Rosa M. Viejo

Mobile epifauna inhabiting the invasive Sargassum muticum and two local seaweeds in northern Spain

Abstract The mobile epifauna colonizing the invasive brown alga Sargassum muticum (Yendo) Fensholt was compared with assemblages inhabiting two locally established seaweeds ( Fucus vesiculosus L. and Cystoseira nodicaulis (Whit.) Roberts) in a sheltered locality of northern Spain. The epifauna colonizing S. muticum was studied in both the shallow subtidal and intertidal pools at mid shore. The main aims of this study were to: (i) determine the host-plant specificity of the mobile epifauna; (ii) estimate the impact of the invasion on these animal assemblages. The mobile epifauna was not highly host-plant specific; very few taxa were found in just one algal species. Herbivorous species (mesograzers) were very abundant, accounting for 58–98 % of the characterized epifauna. Sargassum muticum was successfully colonized by the assemblages inhabiting local seaweeds, and this was probably due to: (i) the presence of epifaunal species that use plant epiphytes as habitat and food rather than the macroalga itself; (ii) morphological and chemical similarities between the invasive and the local seaweeds; (iii) generalist habits of mesograzers consuming the host plant itself. The impact of S. muticum invasion on the epifauna depends on the degree of host-plant specificity of animals as well as on the effect of the invader on macroalgal assemblages. The results of this study support the existence of a labile association between seaweeds and the invertebrates inhabiting them.

The effects of colonization by Sargassum muticum on tidepool macroalgal assemblages

The impact of colonization by Sargassum muticum (Phaeophyta: Fucales) on the resident macroalgal assemblage in tidepools was investigated in northern Spain. Sargassum muticum was allowed to colonize and spread in some tidepools, and was periodically removed in others. Changes in the abundance of macrophytes were recorded at three sites over two years. The colonization by S. muticum and the subsequent changes induced on the native assemblage was extremely variable among sites. Where the invader colonized profusely, it induced changes in the macroalgal composition of tidepools. The leathery group of macrophytes was significantly affected and changes were also observed in the foliose group. It was suggested that S. muticum may take advantage of their perennial and opportunistic traits in the interaction with groups of algae with different survival strategies. However, the spatial patchiness of the colonization may mitigate the impact of the invasion at local and regional scales.

Ecological limits to the invasion of Sargassum muticum in northern Spain

Abstract In northern Spain Sargassum muticum is abundant on sheltered shores and in tide pools on exposed shores, but is rare or absent at low-intertidal levels on exposed shores. An experiment on two shores indicated that S. muticum has an effective dispersal range of less than 5 m and that the presence of fertile plants in the immediate vicinity were required for settlement of large numbers of S. muticum . Few recruits were recorded in quadrats with a canopy of turfing and foliose algae. The results suggest that the lack of free space and supply of propagules play important roles in limiting the invasion of S. muticum at an exposed shore in northern Spain. It therefore appears unlikely that S. muticum will significantly disrupt the ecology of the low-intertidal level on exposed shores in northern Spain.

Physical factors driving intertidal macroalgae distribution: physiological stress of a dominant fucoid at its southern limit

Climate change is driving species range shifts worldwide. However, physiological responses related to distributional changes are not fully understood. Oceanographers have reported an increase in ocean temperature in the northwest Iberian Peninsula that is potentially related to the decline in some cold-temperate intertidal macroalgae in the Cantabrian Sea, namely Fucus serratus. Low tide stress could also play a role in this decline. We performed one mensurative (in situ) and two manipulative (in culture) experiments designed to evaluate the interactive effects of some physical factors. The first experiment analysed field response to low tide stress in marginal (mid-Cantabrian Sea and northern Portugal) versus central (Galicia) populations of F. serratus. Then a second experiment was performed that utilized either harsh or mild summer conditions of atmospheric temperature, irradiance, humidity, and wind velocity to compare the responses of individuals from one marginal and one central population to low tide stress. Finally, the combined effect of sea temperature and the other factors was evaluated to detect interactive effects. Changes in frond growth, maximal photosynthetic quantum yield (Fv/Fm), temperature, and desiccation were found. Three additive factors (solar irradiation, ocean and air temperatures) were found to drive F. serratus distribution, except under mildly humid conditions that ameliorated atmospheric thermal stress (two additive factors). Mid-Cantabrian Sea temperatures have recently increased, reaching the inhibitory levels suggested in this study of F. serratus. We also expect an additive secondary contribution of low tide stress to this species decline. On the northern Portugal coast, ocean warming plus low tide stress has not reached this species’ inhibition threshold. No significant differential responses attributed to the population of origin were found. Mechanistic approaches that are designed to analyse the interactive effects of physical stressors may improve the levels of confidence in predicted range shifts of species.

Effects of wave exposure and intraspecific density on the growth and survivorship of Sargassum muticum (Sargassaceae: Phaeophyta)

In this paper we quantify the interactive effects of wave exposure and intraspecific density on the survivorship and growth of Sargassum muticum in northern Spain. Juveniles settled onto slate plates were experimentally thinned to two densities and transplanted to exposed and sheltered shores. Initially, plants grew faster at the exposed sites but final lengths were similar between exposures after 5 months, largely because of breakages at the top of the main primary lateral of plants at exposed sites. Mean weights of plants did not significantly differ between exposures. The results indicate that S. muticum juveniles growing at high density were longer and thinner but similar in weight to those at low density. Patterns of growth of larger plants transplanted between sheltered sites similarly suggested changes in morphology of S. muticum in response to differences in density. Survivorship of juveniles did not differ between exposures but was significantly poorer at the higher density irrespective of wave e...

An Experimental Evaluation of the Effect of Wave Action on the Distribution of Sargassum muticum in Northern Spain

The invasive brown seaweed Sargassum muticum was first recorded in northern Spain in the 1980s. The species is now patchily distributed in localities that differ in wave-exposure and is most abundant in sheltered areas. A transplant experiment was done on a local scale to test the ability of S. muticum to persist and grow on wave-exposed shores. One month after transplantation, plants moved to exposed areas suffered a high percentage of breakages (> 80%). When only plants without damage were considered, those transplanted to exposed areas exhibited a mean elongation rate significantly less than those on the sheltered shore. The lower rate of elongation in the absence of any apparent injury suggests a grade of phenotypic response of S. muticum to changes in the physical conditions. Nevertheless, the low tolerance exhibited by this species to mechanical stress indicates that its distribution may be limited by physical conditions in wave-exposed environments.

STRUCTURE AND DYNAMICS OF A POPULATION OF PALMARIA PALMATA (RHODOPHYTA) IN NORTHERN SPAIN1

Palmaria palmata (Linnaeus) O. Kuntze (Rhodophyta, Palmariaceae) is a seaweed commercially harvested for human consumption. Its population density, size structure, and frond dynamics were investigated from May 1999 to May 2001 at one intertidal locality in the northern coast of Spain, which is within the southern distributional boundary of the species in the eastern Atlantic coasts. The effect of size, age, and the life‐history phase (haploid vs. diploid) on frond growth and mortality were also evaluated. The study was carried out by mapping and monitoring fronds in the field. New fronds (macroscopic recruits or sprouts) appeared in spring, but subsequent mortality of these young fronds and detachment of the host plant led to lower density values in January. Palmaria palmata exhibited a distinctive seasonal growth cycle, with positive net growth from March to August and breakage from August to March. Interannual differences were also detected, with higher net growth in 2000 than in 1999. Net growth was apparently independent of age, reproductive status (fertile vs. reproductive), and life‐history phase (haploid vs. diploid) but was dependent on size, as longer fronds showed minor growth or greater breakage than small ones. Mortality, on the other hand, was more dependent on age than on size in the period analyzed (March–May 2000). Results of the study indicate that both size and age should be included as state variables and temporal changes in transition probabilities considered in the development of demographic models of the species.

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.