Shield wall: Kelps are the last stand against corals in tropicalized reefs

Communities inhabiting biogeographic transition zones are shifting in composition as a result of progressive warming and heatwaves. In the marine environment, corals are expanding onto higher latitude reefs historically dominated by temperate kelp forests, initiating a shift towards warm affinity coral-dominated states. Although these coral expansions are a global phenomenon, the mechanisms that are underpinning the expansion process remain poorly understood, which limits the projections of the rate and extent of ecosystem reconfiguration. Here, we investigated the interaction between the kelp Ecklonia radiata and the high latitude scleractinian coral Plesiastrea versipora in several of Western Australia's temperate reefs, where coral colony abundance has increased by 50% in recent years. Combining field surveys with field and laboratory experiments, we test the importance of physical (abrasion and light reduction) and chemical (allelopathic) effects of kelp canopies on coral tissue cover, photosynthetic parameters and calcification rates. In the field, kelp cover had a negative effect on coral density that was overwhelming in comparison to other dominant macroalgal taxa. Abrasion by kelp whiplash was the predominant mechanism by which kelp exerted a negative effect on P. versipora fitness, scraping up to 80% of live coenosarc from experimental colonies. In contrast, canopies had no effects on P. versipora photochemical efficiency and laboratory incubations showed that there were no allelochemical effects from kelp on P. versipora. We conclude that E. radiata inhibits P. versipora establishment and development through abrasion, and the survey data confirmed that recent climate-driven kelp loss released corals from this effect, facilitating their expansion on high-latitude reefs in Western Australia. This shows how competitive interactions actively shield against species expansion in biogeographic transition zones and suggests a continued decline of kelp canopies will increase the permeability of temperate reefs to warm affinity species such as scleractinian corals. Read the free Plain Language Summary for this article on the Journal blog.