A demographic comparison of the range-expanding kelp Laminaria ochroleuca across its leading edge

Kelp forests are globally-significant habitats, which underpin a range of critical ecosystem services. The distribution, population structure and ecophysiological performance of kelp species is, however, strongly constrained by temperature and recent ocean warming trends have been linked with widespread ecological changes. Laminaria ochroleuca is a relatively warm-adapted species, distributed from the Mediterranean and Morrocco polewards to the UK and Ireland, which has seemingly proliferated at the leading range edge in recent decades in response to ocean warming. We examined population demography and plant morphology of Laminaria ochroleuca across a gradient of 15° in latitude and ∼5 °C in average sea surface temperature, to explore shifts in ecological structure across range centre and contemporary/historical leading edge populations. Contrary to our predictions, marginal populations located at the extreme leading edge (County Mayo, Ireland) and previously-accepted range edge (Lundy, north Devon, UK) did not exhibit lower population density and/or plant size than those further south at a historical range edge (Plymouth Sound, UK) or towards the range centre (Peniche, Portugal). Rather, some between-location differences were recorded, with higher density/cover of subcanopy plants and higher grazing/tissue damage observed at Peniche (Portugal), and higher epiphyte loading at County Mayo (Ireland). Laminaria ochroleuca was a dominant and conspicuous component of seaweed assemblages at all sites, where it likely plays an important ecological role. Expanded surveys and continued monitoring across a wider range of environmental conditions and sites is needed to assess its utility as an ecological indicator of ocean warming in coastal habitats.