Spatiotemporal variability in the population demography of the golden kelp, <i>Laminaria ochroleuca</i> (Phaeophyceae), at its leading range edge

Ocean warming is driving poleward range shifts for many marine species. For foundation organisms that underpin the wider ecosystem, such as canopy-forming seaweeds, shifts in distributions or population demography can have widespread ecological consequences. The warm water kelp, <i>Laminaria ochroleuca</i>, is found towards its leading edge in Plymouth Sound (southwest England, UK), where it has proliferated in recent years, partially replacing the cool-water congeneric species, <i>L. hyperborea</i>, at some wave-sheltered locations. To determine how ecosystem dynamics have changed due to this substitution and to benchmark population structure, monthly surveys were conducted of <i>L. ochroleuca</i> populations at three sites over 10 months. Canopy-forming <i>L. ochroleuca</i> sporophytes were recorded in all months at all sites, in densities of up to ~8 individuals m^‒2, indicating that this species is now a conspicuous space occupier within this ecosystem. Blade length, weight and fertility followed clear seasonal patterns peaking during late spring/early summer and rapidly declining in autumn/winter. Between-site variability was also observed, with greater densities, blade lengths/widths and standing stock biomass at the most wave-sheltered but tidally influenced site. The most abundant herbivore associated with <i>L. ochroleuca</i> was the blue rayed limpet, <i>Patella pellucida</i>, which was predominantly found in summer months and in greater abundance than reported values for other kelps in the region. Our survey wide estimates of carbon standing stock ranged from 408 ± 67 to 1006 ± 181 g C m^‒2 across sites, with a regional mean of 615 g C m^‒2, which was in line with previous estimates for the congeneric <i>L. hyperborea</i> at nearby sites. Overall, the most detailed demographic assessment of <i>L. ochroleuca</i> to date is provided, which allows for a greater understanding of its ecological role within the wider temperate ecosystem and serves as a robust baseline against which to detect future ecological changes. Kelp population demography at the leading range edge was highly variable.There was pronounced seasonality and site-level variation in demography.The warm-adapted <i>Laminaira ochroleuca</i> is now an abundant and conspicuous habitat-former. Kelp population demography at the leading range edge was highly variable. There was pronounced seasonality and site-level variation in demography. The warm-adapted <i>Laminaira ochroleuca</i> is now an abundant and conspicuous habitat-former.