Canopy-Forming Seaweeds in Urchin-Dominated Systems in Eastern Canada: Structuring Forces or Simple Prey for Keystone Grazers?

Abstract

Models of benthic community dynamics for the extensively studied, shallow rocky ecosystems in eastern Canada emphasize kelp-urchin interactions. These models may bias the perception of factors and processes that structure communities, for they largely overlook the possible contribution of other seaweeds to ecosystem resilience. We examined the persistence of the annual, acidic (H2SO4), brown seaweed Desmarestia viridis in urchin barrens at two sites in Newfoundland (Canada) throughout an entire growth season (February to October). We also compared changes in epifaunal assemblages in D. viridis and other conspicuous canopy-forming seaweeds, the non-acidic conspecific Desmarestia aculeata and kelp Agarum clathratum. We show that D. viridis can form large canopies within the 2-to-8 m depth range that represent a transient community state termed “Desmarestia bed”. The annual resurgence of Desmarestia beds and continuous occurrence of D. aculeata and A. clathratum, create biological structure for major recruitment pulses in invertebrate and fish assemblages (e.g. from quasi-absent gastropods to >150 000 recruits kg−1 D. viridis). Many of these pulses phase with temperature-driven mass release of acid to the environment and die-off in D. viridis. We demonstrate experimentally that the chemical makeup of D. viridis and A. clathratum helps retard urchin grazing compared to D. aculeata and the highly consumed kelp Alaria esculenta. In light of our findings and related studies, we propose fundamental changes to the study of community shifts in shallow, rocky ecosystems in eastern Canada. In particular, we advocate the need to regard certain canopy-forming seaweeds as structuring forces interfering with top-down processes, rather than simple prey for keystone grazers. We also propose a novel, empirical model of ecological interactions for D. viridis. Overall, our study underscores the importance of studying organisms together with cross-scale environmental variability to better understand the factors and processes that shape marine communities.

Mechanistic model identifies increasing light availability due to sea ice reductions as cause for increasing macroalgae cover in the Arctic

“…The death rates, d , were estimated based on the life span of the three species (Table ); 10–50 years for the calcareous algae (Adey ), and 1 ( Desmarestia spp.) or 4 ( Phycodrys rubens ) years for the macroalgae (Schoschina ; Blain and Gagnon ). The death rate, d , was determined in order to reduce an algae population from 100% to 10% in the course of the life span at zero growth rate.…”

Plasticity in the antipredator behavior of the orange-footed sea cucumber under shifting hydrodynamic forces

“…The bedrock seabed at Bread and Cheese Cove is colonized by grazing-resistant, red coralline seaweeds (mainly Lithothamnion glaciale ). The green sea urchin, S. droebachiensis , maintains the biological communities in a “barrens” state ( sensu Lawrence 1975) for most of the year (Blain and Gagnon 2014; Frey and Gagnon 2015). We monitored sea cucumbers naturally distributed along a 110-m-long depth gradient ranging from the turbulent shallow environment at a rocky point on the north shore of the cove to the calmer deeper waters located near the middle of the cove to the south.…”

Thermal and Hydrodynamic Environments Mediate Individual and Aggregative Feeding of a Functionally Important Omnivore in Reef Communities

“…esculenta and Laminaria digitata , dominate the 0–2 m depth range, followed by an extensive urchin ( S . droebachiensis ) barrens to a depth of ~15 m. Transient beds of the annual, acidic, brown seaweed Desmarestia viridis establish every year in this barrens [ 33 ] and intersperse with a few stands of the grazing-resistant kelp Agarum clathratum [ 34 ]. At CBC, an extensive (several 100s of m 2 ) kelp bed dominated by A .…”

“…The unconsumed kelp was wet weighed at the end of trials. We used the equation: Kelp loss = [( T o × C f / C o )— T f ] to obtain the corrected kelp loss in each container with urchins, where T o and T f are the initial and final weights of kelp tissues exposed to urchins, respectively, and C o and C f are the initial and final weights of the corresponding autogenic control, respectively [ 33 ].…”