mangroves and seagrasses) to mitigate greenhouse gas emissions require national assessments of blue carbon resources. Here, we present organic carbon (C) storage in VCE across Australian climate regions and estimate potential annual CO₂ emission benefits of VCE conservation and restoration. Australia contributes 5–11% of the C stored in VCE globally (70–185 Tg C in aboveground biomass, and 1,055–1,540 Tg C in the upper 1 m of soils). Potential CO₂ emissions from current VCE losses are estimated at 2.1–3.1 Tg CO₂-e yr^-1, increasing annual CO₂ emissions from land use change in Australia by 12–21%. This assessment, the most comprehensive for any nation to-date, demonstrates the potential of conservation and restoration of VCE to underpin national policy development for reducing greenhouse gas emissions.

A central question in contemporary ecology is how climate change will alter ecosystem structure and function across scales of space and time. Climate change has been shown to alter ecological patterns from individuals to ecosystems, often with negative implications for ecosystem functions and services. Furthermore, as climate change fuels more frequent and severe extreme climate events (ECEs) like marine heatwaves (MHWs), such acute events become increasingly important drivers of rapid ecosystem change. However, our understanding of ECE impacts is hampered by limited collection of broad scale in situ data where such events occur. In 2011, a MHW known as the Ningaloo Niño bathed the west coast of Australia in waters up to 4°C warmer than normal summer temperatures for almost 2 months over 1000s of kilometers of coastline. We revisit published and unpublished data on the effects of the Ningaloo Niño in the seagrass ecosystem of Shark Bay, Western Australia (24.6–26.6° S), at the transition zone between temperate and tropical seagrasses. Therein we focus on resilience, including resistance to and recovery from disturbance across local, regional and ecosystem-wide spatial scales and over the past 8 years. Thermal effects on temperate seagrass health were severe and exacerbated by simultaneous reduced light conditions associated with sediment inputs from record floods in the south-eastern embayment and from increased detrital loads and sediment destabilization. Initial extensive defoliation of Amphibolis antarctica, the dominant seagrass, was followed by rhizome death that occurred in 60–80% of the bay's meadows, equating to decline of over 1,000 km2 of meadows. This loss, driven by direct abiotic forcing, has persisted, while indirect biotic effects (e.g., dominant seagrass loss) have allowed colonization of some areas by small fast-growing tropical species (e.g., Halodule uninervis). Those biotic effects also impacted multiple consumer populations including turtles and dugongs, with implications for species dynamics, food web structure, and ecosystem recovery. We show multiple stressors can combine to evoke extreme ecological responses by pushing ecosystems beyond their tolerance. Finally, both direct abiotic and indirect biotic effects need to be explicitly considered when attempting to understand and predict how ECEs will alter marine ecosystem dynamics.

The development of early warning indicators that identify ecosystem stress is a priority for improving ecosystem management. As microbial communities respond rapidly to environmental disturbance, monitoring their composition could prove one such early indicator of environmental stress. We combined 16S rRNA gene sequencing of the seagrass root microbiome of Halophila ovalis with seagrass health metrics (biomass, productivity and F_sulphide) to develop microbial indicators for seagrass condition across the Swan-Canning Estuary and the Leschenault Estuary (south-west Western Australia); the former which had experienced an unseasonal rainfall event leading to declines in seagrass health. Microbial indicators detected sites of potential stress that other seagrass health metrics failed to detect. Genera that were more abundant in ‘healthy’ seagrasses included putative methylotrophic bacteria (e.g. Methylotenera and Methylophaga), iron cycling bacteria (e.g. Deferrisoma and Geothermobacter) and N₂ fixing bacteria (e.g. Rhizobium). Conversely, genera that were more abundant in ‘stressed’ seagrasses were dominated by putative sulphur-cycling bacteria, both sulphide-oxidising (e.g. Candidatus Thiodiazotropha and Candidatus Electrothrix) and sulphate-reducing (e.g. SEEP-SRB1, Desulfomonile and Desulfonema). The sensitivity of the microbial indicators developed here highlights their potential to be further developed for use in adaptive seagrass management, and emphasises their capacity to be effective early warning indicators of stress.