The rise of Laminaria ochroleuca in the Western English Channel (UK) and comparisons with its competitor and assemblage dominant Laminaria hyperborea

Abstract

The distribution of species is shifting in response to recent climate change. Changes in the abundance and distributions of habitat-forming species can have knock-on effects on community structure, biodiversity patterns and ecological processes. We empirically examined temporal changes in the abundance of the warm-water kelp Laminaria ochroleuca at its poleward range edge in the Western English Channel. Resurveys of historical sites indicated that the abundance of L. ochroleuca has increased significantly in recent decades. Moreover, examination of historical records suggested that L. ochroleuca has extended its distribution from sheltered coasts on to moderately wave-exposed open coasts, where it now co-exists and competes with the assemblage dominant Laminaria hyperborea. Proliferation of L. ochroleuca at its poleward range edge corresponds with a period of rapid warming in the Western English Channel. Preliminary comparisons between L. ochroleuca and L. hyperborea highlighted some subtle but ecologically significant differences in structure and function. In summer, the average biomass of epiphytic stipe assemblages on L. hyperborea was 86 times greater than on L. ochroleuca whereas, on average, L. ochroleuca had a greater stipe length and its blade supported 18 times as many gastropod grazers (Gibbula cineraria). Differences in summer growth rates were also recorded, with L. ochroleuca being more productive than L. hyperborea throughout July. Comprehensive seasonally replicated comparisons are needed to examine the wider implications of proliferation of L. ochroleuca at its poleward range edge, but our study suggests that local biodiversity patterns and ecological processes (e.g. timing of productivity and trophic pathways) on shallow subtidal reefs may be altered by shifts in the relative abundances of habitat-forming kelp species.

The structure of biogenic habitat and epibiotic assemblages associated with the global invasive kelp Undaria pinnatifida in comparison to native macroalgae

“…For example, a comparison of epibionts attached to farmed U. pinnatifida and S. latissima sporophytes showed that S. latissima generally exhibits higher levels of biofouling (Peteiro and Freire, 2013). L. ochroleuca is also known to produce antifoulants, and the surfaces of the stipe and lamina are generally less fouled by epibionts than other kelps including L. hyperborea (Smale et al, 2015), L. digitata (Blight and Thompson, 2008) and S. latissima (this study). Even so, epiphytic loading and the richness and cover of epibionts on the stipe and lamina habitat of L. ochroleuca were still comparable to or greater than that of U. pinnatifida.…”

“…The shift from long-lived perennial species to short-lived annual species represents a shift in habitat structure from temporally-stable to 'boom and bust' and is likely to have major implications for primary production and biogenic habitat provision. However, the situation is complicated by the proliferation of the 'warm' water kelp L. ochroleuca at its poleward range edge in southwest UK (Smale et al, 2015). On wave-exposed open coastlines L. ochroleuca may support less diverse and abundant assemblages than its cool-water competitor L. hyperborea (Smale et al, 2015; Teagle unpublished data) but in more sheltered locations L. ochroleuca seemingly provides relatively stable and complex biogenic habitat that supports comparatively diverse and abundant epibiont assemblages.…”

“…However, the situation is complicated by the proliferation of the 'warm' water kelp L. ochroleuca at its poleward range edge in southwest UK (Smale et al, 2015). On wave-exposed open coastlines L. ochroleuca may support less diverse and abundant assemblages than its cool-water competitor L. hyperborea (Smale et al, 2015; Teagle unpublished data) but in more sheltered locations L. ochroleuca seemingly provides relatively stable and complex biogenic habitat that supports comparatively diverse and abundant epibiont assemblages. As such, elevated abundances of L. ochroleuca may positively influence local biodiversity within some habitats.…”

“…The ecological structure and functioning of kelp forest ecosystems is, however, being impacted by a range of environmental change stressors, including oceanic warming Vergés et al, 2014), extreme climatic events (Byrnes et al, 2011;Wernberg et al, 2013), reduced water quality (Gorgula and Connell, 2004;Moy and Christie, 2012), altered trophic interactions (Tegner and Dayton, 2000;Ling et al, 2009) and the spread of non-native species (Saunders and Metaxas, 2008). Such stressors can cause shifts in the distributions and/or relative abundances of HFS within kelp forests Smale et al, 2015), which can have knock-on effects for the wider community (Ling, 2008;. As such, documenting changes in the distribution and abundance of HFS and examining their role in structuring communities is a crucial step towards understanding and predicting the effects of rapid environmental change.…”

Global patterns of kelp forest change over the past half-century

“…Kelp forest declines have now been documented in many regions in response to a variety of stressors (18,(31)(32)(33)(34)(35), but kelp abundances have been stable or increasing in other areas (17,36,37). Here, we amass a global database of kelp abundances to provide a comprehensive picture of kelp forest change.…”