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Extreme climatic events including marine heatwaves (MHWs) are becoming more frequent and severe in the Anthropocene. However, our understanding of how these events affect population dynamics of ecologically important species is limited, in part because extreme events are rare and difficult to predict. Here, we quantified the occurrence and severity of MHWs over 60 years in warm range edge kelp forests on both sides of the North Atlantic. The cumulative annual intensity of MHWs increased two-to four-fold during this period, coinciding with the disappearance of kelps. We experimentally demonstrated a relationship between strong and severe 2018 heatwaves and high kelp mortality in both regions. Patterns of kelp mortality were strongly linked to maximum temperature anomalies, which crossed lethal thresholds in both regions. translocation and tagging experiments revealed similar kelp mortality rates on reefs dominated by healthy kelp forests and degraded sediment-laden algal 'turfs', indicating equal vulnerability to extreme events. These results suggest a mechanistic link between MHWs and broad-scale kelp loss, and highlight how warming can make ecosystem boundaries unstable, forcing shifts to undesirable ecosystem states under episodically extreme climatic conditions. Extreme events may pose a stronger, more immediate threat to ecosystem function than shifts in average conditions, and are increasingly manifesting as key drivers of ecosystem reconfiguration as environmental conditions become more variable and extremes more frequent with climate change 1-3. Discrete periods of anomalously high ocean temperatures, known as "marine heatwaves" (MHWs) 4 , can have serious consequences for ecosystems, and are often associated with loss of ecological function and services 5,6. The increased severity and frequency of MHWs threaten biodiversity and ecosystem function on global scales 5 and the total number of MHW days per year has increased by > 50% in recent decades 1,7,8. Yet, the ecological consequences of extreme events such as MHWs have been identified as a key knowledge gap in ecology 9,10 , and our understanding of population dynamics during MHWs is limited 5. This is partly because these processes are not often studied in the field because MHWs are difficult to predict, and their impacts challenging to manipulate in situ 11,12. Compounding these problems, until recently, most ecological research on MHWs has been conducted on local scales using varying definitions of extremity, making broadscale measures and comparisons of the ecological effects of MHWs challenging 4,13. Temperature extremes appear to be particularly damaging for species located at their range edge, because they can abruptly push conditions beyond thermal tolerances, causing direct mortality 5,14. Kelps are cool-water species that are broadly distributed and respond strongly to changing abiotic conditions 15. Temperature is the most important factor controlling the range distribution of kelps 16-18 , and kelp forests in many regions have been shown to ...

Chondrus crispus Stackhouse alternates between two isomorphic life history phases that differ in cellwall phycocolloid composition. It has been long hypothesized that the gametophyte, with strong-gelling kappa-type carrageenans, is mechanically superior to the tetrasporophyte, with nongelling lambda-type carrageenans, which could contribute to the observed gametophytic dominance in many wave-swept environments. Standard mechanical tests were performed on distal tissues of C. crispus sampled from a range of environments in Narragansett Bay, Rhode Island, using a tensometer equipped with a video extensometer. Life history phase was by far the most important determinant of mechanical properties, whereas environmental factors had only modest influence (vertical distribution) or no effect (exposure); gametophytic distal tissues were 43% stronger, 21% more extensible, and 21% stiffer than tetrasporophytic distal tissues. However, the superior strength of gametophytic tissues was not evident at the stipe/holdfast junction (where breakage typically occurs), and the two phases were equally susceptible to dislodgment by a given force. The primary ecophysiological role of carrageenans in C. crispus may not be the provision of a structure to resist wave action.

Humans are rapidly transforming the structural configuration of the planet's ecosystems, but these changes and their ecological consequences remain poorly quantified in underwater habitats. Here, we show that the loss of forest-forming seaweeds and the rise of ground-covering 'turfs' across four continents consistently resulted in the miniaturization of underwater habitat structure, with seascapes converging towards flattened habitats with smaller habitable spaces. Globally, turf seascapes occupied a smaller architectural trait space and were structurally more similar across regions than marine forests, evidencing habitat homogenization. Surprisingly, such habitat convergence occurred despite turf seascapes consisting of vastly different species richness and with different taxa providing habitat architecture, as well as across disparate drivers of marine forest decline. Turf seascapes contained high sediment loads, with the miniaturization of habitat across 100s of km in mid-Western Australia resulting in reefs retaining an additional ~242 million tons of sediment (four orders of magnitude more than the sediments delivered fluvially annually). Together, this work demonstrates that the replacement of marine forests by turfs is a generalizable phenomenon that has profound consequences for the ecology of temperate reefs. | 5263 PESSARRODONA Et Al.

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