Effects of cooling water discharge on the structure and dynamics of epilithic algal communities in the northern Baltic

The Forsmark Biotest Basin is a shallow coastal ecosystem that receives brackish cooling-water discharge from a nuclear power plant. The effects of the discharge on epilithic algal communities were investigated by analysing samples taken every third week throughout one year at 11 sites differentially affected by temperature and/or flow rate enhancement. Community variation was summarized in a canonical correspondence analysis (CCA) of species abundances as a function of site and date. The temperature increase favoured blue-green algae at the expense of red and brown algae. Blue-green algae were however abundant in summer in stagnant water, whether heated or not, and some red and brown algae became abundant in winter in heated sites with flowing water. Green algae and diatoms increased in biomass in the heated sites, but not in relative cover-abundance. The absence of ice and snow cover at sites with heated and/or flowing water caused autumn species to persist into winter, because of the higher light intensity (compared with natural conditions) and the absence of the mechanical abrasion by ice. The thermal discharge lowered species diversity (Shannon-Weaver index) both in summer and winter at sites with flowing water, but not at sites with quiescent or stagnant water. CCA showed alternate periods of stability and rapid change within the seasonal cycle. Individual species were placed according to their optimum; red and brown algae in winter/spring, green algae in spring/summer, blue-green algae in summer, and diatoms at various times. Exceptions to this pattern were species endo- or epiphytic on species of a different group. Analysis of the effects of temperature, flow rate and ice cover on the seasonal pattern of particular species showed that different species respond in individualistic ways to different combinations of these environmental variables.     

Mycorrhizal suppression and phosphorus addition influence the stability of plant community composition and function in a temperate steppe

Nutrient enrichment can reduce ecosystem stability, typically measured as temporal stability of a single function, e.g. plant productivity. Moreover, nutrient enrichment can alter plant–soil interactions (e.g. mycorrhizal symbiosis) that determine plant community composition and productivity. Thus, it is likely that nutrient enrichment and interactions between plants and their soil communities co-determine the stability in plant community composition and productivity. Yet our understanding as to how nutrient enrichment affects multiple facets of ecosystem stability, such as functional and compositional stability, and the role of above–belowground interactions are still lacking. We tested how mycorrhizal suppression and phosphorus (P) addition influenced multiple facets of ecosystem stability in a three-year field study in a temperate steppe. Here we focused on the functional and compositional stability of plant community; functional stability is the temporal community variance in primary productivity; compositional stability is represented by compositional resistance, turnover, species extinction and invasion. Community variance was partitioned into population variance defined as community productivity weighted average of the species temporal variance in performance, and species synchrony defined as the degree of temporal positive covariation among species. Compared to treatments with mycorrhizal suppression, the intact AM fungal communities reduced community variance in primary productivity by reducing species synchrony at high levels of P addition. Species synchrony and population variance were linearly associated with community variance with the intact AM fungal communities, while these relationships were decoupled or weakened by mycorrhizal suppression. The intact AM fungal communities promoted the compositional resistance of plant communities by reducing compositional turnover, but this effect was suppressed by P addition. P addition increased the number of species extinctions and thus promoted compositional turnover. Our study shows P addition and AM fungal communities can jointly and independently modify the various components of ecosystem stability in terms of plant community productivity and composition.     

Temperature and species richness effects in phytoplankton communities

Phytoplankton play an important role as primary producers and thus can affect higher trophic levels. Phytoplankton growth and diversity may, besides other factors, be controlled by seasonal temperature changes and increasing water temperatures. In this study, we investigated the combined effects of temperature and diversity on phytoplankton growth. In a controlled laboratory experiment, monocultures of 15 freshwater phytoplankton taxa (green algae, cyanobacteria, and diatoms) as well as 25 mixed communities of different species richness (2–12 species) and taxa composition were exposed to constant temperatures of 12, 18, and 24 °C. Additionally, they were exposed to short-term daily temperature peaks of +4 °C. Increased species richness had a positive effect on phytoplankton growth rates and phosphorous content at all temperature levels, with maximum values occurring at 18 °C. Overyielding was observed at almost all temperature levels and could mostly be explained by complementary traits. Higher temperatures resulted in higher fractions of cyanobacteria in communities. This negative effect of temperature on phytoplankton diversity following a shift in community composition was most obvious in communities adapted to cooler temperatures, pointing to the assumption that relative temperature changes may be more important than absolute ones.     

Long-term response of the biotic community to fluctuating water levels and changes in water quality in Cootes Paradise Marsh,a degraded coastal wetland of Lake Ontario

During the early 1900s, more than 90% of the surface area of Cootes Paradise Marsh was covered with emergent vegetation; currently, less than 15% of the surface is covered with aquatic vegetation and the remainder is wind-swept, turbid, open water. The loss of emergent cover is significantly correlated with mean annual water levels that increased more than 1.5 m over the past 60 years. Species diversity and the percent cover of the submerged macrophtye community also declined dramatically after the 1940s, coincident with decreased water clarity and increased nutrients from pollution by sewage and stormwater effluent. Phosphorus levels in the marsh dropped ten-fold after the sewage plant was upgraded to a tertiary-treatment facility in 1978; however, there was no measurable improvement in water clarity, in spite of a decrease in chlorophyll concentrations. Long-term changes in the composition of the planktonic, benthic and fish communities accompanied changes in water clarity, nutrient status and macrophyte cover. Phytoplankton changed from a community dominated by diverse taxa of green algae and diatoms during the 1940s, to a less diverse community dominated by a few taxa of green and blue-green algae in the 1970s, then to a much more diverse community recently, including many taxa of green algae, diatoms and chrysophytes; however, because water turbidity continues to be high, and algae tolerant of low light levels are now very abundant. Daphnia, which were prominent during the 1940s (especially in the vegetated sites) were replaced in the 1970s by smaller zooplankton such as the cladoceran, Bosmina, and several rotifer species including Brachionus, Asplanchna and Keratella. In the recent survey conducted in 1993 and 1994, small-bodied forms still dominate the turbid open-water areas, while medium-sized cladocerans such as Moina were common near macrophyte beds. Generally, total herbivorous zooplankton biomass tended to be highest next to Typha beds and declined with increasing distance from the plants. Conversely, biomass of edible algae at these sites increased with distance from the macrophytes. Species diversity of aquatic insects declined dramatically over the past 40 years, from 57 genera (23 families and 6 orders) in 1948, to 9 genera (6 families and 3 orders) in 1978, to only 5 genera (3 families and 2 orders) in 1995. The diverse benthic community present 5 decades ago has now been replaced by a community consisting primarily of chironomid larvae, oligochaetes and other worms associated with low-oxygen environments. These successional changes illustrate the impact of natural (fluctuating water levels) and anthropogenic (deterioration in water quality) stressors on the character of the biotic communities, and reveal the complex interactions among the various trophic levels and the abiotic environment as degradation and remediation proceeded.     

典型红树林生态系统藻类多样性及其在生态过程中的作用

藻类是红树林生态系统重要的生物类群, 根据生态习性可分为浮游植物、底栖微藻和大型藻类三个生态类群, 它们在红树林生态系统生物多样性、初级生产、元素循环等方面起着重要作用。但在红树林生态系统中, 关注重点多集中在红树植物和动物, 对其中的藻类重视不够, 且多数研究集中在近20年以及亚洲的红树林区。事实上, 红树林生态系统藻类非常丰富, 其多样性研究有助于深入揭示红树林生态系统的结构与功能。本文介绍了红树林生态系统藻类的组成类群及其重要性, 重点对红树林区浮游植物、底栖硅藻和大型海藻的种类组成、地理分布及其与初级生产力、水质污染、元素循环、碳库形成等生态过程中的作用的研究动态和进展等进行了总结。根据已有研究, 红树林区浮游植物和底栖硅藻的种类数一般为几十到上百种, 其中硅藻在种类和数量上都占绝对优势, 它们是重要的初级生产者、饵料生物和水质污染指示生物; 红树林区底栖大型藻类主要由红藻、绿藻、褐藻、蓝藻组成, 绿藻的种类较多, 红藻在数量上占优势; 藻类是红树林湿地碳库的重要贡献者, 在红树林湿地生态系统碳汇和碳循环中起重要作用。红树林生态系统是个高度动态和异质的系统, 今后应加强红树林藻类多样性的长周期、大尺度变化及不同生境藻类的综合研究, 关注大陆径流和潮汐对藻类多样性和蓝碳的影响, 借助沉积物藻类记录, 探明红树林区藻类的长周期变化, 反演气候变化和人类活动对红树林生态系统的影响过程和机制。     

Benthic diatom assemblages in mountain streams: community structure in relation to environmental and human pressures

The objective of this study was to determine the environmental factors that best explain the distribution and community composition of benthic diatoms in undisturbed mountain streams in the Eastern Highlands of Zimbabwe. Benthic diatoms were sampled during the dry season from 21 sampling sites established along altitudinal gradient of the study rivers. A total of 119 diatom species belonging to 38 genera and twelve families were recorded for all the 21 sites sampled. No significant differences were observed in species diversity and equitability amongst the three river systems. However, species richness, diversity and equitability decreased significantly along the longitudinal gradient of the rivers, with the highest richness, diversity and evenness being recorded in the upper reaches. Temperature, velocity, NO₃^? and Ca²⁺ levels were strongly associated with changes in diatom communities in the three rivers. Inventory of diatom communities has applications in many fields of biological research including conservation and biological monitoring of ecosystem changes.     

Functional diversity of phytoplankton highlights long‐term gradual regime shift in the middle section of the Danube River due to global warming,human impacts and oligotrophication

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Does functional homogenization accompany taxonomic homogenization of British birds and how do biotic factors and climate affect these processes?

Environmental change has reshuffled communities often causing taxonomic homogenization rather than differentiation. Some studies suggest that this increasing similarity of species composition between communities is accompanied by an increase in similarity of trait composition—functional homogenization—although different methodologies have failed to come to any consistent conclusions. Functional homogenization could have a large effect on ecosystem functioning and stability. Here, we use the general definition of homogenization as “reduced spatial turnover over time” to compare changes in Simpson's beta diversity (taxonomic turnover) with changes in Rao's quadratic entropy beta diversity (functional turnover) in British breeding birds at three spatial scales. Using biotic and climatic variables, we identify which factors may predispose a site to homogenization. The change in turnover measures between two time periods, 20 years apart, was calculated. A null model approach was taken to identify occurrences of functional homogenization and differentiation independent of changes in taxonomic turnover. We used conditional autoregressive models fitted using integrated nested Laplace approximations to determine how environmental drivers and factors relating to species distributions affect changes in spatial turnover of species and functional diversity. The measurement of functional homogenization affects the chance of rejection of the null models, with many sites showing taxonomic homogenization unaccompanied by functional homogenization, although occurrence varies with spatial scale. At the smallest scale, while temperature‐related variables drive changes in taxonomic turnover, changes in functional turnover are associated with variation in growing degree days; however, changes in functional turnover become more difficult to predict at larger spatial scales. Our results highlight the multifactorial processes underlying taxonomic and functional homogenization and that redundancy in species traits may allow ecosystem functioning to be maintained in some areas despite changes in species composition.     

Alpha and beta diversity of connected benthic–subsurface invertebrate communities respond to drying in dynamic river ecosystems

Drying disturbances are the primary determinant of aquatic community biodiversity in dynamic river ecosystems. Research exploring how communities respond to disturbance has focused on benthic invertebrates in surface sediments, inadequately representing a connected community that extends into the subsurface. We compared subsurface and benthic invertebrate responses to drying, to identify common and context‐dependent spatial patterns. We characterized community composition, alpha diversity and beta diversity across a gradient of drying duration. Subsurface communities responded to drying, but these responses were typically less pronounced than those of benthic communities. Despite compositional changes and in contrast to reductions in benthic alpha diversity, the alpha diversity of subsurface communities remained stable except at long drying durations. Some primarily benthic taxa were among those whose subsurface frequency and abundance responded positively to drying. Collectively, changing composition, stable richness and taxon‐specific increases in occurrence provide evidence that subsurface sediments can support persistence of invertebrate communities during drying disturbances. Beta‐diversity patterns varied and no consistent patterns distinguished the total diversity, turnover or nestedness of subsurface compared to benthic communities. In response to increasing drying duration, beta diversity increased or remained stable for benthic communities, but remained stable or decreased for subsurface communities, likely reflecting contrasts in the influence of mass effects, priority effects and environmental filtering. Dissimilarity between subsurface and benthic communities remained stable or increased with drying duration, suggesting that subsurface communities maintain distinct biodiversity value while also supporting temporary influxes of benthic taxa during drying events. As temporary rivers increase in extent due to global change, we highlight that recognizing the connected communities that extend into the subsurface sediments can enable holistic understanding of ecological responses to drying, the key determinant of biodiversity in these dynamic ecosystems.     

Biological quality metrics: their variability and appropriate scale for assessing streams

The concept of spatial scale is at the research frontier in ecology, and although focus has been placed on trying to determine the role of spatial scale in structuring communities, there still is a further need to standardize which organism groups are to be used at which scale and under which circumstances in environmental assessment. This paper contributes to the understanding of the variability at different spatial scales (reach, stream, river basin) of metrics characterizing communities of different biological quality elements (macrophytes, fishes, macroinvertebrates and benthic diatoms) as defined by the Water Framework Directive. For this purpose, high-quality reaches from medium-sized lowland streams of Latvia, Ecoregion 15 (Baltic) were sampled using a nested hierarchical sampling design: (river basin → stream → reach). The variability of metrics within the different groups of biological quality elements confirmed that large-bodied organisms (macrophytes and fish) were less variable than small-bodied organisms (macroinvertebrates and benthic diatoms) at reach, stream and river basin scales. Single metrics of biological quality elements had the largest variation at the reach scale compared with stream and basin scales. There were no significant correlations between biodiversity indices of the different organism groups. The correlation between diversity indices (Shannon’s and Simpson’s) of the biological quality elements (macrophytes, fish, benthic macroinvertebrates and benthic diatoms) and a number of measured environmental variables varied among the different organism groups. Relationships between diversity indices and environmental factors were established for all groups of biological quality elements. Our results showed that metrics of macrophytes and fish could be used for assessing ecological quality at the river basin scale, whereas metrics of macroinvertebrates and benthic diatoms were most appropriate at a smaller scale.     

Effects of a natural flood disturbance on species richness and beta diversity of stream benthic diatom communities

Natural hydrological disturbances in streams may reduce biomass and species richness and change community composition within streams. Disturbances can also affect beta diversity among streams if their effects are species specific or vary across sites. We investigated the effect of a natural flood on species richness, community composition and among-streams beta diversity of benthic diatoms (total community and three functional groups: low profile, high profile and motile) of seven streams in New Zealand. Sampling occurred shortly before, 10 days after and 40 days after the flood. Species richness of the total diatom community did not change after the flood. The high-profile group was the only one whose species richness declined after the flood, whereas species richness of the low-profile group increased. Community composition changed after the flood, mostly as a result of species replacement rather than richness differences over time. Finally, among-streams beta diversity did not change after the flood, suggesting that variation in species composition of benthic diatoms among streams may be maintained in the face of flood disturbances.     

Influence of the temperature regime on the composition of the macrozoobenthos community in a thermal brook in Serbia

In contrast to cold and eurythermal waters, benthic communities of warm brooks in temperate regions have been inadequately studied. In order to investigate the effects of water thermal regime on the benthic communities of warm waters and their relationships with those of cold and eurythermic ones, the macrozoobenthos was studied at eight sites in the Toplica River, and at four sites in its tributary, the Termalni brook. Investigations were carried out seasonally from April 2000 to January 2001. Warm waters of the Termalni brook were characterized by specific macrozoobenthos assemblages that exhibited significant differences to the populations of eurythermal and cold waters of the Toplica River. The dominant taxa in the macrozoobenthos community of warm waters were mainly Gastropoda species. Moreover, benthic communities of warm waters were characterized by lower diversity and greater biomass in comparison with those of cold and eurythermal waters. The gradient of average annual temperatures represented the main ecological factor influencing changes of diversity and biomass along the course of the investigated Termalni brook. Inflow of warm waters at site T₆ lead to a decrease in macrozoobenthos abundance and changes in qualitative and quantitative composition of the benthocoenosis of a highland stream, but did not significantly alter diversity.     

A heatwave increases turnover and regional dominance in microbenthic metacommunities

While the effect of the global biodiversity crisis on local species loss is still debated, there is empirical evidence for major changes in local biodiversity attributed to increased species turnover. In communities exposed to a climate stressor, species turnover can lead to increased dominance of well-adapted species and consequently to an overall decline in species diversity. Despite the known importance of species turnover for community dynamics and functioning, experimental results on the connection between biodiversity loss and species turnover are scarce. We still do not fully understand which specific factors increase the rate of change in species composition, especially when considering natural compared to artificially lab assembled communities. In the present study, we experimentally tested whether a heatwave and dispersal increased species turnover and decreased species diversity in natural benthic diatom communities with different initial species compositions. We found that on the local scale, dispersal had overall positive effects on species richness while the relationship between exposure to the heatwave, species turnover, and diversity depended on initial community composition. However, on the regional (i.e. metacommunity) scale, exposure to the heatwave and dispersal both increased turnover and decreased Shannon diversity by almost 50%. Turnover in these metacommunities was not caused by a loss of species, but rather by a change in dominance patterns leading to homogenization, and consequently decreased diversity. Our study shows that climate change can destabilize community composition and degrade species diversity, but still after ca. 15 generations does not decrease the number of species in the community, demonstrating that the response of species diversity and richness to changing conditions can be fundamentally decoupled on ecological time scales.     

Microbial diversity associated with four functional groups of benthic reef algae and the reef-building coral Montastraea annularis

The coral reef benthos is primarily colonized by corals and algae, which are often in direct competition with one another for space. Numerous studies have shown that coral-associated Bacteria are different from the surrounding seawater and are at least partially species specific (i.e. the same bacterial species on the same coral species). Here we extend these microbial studies to four of the major ecological functional groups of algae found on coral reefs: upright and encrusting calcifying algae, fleshy algae, and turf algae, and compare the results to the communities found on the reef-building coral Montastraea annularis. It was found using 16S rDNA tag pyrosequencing that the different algal genera harbour characteristic bacterial communities, and these communities were generally more diverse than those found on corals. While the majority of coral-associated Bacteria were related to known heterotrophs, primarily consuming carbon-rich coral mucus, algal-associated communities harboured a high percentage of autotrophs. The majority of algal-associated autotrophic Bacteria were Cyanobacteria and may be important for nitrogen cycling on the algae. There was also a rich diversity of photosynthetic eukaryotes associated with the algae, including protists, diatoms, and other groups of microalgae. Together, these observations support the hypothesis that coral reefs are a vast landscape of distinctive microbial communities and extend the holobiont concept to benthic algae.     

Nearshore subtidal community structure compared between inner coast and outer coast sites in Southeast Alaska

Processes that structure subarctic marine communities, particularly in glaciated regions, are not well understood. This understanding is needed as a baseline and to manage these communities in the face of future climate-driven changes. This study investigates two coastal regions of Southeast Alaska with the goals to (a) identify and compare patterns of subtidal community structure for macroalgal, fish, macroinvertebrate (>5?cm), and small epibenthic invertebrate (<5?cm) communities between inner coast and outer coast sites and (b) link patterns of community structure to habitat and environmental parameters. Species assemblage and benthic habitat data were used to compare species diversity and community composition at 6?m and 12?m depths at nine inner coast and nine outer coast sites. Multivariate analysis was applied to reduce environmental variables to major gradients, to resolve community structure, and to relate community structure to habitat and environmental variables. Increased salinity and decreased temperature at outer coast sites compared with inner coast sites were associated with community structure, with greater species diversity at outer coast sites at 6?m depth. Invertebrate community composition was associated with benthic habitat, including crust and coralline algae for macroinvertebrates, and algal cover and substrate for small epibenthic invertebrates. This research suggests that marine communities in glaciated regions are strongly influenced by freshwater input and that future climate-driven changes in freshwater input will likely result in marine community composition changes.     

A comparison of natural and human determinants of phytoplankton communities in the Kentucky River basin,USA

Physical, chemical, and biological characteristics of the Kentucky River and its tributaries were assessed for one year to compare effects of seasonal, spatial, and human environmental factors on phytoplankton. Phytoplankton cell densities were highest in the fall and summer and lowest in the winter. Cell densities averaged 1162 (± 289 SE) cells m1^–1. Cell densities were positively correlated to water temperature and negatively correlated to dissolved oxygen concentration and to factors associated with high-flow conditions (such as, suspended sediment concentrations). Chrysophytes, diatoms, and blue-green algae dominated winter, spring, and summer assemblages, respectively. Ordination analyses (DCCA) indicated that variation in taxonomic composition of assemblages was associated with stream size as well as season.Spatial variation in phytoplankton assemblages and effects of humans was investigated by sampling 55 sites in low flow conditions during August. Phytoplankton density increased with stream size. Assemblages shifted in composition from those dominated by benthic diatoms upstream to downstream communities dominated by blue-green algae and small flagellates. Human impacts were assumed to cause higher algal densities in stream basins with high proportions of agricultural or urban land use than in basins with forested/mined land use. While density and composition of phytoplankton were positively correlated to agricultural land use, they were poorly correlated to nutrient concentrations. Phytoplankton diversity changed with water quality: decreasing with nutrient enrichment and increasing with conditions that probably changed species composition or inhibited algal growth. Human impacts on phytoplankton in running water ecosystems were as great or greater than effects by natural seasonal and spatial factors. Our results indicated that phytoplankton could be useful indicators of water quality and ecosystem integrity in large river systems.     

QUANTITATIVE DIFFERENCES BETWEEN BENTHIC ALGAL COMMUNITIES ALONG A DEPTH GRADIENT IN LAKE MICHIGAN1,2

This study analyzes the impact of conditions associated with depth upon benthic algal communities in Lake Michigan. Diatom abundance was greater at 9.1 and 14.6 m depths than 6.5, 22.6 and 27.4 m. Shallow (6.5 m), mid-depth (9.1 and 14.6 m) and deep (22.6 and 27.4 m) zones were distinguishable on the basis of community composition, structure and abundance. Dominance of benthic species, high diversity and low abundance in shallow communities probably resulted from substantial substrate disturbance by wave action in this productive zone. Dominance of benthic species, high diversity and high abundance characterized mid-depth communities where less wave disturbance enabled algal accumulation. Preponderance of living planktonic taxa, low diversity and low abundance delineated deep communities where planktonic algae accumulated and low light levels reduced growth of benthic species.     

The inshore marine ecosystem off the Vestfold Hills,Antarctica

The planktonic, ice/water interface, and benthic communities at three sites off the coast of the Vestfold Hills, Antarctica, were examined over a complete year.The planktonic flora and fauna were composed predominantly of oceanic species with diatoms and copepods the numerically dominant groups. Primary production was largely restricted to the summer months except for epontic algae which developed in spring. The zooplankton exhibited a similar seasonal cycle but lagged some months behind that of the phytoplankton.The ice/water interface (epontic) fauna consisted of species from the plankton and benthos. Copepods were major contributors; however, two amphipod species dominated. Seasonality of the fauna in this habitat was determined by ice formation and breakout, and development of ice algae.Each of the benthic substrates supported a characteristic macrofaunal assemblage, although infaunal amphipods and tanaids were similar at each site. Infauna exhibited a distinct seasonal cycle related to that of the primary producers whereas macrofauna showed no seasonal changes in abundance.Species composition of each community in this coastal antarctic region was comparable with that of similar habitats in other antarctic coastal areas, supporting the circumpolarity of antarctic marine communities.     

Geographic Variation in Plant Community Structure of Salt Marshes: Species,Functional and Phylogenetic Perspectives

In general, community similarity is thought to decay with distance; however, this view may be complicated by the relative roles of different ecological processes at different geographical scales, and by the compositional perspective (e.g. species, functional group and phylogenetic lineage) used. Coastal salt marshes are widely distributed worldwide, but no studies have explicitly examined variation in salt marsh plant community composition across geographical scales, and from species, functional and phylogenetic perspectives. Based on studies in other ecosystems, we hypothesized that, in coastal salt marshes, community turnover would be more rapid at local versus larger geographical scales; and that community turnover patterns would diverge among compositional perspectives, with a greater distance decay at the species level than at the functional or phylogenetic levels. We tested these hypotheses in salt marshes of two regions: The southern Atlantic and Gulf Coasts of the United States. We examined the characteristics of plant community composition at each salt marsh site, how community similarity decayed with distance within individual salt marshes versus among sites in each region, and how community similarity differed among regions, using species, functional and phylogenetic perspectives. We found that results from the three compositional perspectives generally showed similar patterns: there was strong variation in community composition within individual salt marsh sites across elevation; in contrast, community similarity decayed with distance four to five orders of magnitude more slowly across sites within each region. Overall, community dissimilarity of salt marshes was lowest on the southern Atlantic Coast, intermediate on the Gulf Coast, and highest between the two regions. Our results indicated that local gradients are relatively more important than regional processes in structuring coastal salt marsh communities. Our results also suggested that in ecosystems with low species diversity, functional and phylogenetic approaches may not provide additional insight over a species-based approach.     

Functional Trait Changes,Productivity Shifts and Vegetation Stability in Mountain Grasslands during a Short-Term Warming

Plant functional traits underlie vegetation responses to environmental changes such as global warming, and consequently influence ecosystem processes. While most of the existing studies focus on the effect of warming only on species diversity and productivity, we further investigated (i) how the structure of community plant functional traits in temperate grasslands respond to experimental warming, and (ii) whether species and functional diversity contribute to a greater stability of grasslands, in terms of vegetation composition and productivity. Intact vegetation turves were extracted from temperate subalpine grassland (highland) in the Eastern Pyrenees and transplanted into a warm continental, experimental site in Lleida, in Western Catalonia (lowland). The impacts of simulated warming on plant production and diversity, functional trait structure, and vegetation compositional stability were assessed. We observed an increase in biomass and a reduction in species and functional diversity under short-term warming. The functional structure of the grassland communities changed significantly, in terms of functional diversity and community-weighted means (CWM) for several traits. Acquisitive and fast-growing species with higher SLA, early flowering, erect growth habit, and rhizomatous strategy became dominant in the lowland. Productivity was significantly positively related to species, and to a lower extent, functional diversity, but productivity and stability after warming were more dependent on trait composition (CWM) than on diversity. The turves with more acquisitive species before warming changed less in composition after warming. Results suggest that (i) the short-term warming can lead to the dominance of acquisitive fast growing species over conservative species, thus reducing species richness, and (ii) the functional traits structure in grassland communities had a greater influence on the productivity and stability of the community under short-term warming, compared to diversity effects. In summary, short-term climate warming can greatly alter vegetation functional structure and its relation to productivity.