The effects of an invasive seaweed on native communities vary along a gradient of land-based human impacts

The invasive seaweed Asparagopsis taxiformis erodes the habitat structure and biodiversity of native algal forests in the Mediterranean Sea

Invasive seaweeds are listed among the most relevant threats to marine ecosystems worldwide. Biodiversity hotspots, such as the Mediterranean Sea, are facing multiple invasions and are expected to be severely affected by the introduction of new non-native seaweeds in the near future. In this study, we evaluated the consequences of the shift from the native Ericaria brachycarpa to the invasive Asparagopsis taxiformis habitat on the shallow rocky shores of Favignana Island (Egadi Islands, MPA, Sicily, Italy). We compared algal biomass and species composition and structure of the associated epifaunal assemblages in homogenous and mixed stands of E. brachycarpa and A. taxiformis. The results showed that the biomass of primary producers is reduced by 90% in the A. taxiformis invaded habitat compared to the E. brachycarpa native habitat. The structure of the epifaunal assemblages displayed significant variations among homogenous and mixed stands. The abundance, species richness and Shannon-Wiener diversity index of the epifaunal assemblages decreased by 89%, 78% and 40%, respectively, from homogenous stands of the native E. brachycarpa to the invasive A. taxiformis. Seaweed biomass was the structural attribute better explaining the variation in epifaunal abundance, species richness and diversity. Overall, our results suggest that the shift from E. brachycarpa to A. taxiformis habitat would drastically erode the biomass of primary producers and the associated biodiversity. We hypothesize that a complete shift from native to invasive seaweeds could ultimately lead to bottom-up effects on rocky shore habitats, with negative consequences for the ecosystem structure, functioning, and the services provided.

Potentially combined effect of the invasive seaweed Caulerpa cylindracea (Sonder) and sediment deposition rates on organic matter and meiofaunal assemblages

The seaweed Caulerpa cylindracea (Sonder) is one of the most successful marine bioinvaders worldwide. Caulerpa cylindracea can influence the quantity and biochemical composition of sedimentary organic matter (OM). However, it is still unknown if the effects of C. cylindracea on both OM and small metazoans (i.e. meiofauna) can change according to different sediment deposition rates. To provide insights on this, we investigated the biochemical composition of sediments along with the abundance and composition of meiofaunal assemblages in sediments colonized and not-colonized by the seaweed C. cylindracea under different regimes of sediment deposition. Our results show that the presence of the invasive alga C. cylindracea could alter quantity, biochemical composition, and nutritional quality of organic detritus and influence the overall functioning of the benthic system, but also that the observed effects could be context-dependent. In particular, we show that the presence of C. cylindracea could have a positive effect on meiofaunal abundance wherever the sediment deposition rates are low, whereas the contextual presence of high to medium sedimentation rates can provoke an accumulation of sedimentary organic matter, less favourable bioavailability of food for the benthos, and consequent negative effects on meiofauna.

Synergistic reduction of a native key herbivore performance by two non-indigenous invasive algae

Native generalist grazers can control the populations of non-indigenous invasive algae (NIIA). Here, it was found that the simultaneous consumption of two co-occurring NIIA, Caulerpa cylindracea and C. taxifolia var. distichophylla, hinders the grazing ability of the main Mediterranean herbivorous, the native sea urchin Paracentrotus lividus. The ingestion of any of the two NIIA alone did not produce any difference in sea urchin righting time with respect to usual algal diet. In contrast, the simultaneous consumption of both NIIA, which grow intermingled in nature and are consumed by P. lividus, retarded its righting behavior. Such result reveals substantial physiological stress in the sea urchin, which resulted in reduced motility and coordination. The reported findings reveal the potential of NIIA co-occurrence to escape the supposed control exerted by the main native generalist grazer in Mediterranean sublittoral communities, which in turn can be locked in an "invaded" state.

Habitat associations of an expanding native alga

There are many examples of native macrophytes becoming locally dominant and spreading outside their traditional distributions, but the causes and impacts are often not understood. In New South Wales, Australia, the green alga Caulerpa filiformis is undergoing a range expansion and has transitioned from a subdominant to a dominant alga on several rocky shores around the Sydney coastline. Here we investigated relationships between established patches of C. filiformis, the habitat it occupies and associated algal communities at multiple subtidal sites over the green alga's 700 km range. We tested the following predictions: 1) C. filiformis cover differs among substrata, being greatest on turf-forming algae; 2) C. filiformis cover is positively related to environmental variables linked to increased sedimentation (e.g. reduced reef width, surface slope, increased rugosity and distance from shore); 3) occurrence of C. filiformis is associated with a change in macrophyte community structure and a reduction of macrophyte richness; 4) intact native algal canopies inhibit C. filiformis spread, but turf-forming algae and bare sand are susceptible to invasion. Substratum associations were highly consistent among sites, but contrary to our prediction, C. filiformis was most commonly associated with rock or rock + sand substratum and less frequently associated with turf-forming algae substratum. C. filiformis cover was negatively correlated with reef width, which explained most of the variation observed, although local scale variables distance from shore, reef slope, and water depth were also correlated with C. filiformis cover. Algal diversity and community composition typically differed in the presence of C. filiformis, often with a reduction of algal abundances, in particular Sargassum spp., although results varied among substrata and sites. However, monitoring of borders suggested that C. filiformis does not invade and outcompete undisturbed adjacent canopy-forming algae over a 12 month period. Our results suggest that disturbance processes (possibly linked to sedimentation) acting at the site and quadrat scale are likely important determinants of C. filiformis cover and spread, and hence its potential ecological impacts.

Potential effects of an invasive seaweed (Caulerpa cylindracea, Sonder) on sedimentary organic matter and microbial metabolic activities

Caulerpa cylindracea (Sonder), among the most successful marine bio-invaders on a global scale, poses severe threats to biodiversity. However, the effects of this seaweed on the quantity and the biochemical composition of sedimentary organic matter are still poorly known. Since the whole set of sedimentary features affects the availability of substrates for benthic microbial communities, we: i) investigated the biochemical composition of sediments colonized and not-colonized by C. cylindracea, and ii) compared the metabolic patterns of the microbial communities associated with C. cylindracea and in the sediments colonized and not-colonized by the seaweed. Our results show that C. cylindracea can influence the quantity and biochemical composition of sedimentary organic matter (OM), and that microbial populations associated with colonized sediments do have specific metabolic patterns and degradation capacities. Caulerpa cylindracea can also influence the metabolic patterns of the microbial community specifically adapted to degrade compounds released by the seaweed itself, with possible consequences on C cycling.

Carry over effects of nutrient addition on the recovery of an invasive seaweed from the winter die-back

Nutrient enrichment of coastal waters can enhance the invasibility and regrowth of non-native species. The invasive alga Caulerpa cylindracea has two distinct phases: a well-studied fast-growing summer phase, and a winter latent phase. To investigate the effects of nutrient enrichment on the regrowth of the seaweed after the winter resting-phase, a manipulative experiment was carried out in intertidal rockpools in the North-western Mediterranean. Nutrients were supplied under different temporal regimes: press (constant release from January to May), winter pulse (January to March) and spring pulse (March to May). Independently from the temporal characteristics of their addition, nutrients accelerated the re-growth of C. cylindracea after the winter die-back, resulting in increased percentage covers at the peak of the growing season. Nutrient addition did not influence the number and length of fronds and the biomass. Native components of the algal community did not respond to nutrient additions. Our results show that nutrient supply can favour the spread of C. cylindracea even when occurring at a time of the year at which the seaweed is not actively growing.