Spatial variation in the structure of overwintering, remnant Saccorhiza polyschides sporophytes and their associated assemblages

Nora Salland,Dan Smale

doi:10.1017/s0025315421000692

Abstract

AbstractUnderstanding the structure and richness of natural communities is a fundamental goal of marine ecology, and foundation species such as large macroalgae have a disproportionate role in structuring biodiversity. However, high-resolution information on assemblages associated with macroalgae is lacking for many species and regions. Saccorhiza polyschides is a warm-temperate kelp with a relatively short lifespan (12–18 months), large thallus and bulbous holdfast offering habitat for diverse assemblages. In the UK, S. polyschides populations are thought to have proliferated recently. Here, we quantified the density and habitat structure provided by S. polyschides along a gradient of wave exposure within Plymouth Sound, and examined the composition and diversity of associated faunal assemblages. Density varied significantly between sites but not by wave exposure, while biometric measurements were generally highly variable. Senescing holdfasts from sporophytes offered valuable habitat, with high abundance and richness of associated assemblages, although these varied markedly between sporophytes and sites. Faunal abundance, taxon richness and diversity were significantly higher at fully exposed sites than at moderately exposed sites. Internal volume of holdfasts was positively correlated with faunal abundance and taxon richness. We recorded more than 27 distinct taxa and up to ~600 individuals within a single holdfast. Taxa included three fish species, including a novel observation of the pipefish Nerophis lumbriciformis. Further work is needed to examine seasonality in habitat structure and associated diversity patterns but our study demonstrates that even remnant holdfasts from decaying sporophytes represent a valuable microhabitat that may provide shelter, protection and food during winter.

Highlights

Large macroalgae, such as kelp species, are dominant foundation organisms along much of the global coastline, where they provide complex biogenic habitat for a high diversity of associated flora and fauna (Steneck et al, 2002; Teagle et al, 2017)
The density of S. polyschides sporophytes varied considerably both between and within sites (Figure 2A), with sample-level density ranging from 0–5 individuals per m2
Taxon richness and Shannon diversity varied between moderately (JC, MB) and fully (BS, HB) wave-exposed sites, whereas sporophyte density, biomass and morphology did not differ between levels of wave exposure

Summary

Introduction

Large macroalgae, such as kelp species, are dominant foundation organisms along much of the global coastline, where they provide complex biogenic habitat for a high diversity of associated flora and fauna (Steneck et al, 2002; Teagle et al, 2017). Saccorhiza polyschides (Lightfoot) Batters 1902 is a ‘pseudo-kelp’ species found within several ecoregions (Spalding et al, 2007) in the wider North-east Atlantic, ranging from the Mediterranean and Morocco, to the UK, polewards to Norway and the Faroes (Norton, 1977; Smale et al, 2013). Saccorhiza polyschides is thought to be expanding its leading range edge polewards and is predicted to proliferate with continued ocean warming (Assis et al, 2018), whereas population declines and range contractions at the warm trailing edge have been reported (Smale, 2020 and references therein). Anecdotal evidence suggests that S. polyschides populations have proliferated in recent decades in the south-west of the UK (Birchenough & Bremner, 2010; Smale et al, 2013), which is the focal region of this study

Methods

Results

Conclusion