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 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: Saccorhiza 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 probably 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.

Keywords: Saccorhiza polyschides; Furbelows; foundation species; habitat-former; kelp; macroalgae; marine forests; ocean warming; seasonality; seaweeds; temperate rocky reefs; wave exposure.

Fig. 1.

Survey location and morphological differences between sporophyte age classes of S. polyschides. (A) Map indicating region of study area in southwest England (grey box) and (B) location of three sites along a wave exposure gradient in Plymouth Sound. (C–E) Representative examples of the three age/size classes of S. polyschides: (C) juvenile recruits with a total length < 30 cm and incomplete development of bulbous holdfast; (D) healthy adult ‘plants’ with total length > 30 cm, complete development of closed, bulbous holdfast (bottom right), possible presence of fertile sporophyll tissue (top right) and some limited tissue loss at the distal tips of blades caused by grazing and wave action; (E) senescing, ‘old’ adults exhibiting partial or complete decay of blade, stipe and/or holdfast.

Fig. 2.

Spatiotemporal variability in density and cover of S. polyschides populations in intertidal habitats at three sites (with increasing levels of wave exposure from left, MB-sh, to right, HY-exp) in Plymouth Sound over 15 months. (A) Mean total density (± s.e.) and (B) mean density of each age class at each site and month during the intertidal survey. (C) Mean habitat percentage cover (± s.e.) of S. polyschides (differentiated between three age classes) and other kelps (Laminaria spp., Saccharina latissima, U. pinnatifida) recorded at each site and month.

Fig. 3.

Spatiotemporal variability in density and cover of S. polyschides populations in subtidal habitats. (A) Mean total density (± s.e.), and (B) mean density of age classes at each site (along a gradient of wave exposure) and month. (C) Mean habitat percentage cover (± s.e.) of S. polyschides (differentiated between three age classes) and other kelps (Laminaria spp., Saccharina latissima, U. pinnatifida). Note that subtidal survey months are not consecutive.

Fig. 4.

Spatiotemporal variability in biometric measurements from individuals sampled from intertidal S. polyschides populations. Mean total biomass (A, B) and total length (C, D) (± s.e.). Stacked bar plots indicate values for each structural tissue component.

Fig. 5.

Spatiotemporal variability in sorus biomass from intertidal S. polyschides populations. (A) Mean biomass (± s.e.) of fertile sorus tissue (wet weight) and (B) mean biomass of sorus associated with each structural component during main reproductive months.

Fig. 6.

Spatiotemporal variability in biometric measurements from individuals sampled from subtidal S. polyschides populations. Mean total biomass (A, B) and total length (C, D) (± s.e.). Stacked bar plots indicate values for each structural tissue component. Note that subtidal survey months are not consecutive.

Fig. 7.

Spatiotemporal variability of estimated mean standing stock biomass for S. polyschides populations in both intertidal (lines and symbols) and subtidal (bold symbols only) habitats.