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Kelp and macroalgal forests provide the ecological foundations of many temperate rocky reef ecosystems, but have regionally declined, often due to sea urchin overgrazing and the formation of urchin barrens. Sea urchin removal has long been used to investigate kelp-sea urchin dynamics and is increasingly being promoted for kelp forest restoration. In this review, we assess the methods and outcomes of sea urchin removal experiments to evaluate their potential use and feasibility as a tool for restoring macroalgal forests. Seventy-nine sea urchin removal projects were reviewed from temperate subtidal rocky reef systems between 1975 and 2020. Removal methods were often not reported (35%), but included manual culling, including crushing (25%) and chemical application (quicklime, 9%), or relocating sea urchins (13%). Only a small percentage of removals were large in scale (16% > 10 ha) and 92% of these utilized culling. Culling is often the most practical method of urchin removal, but all methods can be effective and we encourage development of new approaches that harvest and utilize low-quality urchins. Urchin removal led to an increase in macroalgae in 70% of studies, and a further 21% showed partial increases (e.g., at one or more sites or set of conditions). Restoration effectiveness is increased by removing essentially all sea urchins from discrete areas of urchin barrens. Sea urchin removal provides a simple, relatively cheap, and effective method that promotes kelp recovery within urchin barrens. However, sea urchin removal does not address the underlying cause of elevated sea urchin populations and is unlikely to provide a long-term solution to restore kelp forests and full ecosystem function on its own. We therefore suggest that if sea urchin removal is considered as a tool for kelp forest restoration, it should be incorporated with other management measures that aim to increase kelp forest resilience and biodiversity (e.g., marine protected areas, predator protection or enhancement). This will ensure that kelp restoration efforts have the greatest ecological, socio-economic and cultural outcomes in the long-term.

Global climate change, caused by anthropogenic activities, is having significant impacts on species, ecosystems and human live-

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Increasing turbidity in coastal waters reduces benthic irradiance levels and consequently poses a fundamental threat to benthic primary producers and the food webs they support. It is therefore necessary to better understand how important primary producers, such as kelp, can adapt both physiologically and morphologically to these declining light conditions. This study examines how variation in turbidity influences the morphology of the kelp Ecklonia radiata, and how this relates to photosynthetic performance, by measuring multiple morphological characteristics, photosynthesis‐irradiance response, and photosynthetic pigments in adult kelp across a large‐scale turbidity gradient. Kelp morphology was strongly related to variation in benthic irradiance among sites, but this was not consistent with morphological acclimation. Kelp at the most turbid sites had short stipes (< 0.4 m) and lower surface area to volume ratios (SA:V) than kelp at moderate to high‐light sites, which had long stipes (~ 1 m) and high SA:V. Photosynthetic parameters and pigment content also varied considerably in relation to benthic irradiance, with increased photosynthetic efficiency and pigment content at low‐light sites. However, maximum rates of photosynthesis were highest at high‐light sites and strongly positively related to higher SA:V of kelp at these sites. These results demonstrate that while physiological adaptation may increase photosynthetic efficiency at low‐light sites, maximum photosynthetic output is strongly related to thallus morphology rather than cellular acclimation. This suggests that the large‐scale variation in kelp morphology observed across the turbidity gradient likely has important consequences for overall primary production as well as other important ecosystem functions.