Scale of impact determines early post-disturbance assemblage structure in subtidalFucusbeds in the Baltic Sea (Bornholm, Denmark)

Abstract

Disturbances occasionally create gaps of varying sizes in the canopy cover of macroalgae on temperate reefs. This study quantified the size distribution of naturally occurring gaps in a subtidal Fucus serratus canopy, and experimentally tested the hypotheses that (i) early post-disturbance abundance of dominant assemblage-formers in the canopy-dominated habitat will depend on the spatial extent of the impact, and (ii) the effects of disturbance on the abundance of dominant assemblageformers at the edges of large impacted areas will be similar to the effects at small impacted areas. Gaps of up to 6 m diameter accounted for 17% of the habitat and, while small gaps were numerically dominant, all gap size-classes accounted for a similar proportion of habitat. Experimental disturbances (canopy clearing) ranging from 0.6 to 4.8 m diameter caused consistent scale-dependent divergence of assemblage structure after 21 days. Of particular interest was a dense cover ($75%) of filamentous algae in the centres of large (!1.2 m) clearings. Filamentous algae are widely known to have negative effects on Fucus recruitment and growth, and their rapid increase in abundance suggests that future recovery of the Fucus canopy could be adversely affected. The abundance of most taxa at the edges of large clearings was either indistinguishable from abundances in small clearings and intact canopy, or intermediate between abundances in these treatments and in the centres of large clearings. Juvenile Fucus were, however, more abundant around edges than anywhere else. These experimental outcomes support the model that scale-dependent divergence of assemblage structure is conveyed by an escape from processes associated with edges of the original habitat. I propose that, in the subtidal Fucus beds of Bornholm, recovery from large disturbances is restricted to encroachment from the edges because the adjacent canopy suppresses the growth of filamentous algae and thus facilitates regeneration and recruitment of Fucus.

Disturbance intensity, disturbance extent and ocean climate modulate kelp forest understory communities

“…At the warmest location, the lack of response could partly be explained by low propagule supply (there were no fucoids in the control plots) and partly be related to competition from rapidly colonizing turfs. Other studies have shown rapid colonization of turf into disturbed plots and subsequent inhibition of fucoid settlement, survival and growth (Isaeus et al 2004, Wernberg 2006. It is, of course, possible that fucoids, over time, eventually would establish into these plots (Edwards 1998, Worm et al 2001, Eriksson & Johansson 2003.…”

“…30% increase in turf cover and partial canopy loss also increased turfs, but to a lesser extent. Previous studies have shown that increases in resource availability, such as light, nutrients and space, promote rapid colonization by turf (Johnson & Mann 1993, Worm et al 2001, Connell 2005, Wernberg 2006, Russell 2007, Antoniadou et al 2010. However, in contrast to our findings, and those of Wernberg & Connell (2008) in South Australia where turfs also increased in partial clearings, turfs only increased after complete kelp removal in Tasmania (Flukes et al 2014).…”

“…For example, the absence of invertebrates in the intact kelp forest at Ham, articulated corallines at Mar and Jur and fucoids at Kal resulted in no detectable effects of kelp clearing on these groups at these locations, possibly because there were simply too few propagules within the kelp forest to respond to the manipulated disturbances. By contrast, although turf forming algae were only found in very low background densities, high dispersal and rapid colonization traits (Hay 1981, Wernberg 2006, Wernberg & Connell 2008, Antoniadou et al 2010, Wernberg et al 2016a) still allowed turfs to respond rapidly to perturbation and colonize disturbed plots. Thus, we highlight that any responses to disturbances (cf.…”

“…The apparent greater expansion potential of turfs at warmer locations could be related to higher metabolic activity. Alternatively, differences between studies could be related to initial canopy density and/or to kelp morphology and composition (Kennelly 1989, Fowler-Walker et al 2005, 2006, Layton et al 2019, including kelp surface area/volume ratio and measures of stipe width, which influence light penetration and whiplash from fronds.…”

Biogenic habitat structure of seaweeds change along a latitudinal gradient in ocean temperature

“…The overall biogenic habitat structure of each reef was quantified by running ten 25 m transects along haphazard compass bearings and recording the distances (to nearest 10 cm) of habitat change. This technique has previously been successfully applied to describe habitat structure in algal dominated assemblages in other subtidal habitats (Kennelly 1987, Wernberg 2006, Connell & Irving 2008. We distinguished between four major habitat types: (1) mono-specific kelp canopy (>90% Ecklonia radiata), 2fucalean canopy (>50% fucalean algae), (3) mixed kelp and fucalean canopy (50-90% kelp and 10-50% fucalean algae) and (4) canopy-free areas (gaps of < 10% canopy algae), typically dominated by small foliose red algae (<25 cm) (e.g., Hennedya crispa) or articulated coralline algae (e.g., Amphiroa anceps).…”