Dominance by a canopy forming seaweed modifies resource and consumer control of bloom‐forming macroalgae

Abstract

Degradation of ecological resources by large‐scale disturbances highlights the need to demonstrate biological properties that increase resistance to change and promote the resilience of ecosystem regimes. Coastal eutrophication is a global‐scale disturbance that drives ecosystem change by increasing primary production and favouring ephemeral and bloom‐forming life‐forms. Recent synthesis indicates that consumption processes increase the resistance of coastal communities to nutrient loading by controlling the responses of ephemeral macroalgae. Here we suggest a similar ecological function for canopy cover by demonstrating that the presence of a canopy species modifies both resource and consumer control of bloom‐forming algae associated with nutrient enrichment. We tested effects of canopy presence on the interaction between consumer and resource control, by field‐manipulations of a dominant canopy forming seaweed ( Fucus vesiculosus ), grazer presence (dominated by the gastropod Littorina littorea ) and nutrient enrichment (common agricultural NPK fertilizer). Canopy cover and grazers jointly controlled strong increases of ephemeral bloom‐forming algae (dominated by Ulva spp) from nutrient enrichment; nutrients increased ephemeral recruitment almost 10‐fold, but only in the absence of both grazers and canopy cover. Recruitment success of the canopy‐forming seaweed itself decreased additively with 56.1, 71.3 and 50.5% from independent effects of canopy cover, grazers and nutrient enrichment, respectively. A meta‐analysis of nine nutrient enrichment experiments including seaweed, seagrass and stream communities, showed that in the presence of canopies average nutrient effects were reduced by more than 90% compared to without canopies. This corroborates the generality of our finding that dominating canopy species are important for aquatic ecosystems by increasing community resistance to the propagation of nutrient effects.