Ecosystem functioning impacts of the invasive seaweed Sargassum muticum (Fucales, Phaeophyceae)

Beach wrack: Discussing ecological roles, risks, and sustainable bioenergy and agricultural applications

The equilibrium of the marine ecosystem is currently threatened by several constraints, among which climate change and anthropogenic activities stand out. Indeed, these factors favour the growth of macroalgae, which sometimes end up stranded on the beaches at the end of their life cycle, forming what is known as beach wrack. Despite its undeniable important ecological role on beaches, as it is an important source of organic matter (OM), and provides food and habitat for several invertebrates, reptiles, small mammals, and shorebirds, the overaccumulation of beach wrack is often associated with the release of greenhouse gases, negatively impacting tourist activities, and generating economic expenses for its removal. Although currently beach wrack is mainly treated as a waste, it can be used for numerous potential applications in distinct areas. This review aimed at providing a solid point of view regarding the process of wrack formation, its spatiotemporal location, as well as its importance and risks. It also contains the current advances of the research regarding sustainable alternatives to valorise this organic biomass, that range from bioenergy production to the incorporation of wrack in agricultural soils, considering a circular economy concept. Although there are some concerns regarding wrack utilisation, from its variable availability to a possible soil contamination with salts and other contaminants, this review comprises the overall beneficial effects of the incorporation of this residue particularly in the organic agricultural model, strengthening the conversion of this wasted biomass into a valuable resource.

Dramatic changes in the structure of shallow-water marine benthic communities following the invasion by Rugulopteryx okamurae (Dictyotales, Ochrophyta) in Azores (NE Atlantic)

Biological invasions are considered one of the most important drivers of biodiversity loss. Here we use a before-after-control-impact (BACI) design to investigate the impact of Rugulopteryx okamurae on the structure of shallow-water marine benthic communities in São Miguel island, Azores. After its first appearance in 2019, R. okamurae has rapidly invaded much of the southern coast of the island, where it became the dominant algae. This was followed by significant changes in the structure of shallow-water marine benthic communities, with substantial losses of natural variability and species richness. Compared to before, there has been dramatic reductions in the abundances of articulated coralline algae, corticated algae and corticated foliose algae in invaded locations. These results highlight its highly invasive character, not seen with other, more well-known, invasive species. It remains to be investigated if its impacts persist throughout time and to quantify the functional consequences of such dramatic changes.

Thermal Tolerance May Slow, But Not Prevent, the Spread of Sargassum horneri (Phaeophyceae) along the California, USA and Baja California, MEX Coastline

Biological invasions have become increasingly prevalent in marine ecosystems, modifying biodiversity and altering the way ecosystems function. Understanding how variation in environmental factors influences the success of non-native species, especially their early life stages, can be a crucial step in identifying habitats that are under threat of invasion, and in predicting how rapidly and far these species may spread once they arrive in novel habitats. The invasive marine macroalga Sargassum horneri was first observed in Long Beach Harbor, CA, USA in 2003, and has since spread throughout the Southern California Bight and along the Baja California Peninsula, MEX where it now forms dense stands on subtidal rocky reefs and displaces native habitat-forming macroalgae. We examined how variation in temperature, nutrients, and irradiance affect survival, growth, and development in S. horneri early life stages over a three-week period. Our experimental treatments consisted of orthogonally crossed temperatures (10, 15, 20, and 25°C), nutrient concentrations (ambient and nutrient-enriched seawater), and irradiances (50 and 500 µmol photons · m-2 · s-1 ). Overall, temperature exerted the greatest influence on S. horneri's germling and juvenile life stages, with moderate temperatures facilitating their greatest survival, growth, and development. In contrast, fewer germlings developed fully under the lowest or highest temperatures, and juvenile survival and growth were reduced, especially when combined with low irradiances. Together, our data suggest that ocean temperatures of or below 10˚C and of or above 25°C may slow, but likely not stop, S. horneri's northward and southward expansion along the California and Baja California coasts.

Removal of an established invader can change gross primary production of native macroalgae and alter carbon flow in intertidal rock pools

The impact of invasive species on recipient communities can vary with environmental context and across levels of biological complexity. We investigated how an established invasive seaweed species affected the biomass, eco-physiology, carbon and nitrogen storage capacity of native seaweeds at sites with a different environmental setting due to a persistent upwelling in northern Spain. We removed the invasive Japanese wireweed Sargassum muticum from intertidal rock pools once every month during a one-year period and used an in-situ stable isotope pulse-chase labeling to estimate gross primary production (GPP), nitrogen uptake rate, 13C-carbon and 15N-nitrogen storage capacities. Following the addition of 13C-enriched bicarbonate and 15N-enriched nitrate to the seawater in the rock pools during the period of the low tide, we sampled macroalgal thalli at incoming tide to determine label uptake rate. After four days, we sampled macroalgal assemblages to determine both label storage capacity and biomass. After one year of removal there was no change in the macroalgal assemblage. However, both the GPP and 13C-carbon storage capacity were higher in the turf-forming Corallina spp. and, sometimes, in the canopy-forming Bifurcaria bifurcata. Nitrogen uptake rate followed similar, but more variable results. Although S. muticum inhibited carbon storage capacity of native species, the assemblage-level 13C-carbon storage was similar in the S. muticum-removed and control rock pools because the presence of the invasive species compensated for the functional loss of native species, particularly at sites where it was most abundant. No obvious effects were observed in relation to the environmental setting. Overall, the effect of the invasive S. muticum on carbon flow appeared to be mediated both by the effects on resource-use efficiency of native species and by its own biomass. Integrating physiological and assemblage-level responses can provide a broad understanding of how invasive species affect recipient communities and ecosystem functioning.