Macroalgal Assemblages Research Articles

Consistent effects of consumer species loss across different habitats

Our knowledge of the effects of consumer species loss on ecosystem functioning is limited by a paucity of manipulative field studies, particularly those that incorporate inter‐trophic effects. Further, given the ongoing transformation of natural habitats by anthropogenic activities, studies should assess the relative importance of biodiversity for ecosystem processes across different environmental contexts by including multiple habitat types. We tested the context‐dependency of the effects of consumer species loss by conducting a 15‐month field experiment in two habitats (mussel beds and rock pools) on a temperate rocky shore, focussing on the responses of algal assemblages following the single and combined removals of key gastropod grazers (Patella vulgata, P. ulyssiponensis, Littorina littorea and Gibbula umbilicalis). In both habitats, the removal of limpets resulted in a larger increase in macroalgal richness than that of either L. littorea or G. umbilicalis. Further, by the end of the study, macroalgal cover and richness were greater following the removal of multiple grazer species compared to single species removals. Despite substantial differences in physical properties and the structure of benthic assemblages between mussel beds and rock pools, the effects of grazer loss on macroalgal cover, richness, evenness and assemblage structure were remarkably consistent across both habitats. There was, however, a transient habitat‐dependent effect of grazer removal on macroalgal assemblage structure that emerged after three months, which was replaced by non‐interactive effects of grazer removal and habitat after 15 months. This study shows that the effects of the loss of key consumers may transcend large abiotic and biotic differences between habitats in rocky intertidal systems. While it is clear that consumer diversity is a primary driver of ecosystem functioning, determining its relative importance across multiple contexts is necessary to understand the consequences of consumer species loss against a background of environmental change.SynthesisThe roles of species may vary with environmental context, making it difficult to predict how biodiversity loss affects ecosystem functioning across multiple habitats. We tested how natural algal assemblages in two distinct intertidal habitats responded to the removal of different combinations of key consumer species. Despite an initial habitat‐dependent effect of consumer loss, habitat type did not modify the longer‐term responses of algal assemblages to either the identity or number of consumer species removed. Our findings show that, in certain systems, consumer diversity remains a primary driver of ecosystem functioning across widely different environmental contexts.

High prevalence of diffusive uptake of CO2 by macroalgae in a temperate subtidal ecosystem.

Productivity of most macroalgae is not currently considered limited by dissolved inorganic carbon (DIC), as the majority of species have CO2-concentrating mechanisms (CCM) allowing the active uptake of DIC. The alternative, diffusive uptake of CO2 (non-CCM), is considered rare (0-9% of all macroalgal cover in a given ecosystem), and identifying species without CCMs is important in understanding factors controlling inorganic carbon use by eukaryotic algae. CCM activity has higher energetic requirements than diffusive CO2 uptake, therefore when light is low, CCM activity is reduced in favour of diffusive CO2 uptake. We hypothesized that the proportional cover of macroalgae without CCMs (red and green macroalgae) would be low (<10%) across four sites in Tasmania, southern Australia at two depths (4-5 and 12-14 m); the proportion of species lacking CCMs would increase with decreasing depth; the δ(13)C values of macroalgae with CCMs would be more depleted with depth. We found the proportion of non-CCM species ranged from 0 to 90% and included species from all three macroalgal phyla: 81% of red (59 species), 14% of brown (three species) and 29% of green macroalgae (two species). The proportion of non-CCM species increased with depth at three of four sites. 35% of species tested had significantly depleted δ(13)C values at deeper depths. Non-CCM macroalgae are more abundant in some temperate reefs than previously thought. If ocean acidification benefits non-CCM species, the ramifications for subtidal macroalgal assemblages could be larger than previously considered.

THE STRUCTURE OF SUBTIDAL MACROALGAL ASSEMBLAGES AT THE TAMOIOS ECOLOGICAL STATION, A THREATENED CONSERVATION UNIT IN RIO DE JANEIRO, BRAZIL

The structure of subtidal rocky bottom communities at Tamoios Ecological Station (TES), situated in Ilha Grande Bay, Rio de Janeiro State, as well as in other Brazilian marine protected areas, is insufficiently characterized. The present study describes the macroalgal assemblages of shallow subtidal rocky bottoms on two islands of the TES-Imboassica (IM) and Búzios Pequena (BP)adopting species and genera as observational units. Two sites were surveyed on each island in summer 2011. Random 30x30 cm quadrats (n=3) were scraped to collect all macroalgae except crustose species. The subtidal assemblages, in which 58 macroalgal species occurred, were characterized by the high frequency and percent cover of Sargassum vulgare C. Agardh (56.8±8.4%). The sites differed significantly in total number of species and Shannon-Weiner diversity index (PERMANOVA, p&lt;0.05). According to SIMPER, the macroalgal genera that most contributed to the dissimilarities of cover between the sites (contribution &gt;5%) were Sargassum, Laurencia, Wrangelia, Canistrocarpus, Asparagopsis, Hypnea, Ceratodictyon, Gayliella, Spyridia and Chondria.Dissimilarities within and between the islands, as shown by nMDS of the cover data, suggest that different spatial scales should be considered in monitoring the rocky bottom communities of Ilha Grande Bay.

Stressor intensity determines antagonistic interactions between species invasion and multiple stressor effects on ecosystem functioning

Biological invasions, nutrient enrichment and ocean warming are known to threaten biodiversity and ecosystem functioning. The independent effects of these ecological stressors are well studied, however, we lack understanding of their cumulative effects, which may be additive, antagonistic or synergistic. For example, the impacts of biological invasions are often determined by environmental context, which suggests that the effects of invasive species may vary with other stressors such as pollution or climate change. This study examined the effects of an invasive seaweed (Sargassum muticum) on the structure and functioning of a synthetic macroalgal assemblage and tested explicitly whether these effects varied with nutrient enrichment and ocean warming. Overall, the presence of S. muticum increased assemblage productivity rates and warming altered algal assemblage structure, which was characterised by a decrease in kelp and an increase in ephemeral green algae. The effects of S. muticum on total algal biomass accumulation, however, varied with nutrient enrichment and warming, producing antagonistic cumulative effects on total algal biomass accumulation. These findings show that the nature of stressor interactions may vary with stressor intensity and among response variables, which leads to less predictable consequences for the structure and functioning of communities.

Shining light on benthic macroalgae: mechanisms of complementarity in layered macroalgal assemblages.

Phototrophs underpin most ecosystem processes, but to do this they need sufficient light. This critical resource, however, is compromised along many marine shores by increased loads of sediments and nutrients from degraded inland habitats. Increased attenuation of total irradiance within coastal water columns due to turbidity is known to reduce species' depth limits and affect the taxonomic structure and architecture of algal-dominated assemblages, but virtually no attention has been paid to the potential for changes in spectral quality of light energy to impact production dynamics. Pioneering studies over 70 years ago showed how different pigmentation of red, green and brown algae affected absorption spectra, action spectra, and photosynthetic efficiency across the PAR (photosynthetically active radiation) spectrum. Little of this, however, has found its way into ecological syntheses of the impacts of optically active contaminants on coastal macroalgal communities. Here we test the ability of macroalgal assemblages composed of multiple functional groups (including representatives from the chlorophyta, rhodophyta and phaeophyta) to use the total light resource, including different light wavelengths and examine the effects of suspended sediments on the penetration and spectral quality of light in coastal waters. We show that assemblages composed of multiple functional groups are better able to use light throughout the PAR spectrum. Macroalgal assemblages with four sub-canopy species were between 50–75% more productive than assemblages with only one or two sub-canopy species. Furthermore, attenuation of the PAR spectrum showed both a loss of quanta and a shift in spectral distribution with depth across coastal waters of different clarity, with consequences to productivity dynamics of diverse layered assemblages. The processes of light complementarity may help provide a mechanistic understanding of how altered turbidity affects macroalgal assemblages in coastal waters, which are increasingly threatened by diminishing light quantity and altered spectral distributions through sedimentation and eutrophication.

Variations in importance and intensity of competition underpin context dependency in the effects of an invasive seaweed on resident assemblages

Biological invasions are an important component of global biodiversity changes. Distinguishing between intensity and importance of competition may foster our understanding of the mechanisms underpinning context dependency in the ecological impacts of invasions. The importance of competition represents the effect of competition in regulating species performance in relation to that of the whole set of environmental drivers. In contrast, the intensity of competition estimates the difference in the performance of one or more target species between the presence and the absence of competition. We re-analyzed previously published data to evaluate how intensity and importance of competition from the invasive seaweed, Caulerpa cylindracea, on resident macroalgal assemblages changed following nutrient enrichment. Adding nutrients did not alter the intensity of competition from C. cylindracea on species richness, but decreased its importance. In contrast, both the intensity and importance of competition from C. cylindracea on community cover increased following nutrient enrichment. Thus, a greater impact of C. cylindracea at enhanced nutrient levels was not solely the result of increased intensity of competition, as could be concluded without taking into account that the performance of target organisms is generally impacted by multiple drivers. These results suggest that the importance of competition from exotic species depends on the extent to which native species are constrained by biotic interactions versus abiotic factors. Distinguishing between intensity and importance of competition seems crucial for assessing the ecological impacts of invasive species within a multiple stressor context.

Development of a new biotic index for ecological status assessment of Italian coastal waters based on coralligenous macroalgal assemblages

This paper presents a new biotic index (ESCA, Ecological Status of Coralligenous Assemblages), based on analyses of in situ records of coralligenous macroalgal assemblages in the Mediterranean Sea. The ESCA index was developed on the basis of previous impact evaluation studies carried out by traditional destructive methods. It was validated on an independent dataset collected during a 3-year study carried out at five localities in the north-western Mediterranean, and tested on a gradient of anthropogenic stressors. Assemblage descriptors selected as metrics of the ESCA index were: (a) presence/absence and abundance of sensitive taxa/groups (expressed as sensitivity level of assemblages, SL), (b) diversity of assemblages (expressed as α-diversity) and (c) heterogeneity of assemblages (expressed as β-diversity). The three metrics were combined to give a final value for the multimetric ecological index, Ecological Quality Ratio, calculated as the ratio between the measured values and the value obtained in the reference condition. The Ecological Quality Ratio values ranged from 0 to 1 and five ecological status classes were defined in accordance with the Water Framework Directive. The ESCA index could be used to define the ecological status of the studied sites, providing differences between assemblages similar to those shown by multivariate analyses. Moreover, ecological quality values were quite steady throughout the study period. The ESCA index may represent a useful tool for monitoring programmes in coastal waters, since it has been developed in compliance with the Water Framework Directive; it synthesizes complex information in a simple and reliable way; and its application is simple and cost-effective. However, further studies are necessary to complete index development and to properly evaluate its strengths and weaknesses on a larger scale.

The role of disturbance in promoting the spread of the invasive seaweed Caulerpa racemosa in seagrass meadows

Human disturbances, such as anchoring and dredging, can cause physical removal of seagrass rhizomes and shoots, leading to the fragmentation of meadows. The introduced green alga, Caulerpa racemosa, is widely spread in the North-West Mediterranean and, although it can establish in both degraded and pristine environments, its ability to become a dominant component of macroalgal assemblages seems greater in the former. The aim of this study was to estimate whether the spread of C. racemosa depends on the intensity of disturbance to the canopy structure of Posidonia oceanica. A field experiment was started in July 2010 when habitat complexity of a P. oceanica meadow was manipulated to simulate mechanical disturbances of different intensity: rhizome damage (High disturbance intensity = H), leaf removal (Low disturbance intensity = L), and undisturbed (Control = C). Disturbance was applied within plots of different size (40 × 40 cm and 80 × 80 cm), both inside and at the edge of the P. oceanica meadow, according to an orthogonal multifactorial design. In November 2011 (16 months after the start of the experiment), no C. racemosa was found inside the seagrass meadow, while, at the edge, the cover of the seaweed was dependent on disturbance intensity, being greater where the rhizomes had been damaged (H) than in leaf removal (L) or undisturbed (C) plots. The results of this study indicate that physical disturbance at the margin of seagrass meadows can promote the spread of C. racemosa.

Invader traits and community context contribute to the recent invasion success of the macroalga Heterosiphonia japonica on New England rocky reefs

The successful establishment and spread of invasive species may be determined by intrinsic traits of the invader, characteristics of the invaded community, and interactions between the two. The red filamentous macroalga Heterosiphonia japonica has recently invaded and expanded its range in the northwestern Atlantic. We surveyed and compared macroalgal communities at sites with different levels of invasion in Nahant, Massachusetts, and examined potential factors contributing to Heterosiphonia’s successful invasion by assessing its growth rates and rates of herbivory by native grazers in the laboratory. We evaluated growth and herbivory for isolated individuals and for individuals within a macroalgal assemblage comprised of the most abundant species in the native community. We also measured macroalgal nitrate uptake rates to examine a potential mechanism underlying observed differences in growth rate. High abundances of Heterosiphonia were associated with lower native macroalgal species richness, evenness, and diversity and with differences in species composition in the field. Experiments showed that rates of growth and grazing were context dependent. In isolation, Heterosiphonia grew comparably and was grazed similarly to the two native filamentous macroalgae. Within the context of the native macroalgal assemblage, it grew much faster than all common native species, experienced reduced herbivory from one native grazer, and increased herbivory from another. Consistent with its faster growth, Heterosiphonia was the most efficient at nitrate uptake. Our results suggest that multiple interactions between intrinsic traits (e.g., competitiveness for nutrients) and extrinsic factors (e.g., presence of native macroalgal species, identity and relative abundances of native grazers) contribute to Heterosiphonia’s invasion success.

Spatio temporal and environmental factors influencing macroalgal β diversity in the Red Sea, Egypt

Abstract β Diversity is an ecological concept used to describe the turnover of species across a wide range of spatial and temporal scales, but such knowledge is lacking for macroalgal assemblages of the Red Sea. β Diversity patterns for Red Sea macroalgae were evaluated from different sites (Hurghada, Safaga, and Al-Quseir) and seasons across different environmental disturbances. β Diversity was studied by partitioning the total number of species (γ diversity) into additive components. Geographical heterogeneity was more important than seasonal heterogeneity in structuring macroalgae both at species and functional group levels. Species replacement as a component of β diversity produced dissimilarity in species composition and taxonomic structure. Replacement of species between sites was responsible for the presence of new functional groups of macroalgae. High β diversity and taxonomic similarity values were characteristic of the macroalgae of the Red Sea. The occurrence of small macroalgal thalli that have short life cycles induced high species replacement and subsequently high β diversity, with spatial heterogeneity and environmental gradient as drivers of β diversity. Anthropogenic disturbance at the Safaga site was suggested to induce variation of macroalgal assemblages and functional groups. Excluding rare species from the data set did not change the high values of b diversity.

갯닦기 전후 제주도 인공어초의 해조상 및 수심별 감태의 생장양상

Marine algal assemblages on the artificial reefs at three stations (Haengwon, Geumneung, Pyoseon in Jeju-do) and the growth pattern of Ecklonia cava with water depth were studied before and after rocky cleaning. Nine algal species occurred at three artificial reefs before rocky cleaning in July 2012, however, 19 algal species were found at three artificial reefs after rocky cleaning in July 2013. In particular, 13 of 19 species in 2013 were replaced by different species which were not found in July 2012. Algal biomass rapidly increased in July 2013 after rocky cleaning. The nMDS plot based on the presence and absence data of macro-algal assemblages on the artificial reefs showed that the species compositions between artificial reefs were similar to each other before rocky cleaning operations in July of 2012. However, after rocky cleaning, the species composition of macro-algal assemblages in Haengwon region was similar to that in Pyoseon region while that in Geumneung region was different to those in Haengwon and Pyseon regions in July 2013. It is needed to clarify the suitable water depth for transplantation of perennial Phaeophyta E. cava. According to the data on seasonal changes in total length, total weight, blade length, blade width, blade weight of E. cava, it began to grow rapidly from April and reached to the maximum value in June, and then degeneration of the blade occurred in July and continued to early August. Although the total length of E. cava at 10m water depth was larger than those at 5 m and 15 m water depths, there was no significant difference statistically among water depths by ANOVA test. However, in the values of total weight, blade length, blade width, and blade weight, the growth patterns at 5 m water depth were larger than those at 10 m and 15 m. Thus, it is assumed that the most suitable water depth for transplantation and its maximum growth and effective release of zoospores of E. cava will be 7~10 m water depth.

Impacts of natural and manipulated variations in temperature, pH and light on photosynthetic parameters of coralline–kelp assemblages

Increasing absorption of CO2 by the world's oceans is lowering seawater pH and may have severe consequences for marine calcifying organisms. Understanding the ecological consequences of anthropogenic CO2 emissions will require examination of how calcifying organisms and their associated communities respond to natural variation in CO2 concentration. Many macroalgae may respond positively or neutrally to ocean acidification, but calcifying species such as coralline algae are predicted to be some of the most susceptible organisms to changing CO2. Here I test the impacts of temperature and pH variation on important photosynthetic metrics of macroalgal assemblages composed of coralline turf, Corallina vancouveriensis and the associated canopy-forming kelp, Saccharina sessilis using in situ photorespirometry and laboratory mesocosms. In situ photorespirometry was done at two locations on the Oregon (USA) coast, an area with variable upwelling of high CO2, low pH water. To complement in situ measurements, a series of laboratory mesocosms were used to disentangle the effects of pH and temperature on photosynthetic parameters across a light gradient. The acute effects of low pH were also tested across a temperature gradient, revealing an exacerbated effect of short duration, low pH events on respiration rates at increasing temperature. NPP (net primary productivity) was reduced by 10–20% within in situ coralline assemblages across a natural gradient of pH (8.1–7.9), but there was a mostly neutral effect of low pH on NPP of coralline–kelp assemblages. These results indicate varied responses of coralline and coralline–kelp assemblages to temperature and pH gradients, but under limiting light conditions primary production and growth of corallines are likely to decrease under modest scenarios of CO2 increase. Assemblage composition could play an important role in modulating the impacts of ocean acidification on calcifying organisms, and results from this study suggest that canopy and sub-canopy interactions could determine the response of susceptible species to changing climatic parameters.

갯닦기 전후의 해중림초의 해조상과 시험 부착판에서의 해조류 초기 천이

The marine algal flora assemblages settled on the artificial seaweed reefs in Jeju-do before and after rocky cleaning were investigated during summer and the early succession of seaweeds attached to the test panels on the structures of the artificial reef after rocky cleaning were observed by visual observations and under the dissecting microscope in order to evaluate the effects of this rocky cleaning operation. Species composition and algal biomass on the Bboolsamgak-typed seaweed artificial reefs before and after rocky cleaning were measured and compared before and after rocky cleaning. Before rocky cleaning in July 2012, only nine algal species had existed on the Bboolsamgak-typed seaweed artificial reefs, however, twenty algal species occurred on the seaweed artificial reefs after rocky cleaning in July 2013. Particularly, 15 species among them in July 2013 were not found in July 2012 and they seemed to occur newly on the artificial reefs. A large quantity of algal biomass increased in July 2013 after rocky cleaning. Based on the importance value, there was a drastic change in the dominant species between July 2012 and July 2013 after rocky cleaning. By dissecting micrographs, an epiphytic diatom, Licmophora sp. and flagellates, were observed on the 9th day. A chlorophyte, Ulothrix flacca was observed after 14 days, Ulva pertusa, Bryopsis pulmosa, crustose coralline algae, and Cladophora spp. were after one month. After two months, a significant species succession was observed from three previous chlorophyte species to crustose coralline algae existed as a dominant species on the panels. In this study, however, any brown algae were not found until three months in the early seaweed succession. All above results indicates that the rocky cleaning is a very efficient method for the establishment of macroalgal assemblages on the artificial reefs.

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

Ongoing changes in natural diversity due to anthropogenic activities can alter ecosystem functioning. Particular attention has been given to research on biodiversity loss and how those changes can affect the functioning of ecosystems, and, by extension, human welfare. Few studies, however, have addressed how increased diversity due to establishment of nonindigenous species (NIS) may affect ecosystem function in the recipient communities. Marine algae have a highly important role in sustaining nearshore marine ecosystems and are considered a significant component of marine bioinvasions. Here, we examined the patterns of respiration and light-use efficiency across macroalgal assemblages with different levels of species richness and evenness. Additionally, we compared our results between native and invaded macroalgal assemblages, using the invasive brown macroalga Sargassum muticum (Yendo) Fensholt as a model species. Results showed that the presence of the invader increased the rates of respiration and production, most likely as a result of the high biomass of the invader. This effect disappeared when S.muticum lost most of its biomass after senescence. Moreover, predictability-diversity relationshipsof macroalgal assemblages varied between native and invaded assemblages. Hence, the introduction of high-impact invasive species may trigger major changes in ecosystem functioning. The impact of S.muticum may be related to its greater biomass in the invaded assemblages, although species interactions and seasonality influenced the magnitude of the impact.

The indirect role of nutrients in enhancing the invasion of Caulerpa racemosa var cylindracea

The effects of nutrients dynamics on biological invasions in marine habitats have not been fully investigated. The present study aimed to evaluate the role of nutrient pollution in determining the successful invasion of the introduced seaweed Caulerpa racemosa var. cylindracea in a Mediterranean subtidal rocky habitat. To this end, a manipulative field experiment was carried out by supplying the water column with nutrients for a one-year period. Afterwards C. racemosa was transplanted in both nutrient-enriched and control macroalgal assemblages. Results showed that the penetration and biomass of C. racemosa in the assemblages previously treated with nutrients were higher than in control ones, indicating that communities stressed by nutrient pollution are more vulnerable to invasion. The nutrient enrichment changed the structure of natural macroalgal assemblages, mainly by encouraging the growth of turf species. No significant effects were detected for the erect and prostrate layers and for the number of species, suggesting that the susceptibility of the community to invasion may depend more on the morpho-functional identity of macroalgae rather than on the diversity of assemblages. Changes in the availability of seawater nutrients may foster the spread of C. racemosa, both by enhancing its growth and eroding the natural resistance of macroalgal assemblages. Thus, the maintenance of good water quality may play an important role in containing both C. racemosa and other introduced seaweeds.

Long-term changes in sublittoral macroalgal assemblages related to water quality improvement

Abstract Sampling of sublittoral macroalgal assemblages was carried out in 1982 and 1999 along the coast of Catalonia (Northwestern Mediterranean, Spain) in order to study long-term changes in species composition. Ordination analysis revealed significant differences in species composition and abundance between the two study periods. The number of stations with indicators of eutrophication, such as species of Chaetomorpha, Cladophora, and Ulva, decreased from 1982 to 1999. This decrease was not balanced by an increase in species typical of pristine environments, such as large perennial brown algae (in particular, Cystoseira mediterranea), but rather by an increase of stress-resistant species such as Corallina elongata. The observed shifts seem to be driven by the decrease in nutrient loadings in the water column, resulting from the progressive implementation of sewage treatment management in the study region since the mid-1970s.

Usefulness of tissue nitrogen content and macroalgal community structure as indicators of water eutrophication

We tested the hypothesis that the community structure and biochemical composition of macroalgae reflect the degree of nutrient concentrations in the water column. Benthic community structure and tissue nitrogen (N) content of macroalgae on intertidal rocky shores at three sites were investigated in relation to sewage effluents on Mireuk Island, Tongyeong city, on the southern coast of Korea. Ulva australis clearly dominated at site 1, which was close to a sewage treatment plant, where higher dissolved inorganic N and dissolved inorganic phosphate concentrations were observed. U. australis-dominated communities also appeared at site 2 (intermediate levels of nutrient enrichment). The macroalgal assemblage at site 3 (unimpacted site) was significantly different from those at sites 1 and 2. Five species (U. australis, Sargassum fusiforme, Grateloupia elliptica, Gelidium amansii, and Sargassum horneri) were dominant at site 3, representing 87 % of the total coverage throughout the study period. Species richness (d), evenness (J'), and diversity index (H') were highest at site 3, intermediate at site 2, and lowest at site 1, showing a negative relationship with nutrient levels. These results indicate that macroalgal community structure can be used as a bioindicator in water quality assessment. The tissue N content of green and red algae was responsive to nutrient availability, while the tissue N content of brown algae was relatively unchanged among the sites. This suggests that tissue N content as a bioindicator for detecting the influence of sewage effluent should be considered to reflect the N storage capacity of macroalgae.

Distribution patterns of the invasive herbivore Siganus luridus (Rüppell, 1829) and its relation to native benthic communities in the central Aegean Sea, Northeastern Mediterranean

AbstractThe present study explores the distribution of the invasive herbivore fish Siganus luridus (Rüppell 1829) and its relationship to native herbivores and macroalgal assemblages in the shallow sublittoral of the Cyclades Archipelago, Aegean Sea, Greece. In situ underwater surveys of herbivore abundance (fishes and sea urchins) and algal coverage were carried out at 180 sampling sites in 18 islands. Siganus luridus accounted for 17% of the total herbivore abundance, with a decrease in relative abundance from the southeastern to the northwestern islands. In Santorini Island (in the southeast of the study area) S. luridus abundance accounted for 90% of the total herbivore fish abundance, while in Kea Island (at the northwestern limit of the study area), S. luridus was absent. The spatial variation of minimum sea surface temperature is possibly the reason for its distributional pattern. Siganus luridus abundance was found to be positively correlated to the native herbivore Sparisoma cretense (Linnaeus 1758). A significant negative relationship was found between the abundance of the invasive species and the sum of erect and canopy algae cover (Dictyotales and Cystoseira spp.), which are the main components of its diet in the region. On the other hand, its occurrence was particularly high in barren sites. The results arising from this study reinforce evidence from studies in the Eastern Mediterranean Basin that the expansion of the invasive species S. luridus may have profound impacts on native communities in the Mediterranean infralittoral zone.

Impacts of temperature on primary productivity and respiration in naturally structured macroalgal assemblages.

Rising global temperatures caused by human-mediated change has already triggered significant responses in organismal physiology, distribution and ecosystem functioning. Although the effects of rising temperature on the physiology of individual organisms are well understood, the effect on community-wide processes has remained elusive. The fixation of carbon via primary productivity is an essential ecosystem function and any shifts in the balance of primary productivity and respiration could alter the carbon balance of ecosystems. Here we show through a series of tests that respiration of naturally structured algal assemblages in southern New Zealand greatly increases with rising temperature, with implications for net primary productivity (NPP). The NPP of in situ macroalgal assemblages was minimally affected by natural temperature variation, possibly through photo-acclimation or temperature acclimation responses, but respiration rates and compensating irradiance were negatively affected. However, laboratory experiments testing the impacts of rising temperature on several photosynthetic parameters showed a decline in NPP, increasing respiration rates and increasing compensating irradiance. The respiration Q10 of laboratory assemblages (the difference in metabolic rates over 10°C) averaged 2.9 compared to a Q10 of 2 often seen in other autotrophs. However, gross primary productivity (GPP) Q10 averaged 2, indicating that respiration was more severely affected by rising temperature. Furthermore, combined high irradiance and high temperature caused photoinhibition in the laboratory, and resulted in 50% lower NPP at high irradiance. Our study shows that communities may be more severely affected by rising global temperatures than would be expected by responses of individual species. In particular, enhanced respiration rates and rising compensation points have the potential to greatly affect the carbon balance of macroalgal assemblages through declines in sub-canopy NPP, the impacts of which may be exacerbated over longer time-scales and could result in declines in sub-canopy species richness and abundance.

Introduced and native species on rocky shore macroalgal assemblages: Zonation patterns, composition and diversity

The study of the natural and anthropogenic changes in patterns of species and habitat diversity is important for understanding the organization of natural species assemblages. One of the major threats to native marine biodiversity and resource values is the introduction of exotic species. Rocky intertidal shores are considered among the environments that are susceptible to the introduction of organisms like macroalgae. The scope of this work is to study the spatial variation of the native and introduced intertidal macroalgae in rocky shores of northern Patagonia, Argentina. Seasonal sampling of cover, abundance, richness, biomass and diversity of native and introduced macroalgae at three intertidal levels (high, middle and low) was carried out at four wave-protected rocky shores during one year. We found a conspicuous zonation pattern of the dominant species of algae and invertebrates, with the greatest richness, abundance and algal diversity at the low intertidal level, but these variables were heterogeneous through time and space. These differences were mainly due to the variations in the abundance of ephemeral algae. Introduced species represented around 20–25% of the total richness of each locality, being most abundant in those localities that also showed a greater total diversity. This study provides the first assessment of rocky shore macroalgae assemblages from Argentina that incorporates the presence of introduced species and shows that the number of introduced algae species along Patagonian rocky shores had been underestimated. We also encourage the monitoring of the biodiversity and the study of the processes that are involved in the role that introduced species plays in these environments.