Dynamics of recruitment and establishment of the invasive seaweed Codium fragile within an eelgrass habitat

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 the 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 recognized as potentially important to introduction into susceptible habitats.

METHODS

We assessed a novel mechanism of spread - limpets as transporters of an invasive alga, 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-year 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, to cobblestones, to 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 responses of Z. marina to S. muticum were 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 previously unreported transport mechanism introducing invaders into sensitive habitats. S. muticum reduced production of phenolics 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.

Spatio-temporal dynamics of Codium populations along the rocky shores of N and NW Spain

The green alga Codium fragile ssp. fragile (hereafter C. fragile) has long been assumed to outcompete and displace its native congeners via cryptic invasion. We analysed the population dynamics of the exotic C. fragile and native congeners and their relative abundance on intertidal shores in N-NW Spain. Our results did not support the existence of current competitive displacement by the exotic species. The presence of C. fragile was clearly seasonal, while the native C. tomentosum was more persistent throughout the year, due to a higher frequency of frond sprouting from perennial basal fragments. However, our results also indicated an increase in the proportion of C. fragile relative to native species towards inner areas of the Bay of Biscay, which was correlated with environmental gradients. The greater tolerance of C. fragile to environmental stress and its opportunistic abilities may favour establishment and spread of the species under a warming scenario.

An overview of recent research in marine biological invasions

The Topical Collection on Invasive Species includes 50 articles addressing many tenets of marine invasion ecology. The collection covers important topics relating to propagule pressure associated with transport vectors, species characteristics, attributes of recipient ecosystems, invasion genetics, biotic interactions, testing of invasion hypotheses, invasion dynamics and spread, and impacts of nonindigenous species. This article summarizes some of the collection's highlights.