Rapid collapse of extensive kelp forests and a regime shift to tropicalized temperate reefs followed extreme heatwaves and decades of gradual warming. Abstract:Ecosystem reconfigurations arising from climate driven changes in species distributions are expected to have profound ecological, social and economic implications. Here, we reveal a rapid climate driven regime shift of Australian temperate reef communities, which lost their defining kelp forests and became dominated by persistent seaweed turfs. Following decades of ocean warming, extreme marine heatwaves forced a 100 km range contraction of extensive kelp forests, and saw temperate species replaced by seaweeds, invertebrates, corals and fishes characteristic of subtropical and tropical waters. This community wide tropicalization fundamentally altered key ecological processes, suppressing the recovery of kelp forests. Main Text:Broad scale losses of species which provide the foundations for habitats cause dramatic shifts in ecosystem structure because they support core ecological processes (1-3). Such habitat loss can lead to a regime shift where reinforcing feedback mechanisms intensify to provide resilience to an alternate community configuration, often with profound ecological, social and economic consequences (4-6). Benthic marine regime shifts have been associated with the erosion of ecological resilience through overfishing or eutrophication, altering the balance between consumers and resources, rendering ecosystems vulnerable to major disturbances (1, 2,6,7). Now, climate change is also contributing to the erosion of resilience (8,9), where increasing temperatures are modifying key physiological, demographic and community scale processes (8, 10), driving species redistribution at a global scale and rapidly breaking down long-standing biogeographic boundaries (11,12). These processes culminate in novel ecosystems where tropical and temperate species interact with unknown implications (13). Here we document how a marine heatwave caused the loss of kelp forests across ~2,300 km 2 of Australia's Great Southern Reef, forcing a regime shift to seaweed turfs. We demonstrate a rapid 100 km rangecontraction of kelp forests and a community-wide shift toward tropical species with ecological processes suppressing kelp forest recovery.To document ecosystem changes we surveyed kelp forests, seaweeds, fish, mobile invertebrates and corals at 65 reefs across a ~2,000 km tropical to temperate transition zone in western Australia (14). Surveys were conducted between 2001 to 2015, covering the years before and after an extreme marine heatwave impacted the region.The Indian Ocean adjacent to western Australia is a 'hotspot' where the rate of ocean warming is in the top 10% globally (15), and isotherms are shifting poleward at a rate of 20 -50 km per decade (16). Until recently, kelp forests were dominant along >800 km of the west coast (8), covering 2,266 km 2 of rocky reefs between 0 -30 m depth south of 27.7°S (Fig. 1). Kelp forests along the midwest section of this ...

Article type: Letter 41 42 One Sentence Summary: Marine heatwaves alter ecosystem structure and functioning at 43 global scales. 44 45 46 47 48 49 implications for marine ecosystems 1 . Concurrent with long-term persistent warming, 50 discrete periods of extreme regional ocean warming (marine heatwaves, 'MHWs') have 51 increased in frequency 2 . Here we quantify trends and attributes of MHWs across all 52 ocean basins and examine their biological impacts from species to ecosystems. Multiple 53 regions within the Pacific, Atlantic and Indian Oceans are particularly vulnerable to 54 MHW intensification, due to the co-existence of high levels of biodiversity, a prevalence 55 of species found at their warm range edges, or concurrent non-climatic human impacts. 56 The physical attributes of prominent MHWs varied considerably, but all had 57 deleterious impacts across a range of biological processes and taxa, including critical 58 foundation species (corals, seagrasses and kelps). MHWs, which will likely intensify 59 with anthropogenic climate change 3 , are rapidly emerging as forceful agents of 60 disturbance with the capacity to restructure entire ecosystems and disrupt the provision 61 of ecological goods and services in coming decades. 62 63 Anthropogenic climate change is driving the redistribution of species and reorganization of 64 natural systems and represents a major threat to global biodiversity 4,5 . The biosphere has 65 401 working group on marine heatwaves (www.marineheatwaves.org).

Extreme climatic events, such as heat waves, are predicted to increase in frequency and magnitude as a consequence of global warming but their ecological effects are poorly understood, particularly in marine ecosystems. In early 2011, the marine ecosystems along the west coast of Australia - a global hotspot of biodiversity and endemism - experienced the highest-magnitude warming event on record. Sea temperatures soared to unprecedented levels and warming anomalies of 2-4C persisted for more than ten weeks along \u3e2,000 km of coastline. We show that biodiversity patterns of temperate seaweeds, sessile invertebrates and demersal fish were significantly different after the warming event, which led to a reduction in the abundance of habitat-forming seaweeds and a subsequent shift in community structure towards a depauperate state and a tropicalization of fish communities. We conclude that extreme climatic events are key drivers of biodiversity patterns and that the frequency and intensity of such episodes have major implications for predictive models of species distribution and ecosystem structure, which are largely based on gradual warming trends

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