Consolación Fernández

The retreat of large brown seaweeds on the north coast of Spain: the case of Saccorhiza polyschides

Recent changes in kelp distribution along the north coast of Spain are described and analysed through a long-term population study focused on Saccorhiza polyschides. The main purpose of this work was to understand which population processes are more sensitive to increased sea surface temperatures and reductions in the intensity of upwelling episodes in the current scenario of global warming. Data on the distribution of kelp species (old and recent data) were obtained from the literature and compared with current species distributions assessed by field sampling between 2007 and 2010 and covering a transition coastline of 200 km. The long-term population study of Saccorhiza polyschides was conducted at a site close to the edge of its current distribution. Data for recruitment, growth and survival, as well as density and supported biomass of populations collected since the 1990s were analysed using data from the late 1970s for comparison. Kelps on the north coast of Spain have shown a westward retreat since the 1980s. Dense populations of Saccorhiza polyschides, the most important species, and Laminaria ochroleuca have been reduced to small patches and isolated individuals east of Peñas Cape (43° 39.4′ N; 5° 50.8′ W). The long-term study of Saccorhiza polyschides populations showed a collapse in the growth of the sporophyte and very low recruitment from the beginning of this century. The possible causal factors of this population decline and its consequences are considered, suggesting that long warm summer periods (more than 30 consecutive days of seawater temperature > 20°C) could alter kelp performance.

LONG‐TERM CHANGES IN THE STRUCTURE OF INTERTIDAL ASSEMBLAGES AFTER INVASION BY SARGASSUM MUTICUM (PHAEOPHYTA)1

The macroalgal assemblages at the low intertidal zone were studied at three localities on the north coast of Spain between 1977 and 2002. Two of these localities were invaded at the end of the 1980s by the brown seaweed Sargassum muticum (Yendo) Fensholt (Phaeophyta, Sargassaceae), whereas the third locality remained free of the invader. In 2002, distinct algal assemblages were noticed in invaded and noninvaded localities. No major changes were detected in the noninvaded locality. Apart from the obvious presence of S. muticum, the changes observed in the invaded localities included a significant reduction in abundance of the previous dominant species (the red alga Gelidium spinosum (S. G. Gmelin) P. C. Silva) as well as an increased number of species and diversity, increased primary productivity, and variations in the seasonal abundance patterns of some species. We speculate that the arrival of S. muticum had a negative effect on the dominant native G. spinosum, probably related to competition for light. This resulted in indirect positive effects on other species of the assemblage (such as Bifurcaria bifurcata R. Ross). Other small epiphytic opportunistic species might also have been benefited from the presence of S. muticum, because the invader has a rich associated epiphytic assemblage.

SIZE STRUCTURE AND DYNAMICS IN A POPULATION OF SARGASSUM MUTICUM (PHAEOPHYCEAE)

Sargassum muticum (Yendo) Fensholt is an introduced brown seaweed with a very distinctive seasonal growth cycle on European shores. The present study links the dynamics of a population of S. muticum with the seasonal growth cycle of the species and the density‐dependent processes operating throughout this cycle. Results indicate that both growth cycle and intraspecific competition influenced the structure and population dynamics. Size inequality increased during the slow growth phase (autumn–winter) of the 2‐year study. Mechanisms generating inequality of size could be the existence of asymmetric competition and the inherent differences in growth rates between old (regenerated) and new thalli (recruits). Inequality of size distributions decreased progressively during the last months of the growth phase (spring–summer) and could be related to a process of self‐thinning. There was a negative biomass–density relationship (as a measure of biomass accumulation‐driven mortality) that confirms the importance of self‐thinning as a major demographic factor in the S. muticum population.

IMPACT OF THE INVASIVE SEAWEED SARGASSUM MUTICUM (PHAEOPHYTA) ON AN INTERTIDAL MACROALGAL ASSEMBLAGE1

The impact of the invasive seaweed Sargassum muticum (Yendo) Fensholt on a low intertidal macroalgal assemblage was assessed at a semiexposed rocky shore in northern Spain between 2002 and 2004. Sargassum muticum plants were removed from the mature macroalgal assemblage and from those occurring along the successional process of the assemblage. Biomass, richness, diversity, and percentage cover of macroalgae in experimental plots were compared with unmanipulated controls. The effect of S. muticum removal on the macroalgal assemblage more than 2 years after the beginning of the experiment was negligible. Moreover, no differences between treatments were detected in the general patterns of succession. Only significant differences in S. muticum abundance were detected between treatments at the end of the experiment. We suggest that the low abundance of S. muticum at this intertidal level and its pseudoperennial life cycle may limit competition with native macroalgae. However, long‐term removal experiments may be a more indicator of the impact of S. muticum at the upper limit of its vertical distribution.

Ecology of Sargassum muticum on the North Coast of Spain. Preliminary Observations

The first occurrence and establishment of a population of Sargassum muticum in a rocky intertidal area on the Asturian coast of northern Spain is reported. Some competitive attributes of Sargassum muticum, such as photosynthetic and growth rates were compared with other indigenous macrophytes.

WATER MOTION AND MORPHOLOGY IN CHONDRUS CRISPUS (RHODOPHYTA)1

Morphological variability of intertidal Chondrus crispus Stackh. fronds along a small open rocky coast was related to wave exposure and emersion. Cluster analysis revealed two well‐defined morphologies: filiform and planiform, named the N morphotype and B morphotype, respectively. We propose a rapid method of classifying fronds based on the morphology of the cross section at half the height on the thallus. The N morphotype is characterized by fewer dichotomies per unit length, a circular cross section with a large inner cortex, and narrow fronds. It is abundant at low intertidal and exposed sites. The B morphotype is characterized by more dichotomies, smaller sizes, a subelliptical or flattened cross section, and broad fronds. It is abundant at high intertidal sites in sheltered areas. Regression analysis revealed a major effect of water movements on frond morphology with respect to tidal level, which was more evident at high intertidal levels. No relationships were observed between morphology and life history phases.

Ecology of Sargassum muticum (Phaeophyta) on the north coast of Spain: III. Reproductive ecology

On the north coast of Spain, Sargassum muticum is fertile between spring and summer. Maturation occurs gradually from April to September. Senescence occurs simultaneously with full maturity increasing from June to September. Differences between two consecutive years can be explained by changes in environmental factors, such as seawater temperature and nutrient concentrations. Assuming that receptacles are the only reproductive structures, standing reproductive allocation (standing R A) in the population varied between 20-24% and the maximum value recorded for a single plant was 55%. There was a positive relationship between standing RA and the size of the plant, but this allometric relationship was much clearer when the number of receptacles (NR) was plotted against the weight of the plant. This suggests that NR is fixed for a given plant size but that actual allocation (standing RA) varied among plants. Inter-annual differences between reproductive investment were related to environmental factors and other factors intrinsic to the population.

Seasonal nitrogen metabolism in an intertidal population of Gelidium latifolium (Gelidiaceae, Rhodophyta)

The nitrogen status of Gelidium latifolium on the north coast of Spain was monitored for a 2 year period from 1990 to 1991. Each summer, the population showed obvious symptoms of nitrogen (N) limitation, such as thallus ‘bleaching’. Minimum N content (as % of dry weight), highest C/N ratio (above 45) and lowest protein (< 10% dry weight) and phycobilin contents were observed during summer, when nitrate availability in seawater was lowest, and temperature and light levels were highest. However, tissue N content did not respond to a short local upwelling period that increased nitrate levels in seawater more than ten-fold. The effect of seawater N concentration and increased light levels on pigment content was established experimentally. N-starved plants placed in N-enriched seawater during summer 1990 showed increased levels of phycobilin and chlorophyll a compared with an unenriched control. Screens which transmitted different proportions of natural light placed over a Gelidium latifolium population during...

Fucoid assemblages on the north coast of Spain: past and present (1977–2007)

Abstract Fucoid assemblages dominated by Pelvetia canaliculata, Fucus spiralis, F. vesiculosus, and Ascophyllum nodosum located at two sites on the central coast of Asturias (northern Spain) were sampled monthly in 1977. Repeating the same sampling methodology, a resurvey was done in 2007 to detect changes in the abundances of the species using the previous data as a baseline. Annual net primary production was lower for all the species in 2007, and there were differences in the timing of maximum biomass for P. canaliculata and F. vesiculosus, as well as a shortening of the growth period in 2007. Associated fauna also differed between dates. Higher abundances of Gibbula spp. occurred in 2007, whereas littorinid species densities in the upper intertidal were reduced in that year. As a result, the fucoid-grazers balance changed, with P. canaliculata and particularly F. vesiculosus, being the assemblages most sensitive to change. Observed modifications were not merely fluctuations in the biomass patterns of these species, but also responses to increases in air temperature and sea surface temperature and to a shift in the frequency and seasonality of upwelling episodes. Other long-term abiotic fluctuations not directly related to global warming should also be considered as possible drivers for these changes.

Ecology of Fucus vesiculosus (Phaeophyceae) at its southern limit of distribution: Growth and production of the early stages of development.

Abstract Growth and survival of two populations of Fucus vesiculosus were studied at its southern limit of distribution on the eastern Atlantic coast. Experimentally denuded areas at an estuarine and a semi-exposed site in an upwelling area (NW Spain) were followed for 17 months. Three different cohorts were detected during the sampling period. Differences among the three cohorts in terms of growth, reproduction and survival were detected. These differences may be due to the different time of appearance of the different cohorts or the presence of the previously established individuals when the second and third cohorts were recruited. Although growth of the cohorts recruiting in autumn was faster than for the cohorts recruiting in spring and summer, individual growth was represented in all cases by a logistic function, as the fastest rates of increase in length occurred during the first 6 months of life, and maximum length was attained after the thallus reached 1 year in age. In the same way, production was maximum for the first cohort, recruiting in autumn, even when it had the lowest survival rate, because of the rapid growth of survivors during spring and summer. For both populations, reproduction was continuous through the year but it was maximal during spring and summer. Protection from waves might have been thought to favour higher production and standing stock biomass values at the estuarine site compared with the semi-exposed site, while turnover rates of biomass were higher at the latter. Contrary to expectations, most of the nutrients available for the studied populations were not related to upwelling. Despite the fast initial growth of new recruits, both populations appeared to be very sensitive to clearance.