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 ...

Global seagrass losses parallel significant declines observed in corals and mangroves over the past 50 years. These combined declines have resulted in accelerated global losses to ecosystem services in coastal waters. Seagrass meadows can be extensive (hundreds of square kilometers) and longlived (thousands of years), with the meadows persisting predominantly through vegetative (clonal) growth. They also invest a large amount of energy in sexual reproduction. In this article, we explore the role that sexual reproduction, pollen, and seed dispersal play in maintaining species distributions, genetic diversity, and connectivity among seagrass populations. We also address the relationship between long-distance dispersal, genetic connectivity, and the maintenance of genetic diversity that may enhance resilience to stresses associated with seagrass loss. Our reevaluation of seagrass dispersal and recruitment has altered our perception of the importance of long-distance dispersal and has revealed extensive dispersal at scales much larger than was previously thought possible.

Summary1. Extreme climatic events will dictate the response of ecosystems to climate change, yet are understudied in marine ecosystems. The interaction of stressors from such events has the potential to amplify negative impacts and drive ecosystems into alternate states. 2. Here, we show a drastic response of a temperate seagrass species (Amphibolis antarctica) in Shark Bay -a World Heritage Site in Western Australia at a temperate-tropical transition zone -to two stressors driven by concurrent extreme climatic events: a marine heatwave (Ningaloo Niña) and the Gascoyne floods that impacted the west coast of Australia in the austral summer of 2010-2011. 3. Widespread defoliation (leaf loss) of A. antarctica was observed in the months following the extreme events and was highest at sites affected by flooding (Wooramel River floods). We propose that the negative impact was magnified by the synergistic interactions both stressors had on the carbon balance of the plant. The elevated temperatures increased plant demand for carbon, which could not be met through photosynthesis due to turbid floodwaters reducing light availability, resulting in the plant having a negative carbon balance. 4. Two years following the extreme events, recovery of leaf biomass was evident, though still 7-20% of historical averages. In contrast, below-ground biomass decreased by an order of magnitude in the two years following the events. As below-ground reserves underpin the tolerance of large seagrass species like A. antarctica to disturbances, the declining trajectory of below-ground biomass will likely manifest as a loss of resilience in A. antarctica to future disturbances. 5. Synthesis. Given the ecological importance of Amphibolis antarctica in Shark Bay as a foundation species -accounting for 85% (~3700 km 2 ) of the cover of seagrasses in Shark Bay -predicted increases in the frequency and magnitude of similar climatic events could have catastrophic implications for the future of this World Heritage embayment. Where extreme climatic events overlap and cause multiple, synergistic stressors to plant communities, ecological responses are likely to be more extreme, particularly in ecosystems where foundation species exist near upper thermal tolerance limits.

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.

Mesophotic coral ecosystems (MCEs) occur at depths beyond those typically associated with coral reefs. Significant logistical challenges associated with data collection in deep water have resulted in a limited understanding of the ecological relevance of these deeper coral ecosystems. We review the trends in this research, covering the geographic spread of MCE research, the focus of these studies, the methods used, how MCEs differ in terms of species diversity and begin to assess connectivity of coral populations. Clear locational biases were observed, with studies concentrated in a few discrete areas mainly around the Atlantic region. The focus of MCE studies has diversified in recent years and more detailed aspects of MCE ecology are now being investigated in particular areas of research. Advances in technology are also reflected in the current range of research, with a wider variety of methods now employed. However, large information gaps are present in entire regions and particularly in relation to the threats, impacts and subsequent management of MCEs. Analysis of species diversity shows that initial definitions based on depth alone may not be appropriate globally, while further taxonomic resolution may also be required to deduce the full biodiversity of major groups in certain regions. Genetic studies to date show species-specific results, although distinct deeper populations do appear to exist, which raises questions regarding the potential of MCEs to act as refugia.

Accurate estimation of connectivity among populations is fundamental for determining the drivers of population resilience, genetic diversity, adaptation and speciation. However the separation and quantification of contemporary versus historical connectivity remains a major challenge. This review focuses on marine angiosperms, seagrasses, that are fundamental to the health and productivity of temperate and tropical coastal marine environments globally. Our objective is to understand better the role of sexual reproduction and recruitment in influencing demographic and genetic connectivity among seagrass populations through an integrated multidisciplinary assessment of our present ecological, genetic, and demographic understanding, with hydrodynamic modelling of transport. We investigate (i) the demographic consequences of sexual reproduction, dispersal and recruitment in seagrasses, (ii) contemporary transport of seagrass pollen, fruits and seed, and vegetative fragments with a focus on hydrodynamic and particle transport models, and (iii) contemporary genetic connectivity among seagrass meadows as inferred through the application of genetic markers. New approaches are reviewed, followed by a summary outlining future directions for research: integrating seascape genetic approaches; incorporating hydrodynamic modelling for dispersal of pollen, seeds and vegetative fragments; integrating studies across broader geographic ranges; and incorporating non-equilibrium modelling. These approaches will lead to a more integrated understanding of the role of contemporary dispersal and recruitment in the persistence and evolution of seagrasses.

A feedback between seagrass presence, suspended sediment and benthic light can induce bistability between two ecosystem states: one where the presence of seagrass reduces suspended sediment concentrations to increase benthic light availability thereby favoring growth, and another where seagrass absence increases turbidity thereby reducing growth. This literature review identifies (1) how the environmental and seagrass meadow characteristics influence the strength and direction (stabilizing or destabilizing) of the seagrass-sediment-light feedback, and (2) how this feedback has been incorporated in ecosystem models proposed to support environmental decision making. Large, dense seagrass meadows in shallow subtidal, noneutrophic systems, growing in sediments of mixed grain size and subject to higher velocity flows, have the greatest potential to generate bistability via the seagrass-sediment-light feedback. Conversely, seagrass meadows of low density, area and height can enhance turbulent flows that interact with the seabed, causing water clarity to decline. Using a published field experiment as a case study, we show that the seagrass-sedimentlight feedback can induce bistability only if the suspended sediment has sufficient light attenuation properties. The seagrass-sediment-light feedback has been considered in very few ecosystem models. These models have the potential to identify areas where bistability occurs, which is information that can assist in spatial prioritization of conservation and restoration efforts. In areas where seagrass is present and bistability is predicted, recovery may be difficult once this seagrass is lost. Conversely, bare areas where seagrass presence is predicted (without bistability) may be better targets for seagrass restoration than bare areas where bistability is predicted.

Understanding the extent and impact of factors influencing the levels and structuring of genetic diversity within natural populations is a key objective of ecological genetics. For marine angiosperms, variation in abiotic environmental factors at the local scale can have a major influence on levels of clonality and spatial genetic structure, and thus influence mating systems, sexual reproduction, and recruitment. Identifying the key drivers of genetic structuring is critical for genetic management of ecological restoration success, especially in systems where the nature and extent of clonality is highly variable. Here, we quantify clonality and patterns of genetic structure in the temperate Australian seagrass Posidonia australis. We examine the location of meadows in relation to water movement and prevailing winds to assess their relative influence on local spatial genetic structuring. Measures of genetic diversity, assessed with 7 polymorphic microsatellite loci, were highly variable across 13 meadows sampled within and around a natural embayment on the west coast of Australia. The overall structure of P. australis meadows across this region is best explained as one of 'chaotic' genetic patchiness, with significant differentiation among most meadows (pairwise F ST values), high levels of genetic diversity in meadows that are in more open waters, and lower genetic diversity at inshore sites facing strong prevailing winds at the time of seed dispersal or that have little water movement. A strong isolation by distance relationship within the embayment is consistent with prevailing winds (which create surface currents) at the time of peak pollen and seed release, strongly influencing dispersal direction.