Spatiotemporal variability in population demography and morphology of the habitat-forming macroalga Saccorhiza polyschides in the Western English Channel

Abstract

Background and Aims Large brown macroalgae serve as foundation organisms along temperate and polar coastlines, providing a range of ecosystem services. Saccorhiza polyschides is a warm-temperate kelp-like species found in the northeast Atlantic, which is suggested to have proliferated in recent decades across the southern United Kingdom (UK), possibly in response to increasing temperatures, physical disturbance, and reduced competition. However, little is known about S. polyschides with regard to ecological functioning and population dynamics across its geographical range. Here we examined the population demography of S. polyschides populations in southwest UK, located within the species’ range centre, to address a regional knowledge gap and to provide a baseline against which to detect future changes. Methods Intertidal surveys were conducted during spring low tides at three sites along a gradient of wave exposure in Plymouth Sound (Western English Channel) over a period of 15 months. Density, cover, age, biomass, and morphology of S. polyschides were quantified. Additionally, less frequent sampling of shallow subtidal reefs was conducted to compare intertidal and subtidal populations. Key Results We recorded pronounced seasonality, with fairly consistent demographic patterns across sites and depths. By late summer, S. polyschides was a dominant habitat-former on both intertidal and subtidal reefs, with maximum standing stock exceeding 13,000 g wet weight m-2. Conclusions S. polyschides is a conspicuous and abundant member of rocky reef assemblages in the region, providing complex and abundant biogenic habitat for associated organisms and high rates of primary productivity. However, its short-lived pseudo-annual life strategy is in stark contrast to dominant long-lived perennial Laminarian kelps. As such, any replacement or reconfiguration of habitat-forming macroalgae due to ocean warming will likely have implications for local biodiversity and community composition. More broadly, our study demonstrates the importance of high-resolution cross-habitat surveys to generate robust baselines of kelp population demography, against which the ecological impacts of climate change and other stressors can be reliably detected.