Thinning of kelp canopy modifies understory assemblages: the importance of canopy density

Abstract

Kelp forests in southeastern Australia form canopies that support complex understory assemblages. Predicted levels of climate change in this region are likely to impact the health and distribution of these forests, potentially resulting in large-scale reductions in canopy cover. This study determined the impacts of a permanent reduction in canopy cover of the dominant kelp in this region, Ecklonia radiata, on the structure of understory algal and sessile invertebrate community assemblages. Changes in assemblages were determined over 12 mo in 3 treatments: unmanipulated, 33% canopy reduction and 66% canopy reduction. Clearance treatments were maintained to simulate the predicted effects of long-term climatedriven canopy reduction. Thinning of E. radiata canopy (especially 66% loss) caused a shift towards a foliose algal-dominated understory, with an associated loss of sponges, bryozoans, and encrusting algae. Canopy loss homogenised existing patchiness in understory assemblages, and high recruitment of E. radiata occurred at both levels of thinning. A 66% reduction in kelp canopy increased understory community diversity, but did not affect species richness. Thus, changes to understory assemblages occurred in a density-dependent manner, with 66% canopy loss required to alter the structure of assemblages at the community scale. Changes at this scale were subtle but important (with stability attributed to a combination of biogeography and resistance to perturbation driven by high diversity); and indicate that partial loss of kelp canopy under future climate change scenarios will shift understory communities towards a foliose algal-dominated state, which has important implications for sessile invertebrates and potentially future recruitment of kelp.

Phase-Shift Dynamics of Sea Urchin Overgrazing on Nutrified Reefs

“…Importantly, recruitment was notably greater in the presence of adult sporophytes. This is contrary to the studies of Carnell and Keough [49] and Flukes, Johnson and Wright [50], who showed greater recruitment of E . radiata into patches cleared of canopy-forming algae.…”

“…The first addresses the effects of the scale of the kelp bed patch and extent of intraspecific competition. Carnell and Keough [49] and Flukes, Johnson and Wright [50] worked in large beds of dense kelp where the benthos was heavily shaded (>100% canopy cover) and there was (ostensibly) an abundance of spore production. In contrast, since the small patch reefs initiated in the ‘kelp bed state’ supported only 3 adult sporophytes defining 50% canopy cover, light levels on these patches were likely higher than on larger kelp patches with a similar density of individuals (C. Layton, unpublished data), and thus the effects of intraspecific competition are likely much reduced relative to larger kelp bed patches.…”

“…Kennelly [51] showed that timing of canopy clearance has a large influence on recruitment of E . radiata , and both Carnell and Keough [49] and Flukes, Johnson and Wright [50] cleared patches within established kelp canopies just before the seasonal peak (during autumn) in E . radiata sorus development and spore release.…”

Family‐level variation in early life‐cycle traits of kelp

“…data). Thus although canopy thinning in the field often results in an increase in juvenile sporophyte abundance (Kirkman 1981, Kennelly 1987, Flukes et al 2014, the response of microscopic and juvenile sporophytes to the increased light appears complex and likely to interact with other environmental stressors. There is high phenotypic plasticity in E. radiata to environmental factors Thomsen 2005, Fowler-Walker et al 2006), but the possible adaptive responses are not known.…”

Decadal losses of canopy‐forming algae along the warm temperate coastline of Brazil

“…The predictive ability of the model was not improved by considering other non-climatic proxies of coastal urbanization: local water temperature, turbidity or nitrogen concentration. been shown as the prelude to changes in canopy assemblages and associated species (Flukes, Johnson, & Wright, 2014). Once established, turfs can also begin to alter the chemical microenvironment of rocky substrate to have negative impacts on subsequent canopy recruitment (Layton et al, 2019).…”