Intra‐ and Inter‐Decadal Scale Variability in Kelp Population Structure Reveal Both Stability and Decline in a Critical Foundation Species

Kelp forests support high biodiversity, primary productivity and ecosystem service value but exhibit high levels of spatiotemporal variability, which can complicate management and conservation efforts. A better understanding of multiscale temporal variability is needed to improve understanding of kelp forest dynamics. Location: West Scotland and southwest England, United Kingdom. Methods: We utilised high-resolution quantitative historical records from the 1940s and 1980s, combined with robust multiyear contemporary field surveys, to explore intra- and inter-decadal scale variability in population structure of the kelp Laminaria hyperborea in two regions of the UK (‘cool’ conditions in west Scotland, versus ‘warm’ conditions in southwest England). Results: Over decadal timescales, the abundance (i.e., density, standing stock) of a kelp population in southwest England declined, which aligns with a concurrent increase in sea temperatures above the optimum for the performance of this species. Conversely, a population in western Scotland exhibited stability, with plant density and size remaining consistent over multiple decades. Over shorter timescales (across 10 years of contemporary surveys), kelp populations in both regions exhibited minimal temporal variability, in terms of plant density, cover and standing stock, indicating that this species underpins stable and abundant habitat. Main Conclusions: Compared to many kelp ecosystems globally, and contrary to the widely accepted highly dynamic nature of these habitats, L. hyperborea populations exhibited marked stability across years and (in the case of west Scotland) decades, although gradual ocean warming is likely eroding the resilience of populations in warmer waters located towards this species upper thermal limit. While historical evidence allowed us to reconstruct ecological baselines, patterns must be interpreted with caution as the number of available sites was very limited. Improved monitoring of these vital coastal habitats is needed to better understand patterns and drivers of temporal variability over multiple time scales.