Invasive macroalgae in native seagrass beds - vectors of spread and impacts.

BACKGROUND AND AIMS

Worldwide, invasive species are spreading through marine systems at an unprecedented rate with both positive and negative consequences for ecosystems and biological functioning of organisms. Human activities from shipping to habitat damage and modification are known vectors of spread, although biological interactions including epibiosis are increasingly recognised as potentially important to introduction into susceptible habitats.

METHODS

We assessed a novel spread mechanism - limpets as transporters of invasive algae, Sargassum muticum into beds of the seagrass Zostera marina - and the physiological impact of its invasion. The association of S. muticum with three limpet species and other habitats was assessed using intertidal surveys on rocky shores and snorkelling at two seagrass sites in the UK. A 4-yr field study tested the effect of S. muticum on Z. marina shoot density, dry weight and phenolic compounds (caffeic and tannic acid) content, and a laboratory experiment tested the impact of S. muticum on nutrient partitioning (C/H/N/P/Si), photosynthetic efficiency (Fv/Fm) and growth of Z. marina.

RESULTS

On rocky shores 15% of S. muticum occurrences were attached to the shells of live limpets. In seagrass beds 5% of S. muticum occurrences were attached to the shells of dead limpets. The remainder were attached to rock, cobblestones, the seagrass matrix or embedded within the sand. Z. marina density and phenolics content was lower when S. muticum co-occurred with it. Over 3-years, photosynthetic response of Z. marina to S. muticum was idiosyncratic, and S. muticum had no effect on nutrient partitioning in Z. marina.

CONCLUSIONS

Our results show limpets support S. muticum as an epibiont and may act as a previous unreported transport mechanism introducing invaders into sensitive habitats. S. muticum reduced phenolics production in Z. marina which may weaken its defensive capabilities and facilitate proliferation of S. muticum. The effect of S. muticum on Z. marina photosynthesis requires further work but having no effect on the capacity of Z. marina to sequester nutrients suggests a degree of resilience to this invader.

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