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Canopy-forming algae characterise most temperate coasts (e.g. kelp forests pictured on the left), but some forests have been replaced by extensive covers of turf-forming algae (e.g. the Adelaide metropolitan coast on the right). Models that account for this switch from topographically complex and productive habitat to simpler and less productive habitat are needed if we are predict or manage against future change. Photographs by S.D.C.  

Canopy-forming algae characterise most temperate coasts (e.g. kelp forests pictured on the left), but some forests have been replaced by extensive covers of turf-forming algae (e.g. the Adelaide metropolitan coast on the right). Models that account for this switch from topographically complex and productive habitat to simpler and less productive habitat are needed if we are predict or manage against future change. Photographs by S.D.C.  

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There is concern about historical and continuing loss of canopy-forming algae across the world's temperate coastline. In South Australia, the sparse cover of canopy-forming algae on the Adelaide metropolitan coast has been of public concern with continuous years of anecdotal evidence culminating in 2 competing views. One view considers that current...

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... A wide range of factors are responsible for global kelp forest decline, with most of them being the result of anthropogenic activities [10]. Increasing ocean temperature is a global stressor that is negatively influencing the distribution and productivity of kelp dominated ecosystems at the warm leading edges of their range [11][12][13]. Temperature plays a pivotal role in the physiology, ecology and the geographical distribution of kelp forest species. However, the mechanistic effects by which temperature exerts this controlling effect is often poorly understood [14,15]. ...
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Rising ocean temperature is a major driver of kelp forest decline worldwide and one that threatens to intensify over the coming decades. What is not particularly well understood are the mechanisms that drive loss and how they operate at differing life stages. This study aimed to establish an understanding of the effects of increasing temperature on the early developmental stages of the giant kelp, Macrocystis pyrifera . Sporulation was carried out across 10 temperature treatments from 9.5 to 26.2°C ± 0.2°C at approximately 2°C intervals. Spores were then incubated at these temperatures under a 20.3±1.7 μmol photons m ⁻² s ⁻¹ , 16L:8D photoperiod for 5 days. Results indicate that spore release was positively correlated with increasing temperature, whereas an inverse trend was observed between temperature and the growth of germ-tube. The thermal threshold for spore and germling development was determined to be between 21.7°C and 23.8°C. Spore settlement was the most drastically effected developmental phase by increasing temperature. This study highlights the vulnerability of early life stages of M . pyrifera development to rising ocean temperature and has implications for modelling future distribution of this valuable ecosystem engineer in a changing ocean.
... In seagrass meadows, light attenuation associated with suspended sediment inhibits leaf growth and increases seagrass mortality (Moore et al., 1997;Terrados et al., 1999;Zabarte-Maeztu et al., 2020). Kelp beds and forests are likewise affected by suspended sediments, but deposited sediments can directly smother juvenile and adult kelps (Neushul et al., 1976;Roleda and Dethleff, 2011) as well as indirectly make the environment more suitable for sediment-tolerant turf algae that can outcompete kelps (Connell et al., 2008;Moy and Christie, 2012;Filbee-Dexter and Wernberg, 2018). ...
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... Multiple factors can cause macroalgal deforestation. For example, declines in Australia have been caused by eutrophication, overgrazing, warming, and marine heatwaves [30,274,[304][305][306][307]; declines in the inner basin of the Salish Sea have been caused by elevated temperature, lower nutrient concentrations, and generally low current velocities [308]. In the time since the first bleaching event was observed off the coast of Jeju Island, South Korea, many macroalgal forests have been reduced by the seawalls constructed to protect reclaimed land, coastal pollutants accumulated during recent decades, and warm lownutrient water from the Kuroshio Extension [7,309]. ...
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... Multi-habitat reef restoration: a case-study A multi-habitat restoration is underway along South Australia's most urbanised coastline (Adelaide's metropolitan coast). Over the last 200 years, this coastline's seafloor has experienced considerable re-structuring (Tanner, 2005); overfishing eradicated oyster reefs from 1,500 km of coastline (Alleway and Connell, 2015), and extensive seagrass meadows (6,200 ha) and kelp forests (70 km) were lost due to anthropogenic nutrient inputs (Connell et al., 2008;Tanner et al., 2014). In recent decades, water quality improvements have resulted in net recovery of 11,000 ha of seagrass in water deeper than 10 m (Fernandes et al., 2022). ...
... Bringing diverse research and management expertise together to inform these reef restorations (McAfee et al., 2022) helped realise the opportunities for recovering multiple lost habitats. For example, these reefs provide suitable substrata to also recover lost kelp forests (Ecklonia radiata; Connell et al., 2008), and they reduce hydrodynamic energy and stabilize sediment around the reef base, providing conditions for seagrass recovery. Such hydrodynamic dampening likely underpins the positive interaction between seagrass and oyster reefs commonly observed at restorations elsewhere (e.g., Milbrandt et al., 2015;Sharma et al., 2016; Figure 1). ...
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... Across subtropical and temperate Australia, the laminarian kelp Ecklonia radiata is a major provider of habitat on reefs down to 40-m depth (Marzinelli et al., 2015;Wernberg et al., 2011;Wernberg et al., 2019). In recent decades, Ecklonia kelp forests in mid-and high-latitude regions in Australia have been losing ground to turf macroalgae and corals due to progressive warming, marine heatwaves and the migration of tropical grazers increasing consumer pressure on kelp biomass (Bennett, Wernberg, De Bettignies, et al., 2015;Connell et al., 2008;Vergés et al., 2016;Wernberg et al., 2016). The temperate coast of Western Australia (WA), where coastal and offshore reefs are particularly influenced by the southward-flowing Leeuwin Current, has experienced one of the most significant kelp losses recorded. ...
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... In many places throughout the world, habitatforming macroalgae are being lost due to various, often local-scale stressors (Krumhansl et al. 2016). Persistent large-scale losses of native macroalgae have occurred in North America (Steneck et al. 2002), Europe (Airoldi and Beck 2007), Japan (Watanuki et al. 2010) and Australia (Connell et al. 2008;Coleman et al. 2008). Abiotic disturbances such as increased temperature, sedimentation, or eutrophication can often result in native macroalgae being replaced by smaller turf-forming algae (Airoldi and Beck 2007;Connell et al. 2008;Filbee-Dexter and Wernberg 2018). ...
... Persistent large-scale losses of native macroalgae have occurred in North America (Steneck et al. 2002), Europe (Airoldi and Beck 2007), Japan (Watanuki et al. 2010) and Australia (Connell et al. 2008;Coleman et al. 2008). Abiotic disturbances such as increased temperature, sedimentation, or eutrophication can often result in native macroalgae being replaced by smaller turf-forming algae (Airoldi and Beck 2007;Connell et al. 2008;Filbee-Dexter and Wernberg 2018). Alternatively, the bare space created by the loss of native macroalgae can provide an opportunity for colonization by invasive or non-native macroalgae (Valentine and Johnson 2003). ...
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The green macroalga Caulerpa filiformis has been spreading on shallow soft sediment habitats along the Peruvian coast, colonizing previously unvegetated sediments to create monospecific meadows. We examined the nature of the impact of C. filiformis meadows on the density, taxonomic richness and assemblage structure of epifaunal and infaunal benthic macroinvertebrates. Specifically, we tested whether the spread of C. filiformis has resulted in different macroinvertebrate assemblages than those formed by the dominant native macroalgae (i.e., Rhodymenia spp.) and unvegetated sediments. Surveys were undertaken in two bays in each of two locations, in central and southern Peru, during winter 2017 and summer 2018. In general, our results show that macroinvertebrate assemblages were similar across all three habitats, although there were some differences , related to location and time, but with no clear patterns observed. Taxonomic richness and density was generally higher in the vegetated habitats than the unvegetated habitat, and where there were differences between the two vegetated habitats there was no consistent pattern of which habitat supported the highest richness or density. Given invading C. fili-formis is primarily colonizing unvegetated habitats it would appear that this species is creating a new niche which supports similar assemblages, but higher taxo-nomic richness and density than unvegetated habitats. While our study suggests that C. filiformis is having a limited ecological impact we recommend that actions be put in place to limit the spread of this invasive species at the same time as increasing monitoring of the ecological impacts of this species as lags in the ecological impacts of invasive species are common.
... Early declines of kelp forests in the 1800s have been linked to population expansion of sea urchins, most often facilitated by the removal of urchin predators from the ecosystem (Roberts, 2007). Subsequent kelp population declines in the 20 th century were driven by threats such as direct harvest of kelp or high levels of water pollution from urban areas (Wilson and North, 1983;Vogt and Schramm, 1991;Coleman et al., 2008;Connell et al., 2008). ...
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This chapter concentrates on understanding whether restoration is needed and appropriate. It presents a set of core questions to help address this decision-making process, including: Have kelp forests declined in your area, and if so, why? Is restoration achievable, given the cause of kelp decline and the resources available? And what are the opportunities, risks, and challenges of restoration?
... Early declines of kelp forests in the 1800s have been linked to population expansion of sea urchins, most often facilitated by the removal of urchin predators from the ecosystem (Roberts, 2007). Subsequent kelp population declines in the 20 th century were driven by threats such as direct harvest of kelp or high levels of water pollution from urban areas (Wilson and North, 1983;Vogt and Schramm, 1991;Coleman et al., 2008;Connell et al., 2008). ...
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This chapter outlines the concept of ‘future-proofing’ in restoration, which encompasses a range of novel approaches in response to kelp loss due to ongoing climate change. These situations are complex because the driver of kelp forest decline cannot be directly ameliorated, and so innovative solutions are needed to boost resilience and optimise restoration success.
... Early declines of kelp forests in the 1800s have been linked to population expansion of sea urchins, most often facilitated by the removal of urchin predators from the ecosystem (Roberts, 2007). Subsequent kelp population declines in the 20 th century were driven by threats such as direct harvest of kelp or high levels of water pollution from urban areas (Wilson and North, 1983;Vogt and Schramm, 1991;Coleman et al., 2008;Connell et al., 2008). ...
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This chapter outlines the motivations and methods for monitoring and evaluating kelp restoration efforts, including what to consider before monitoring; important considerations when designing and implementing a monitoring program; and common methods for collecting kelp forest monitoring data.
... Early declines of kelp forests in the 1800s have been linked to population expansion of sea urchins, most often facilitated by the removal of urchin predators from the ecosystem (Roberts, 2007). Subsequent kelp population declines in the 20 th century were driven by threats such as direct harvest of kelp or high levels of water pollution from urban areas (Wilson and North, 1983;Vogt and Schramm, 1991;Coleman et al., 2008;Connell et al., 2008). ...
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Despite a relatively long history in places like Japan, Korea, and California, the science and practice of kelp forest restoration is still in its infancy, and there remains a wealth of knowledge to be learned and shared from our collective failures and successes. To date, many projects have remained disconnected and had limited opportunities to share their experiences and learnings. The practice of kelp forest restoration will be greatly enhanced with collaborative, science-based efforts, where all stakeholders and custodians are engaged in decision-making and even short-term failures can yield insights that contribute to longer-term success. The development of the Kelp Restoration Guidebook was informed by a series of global workshops, and an expert panel of authors and editors, with the aim to share and distil lessons learned from kelp restoration efforts globally. The intent is for this guidebook to serve as a starting point for practitioners, researchers, managers, and custodians to learn about the steps of restoration and access an active community of practice—all to improve the likelihood of success for future restoration projects. The broad lessons contained herein can then be extended and refined to suit local kelp species and circumstances. Ultimately, by cultivating an alliance of kelp forest restoration practitioners around the world, we can work together to ensure that kelp forests flourish in our planet’s changing seas.