Early-20th Century Environmental Changes Inferred Using Subfossil Diatoms from a Small Pond on Melville Island, N.W.T., Canadian High Arctic

Diatom-based paleolimnological studies are being increasingly used to track long-term environmental change in arctic regions. Little is known, however, about the direction and nature of such environmental changes in the western Canadian high Arctic. In this study, shifts in diatom assemblages preserved in a ²¹⁰Pb-dated sediment core collected from a small pond on Melville Island, N.W.T., were interpreted to record marked environmental changes that had taken place since the early 20th century. For most of the history of the pond recorded in this core, the diatom assemblage remained relatively stable and was dominated by Fragilaria capucina. A major shift in species composition began in the early-20th century, with a sharp decline in F. capucina and a concurrent increase in Achnanthes minutissima. In the last 20 years, further changes in the diatom assemblage occurred, with a notable increase in the Nitzschia perminuta complex. The assemblage shifts recorded at this site appear to be consistent with environmental changes triggered by recent climatic warming.     

Mediterranean karst springs: diatom biodiversity hotspots under the pressure of hydrological fluctuation and nutrient enrichment

Abstract

Mediterranean karst springs are affected by strong climatic seasonality, with long, dry and hot summers and are increasingly threatened by anthropogenic pressures. In Sardinia (western Mediterranean Sea, Italy), they are largely unexplored and their biocoenoses are mostly unknown. The diatom flora from two substrates (cobbles and macrophytes) in eight springs of different areas of the island was investigated in summer 2016 and winter 2017. A total of 162 diatom taxa (58 genera) were found of which 27 (17 genera) only on cobbles and 26 (18 genera) only on macrophytes. The most abundant species from both substrates were Achnanthidium minutissimum, Amphora indistincta, Amphora pediculus, Cocconeis euglypta, Planothidium frequentissimum and P. lanceolatum. Overall, 67 taxa (40 genera) were recorded in single sites and some of these taxa showed high affinity with specific environmental conditions. Hydrological stability (water flow permanence), discharge and nutrients were the main environmental factors influencing diatom assemblages. Our results suggest that diatoms can reflect important local factors related to the vulnerability of these spring ecosystems and underline the importance of their preservation both for biodiversity and water quality maintenance.     

Some factors affecting distribution of diatom assemblages in Pyrenean springs

SUMMARY.

• 1 The composition and abundance of diatom assemblages were studied together with physiographical and physicochemical variables in twenty-eight springs in the Central Pyrenees during the period 1987–88.
• 2 Characteristic groupings of diatom taxa are revealed by Principal Components Analysis. One group, made up of assemblages of the taxa Navicula cincta, Caloncis spp. and Niizschia elliptica, is characteristic of springs with waters of high ionic strength. Certain diatom taxa are characteristic of hard water springs, either in quiet waters (Denticula tenuis, Achnanthes minutissima, Eunotia spp.) or fast flowing ones (Fragilaria spp., Diatoma hiemale). Other taxa are representative of relatively quiet, softwater springs: Anomoeoneis bracfiysira, Tabellaria flocculosa, Aulacoseira distans and Fragilaria vaucheriae.
• 3 Five different spring types were identified, with respect to diatom assemblages, using discriminant analysis. The first consisted of high mountain springs with cold, fast waters and low ionic strength. The second group, distributed throughout the Pyrenees, had similar environmental characteristics to the first group but with intermediate water velocity and slightly higher mineral content. The third contained springs in calcareous regions of the Pyrenees, mainly slow-flowing and with hard waters. The fourth was composed of softwater springs, whilst the fifth consisted of two springs which were subject to stressful conditions (limitation of light, abundance of nitrates or high salinity).
• 4 Ionic strength and current velocity appeared to be the dominant environmental factors affecting diatom distribution in Pyrenean springs. These factors also seem to determine diatom communities in some other geographical areas. In some springs, however, peculiar environmental factors (dim light, salinity, and high temperature) affect the structure of diatom assemblages.

     

Seasonal and interannual dynamics of diatom assemblages in Sacrower See (NE Germany): a sediment trap study

Diatom assemblages from sediment trap samples collected during ten intervals between October 2003 and October 2005 in Sacrower See (NE Germany) were related to limnological and meteorological data. Sacrower See is a dimictic, 38 m deep, hypertrophic lowland lake (29.5 m a.s.l.). We identified distinct seasonal and interannual changes of diatom assemblages for the studied period. Diatoms showed a typical seasonal succession for temperate, dimictic, and eutrophic lakes. Stephanodiscus parvus, S. hantzschii, S. neoastraea, and S. alpinus had high accumulation rates during winter and spring, whereas species of the genera Stephanodiscus, Fragilaria, and Nitzschia were the predominant diatoms during summer and autumn. In a Canonical Correspondence Analysis, precipitation, air and water temperatures, epilimnetic calcium, pH, and total phosphorus concentrations together explained 70% of the variance of the diatom data. Interannual variability in the diatom assemblages during the two sampled years mainly seems to reflect changes in the total phosphorus concentration and temperature and secondarily the onset of the growing season and of stratification. Handling editor: J. Saros     

Biofilm and capsule formation of the diatom Achnanthidium minutissimum are affected by a bacterium

Photoautotrophic biofilms play an important role in various aquatic habitats and are composed of prokaryotic and/or eukaryotic organisms embedded in extracellular polymeric substances (EPS). We have isolated diatoms as well as bacteria from freshwater biofilms to study organismal interactions between representative isolates. We found that bacteria have a strong impact on the biofilm formation of the pennate diatom Achnanthidium minutissimum. This alga produces extracellular capsules of insoluble EPS, mostly carbohydrates (CHO), only in the presence of bacteria (xenic culture). The EPS themselves also have a strong impact on the aggregation and attachment of the algae. In the absence of bacteria (axenic culture), A. minutissimum did not form capsules and the cells grew completely suspended. Fractionation and quantification of CHO revealed that the diatom in axenic culture produces large amounts of soluble CHO, whereas in the xenic culture mainly insoluble CHO were detected. For investigation of biofilm formation by A. minutissimum, a bioassay was established using a diatom satellite Bacteroidetes bacterium that had been shown to induce capsule formation of A. minutissimum. Interestingly, capsule and biofilm induction can be achieved by addition of bacterial spent medium, indicating that soluble hydrophobic molecules produced by the bacterium may mediate the diatom/bacteria interaction. With the designed bioassay, a reliable tool is now available to study the chemical interactions between diatoms and bacteria with consequences for biofilm formation.     

Palaeolimnological evidence of environmental change over the last 400 years in the Rwenzori Mountains of Uganda

Tropical alpine areas may be highly sensitive to climate change. Yet, because high-resolution palaeoenvironmental studies in these regions are scant, patterns of environmental change over the last few centuries, and linkages with regional changes, remain poorly resolved. This article presents a 400-year palaeolimnological record from Lower Kitandara Lake (3,989 m above m.a.s.l.), located in the Rwenzori Mountains of Uganda, where marked glacial recession has been recorded over much of the twentieth century. An age model is produced for a 57.5 cm sediment core based on ²¹⁰Pb and ¹⁴C dating, suggesting a basal date of approximately 1600 AD. Diatom and organic geochemistry (%TOC, C/N ratios, δ¹³C) analyses are carried out at an approximately decadal resolution. Twentieth-century glacial recession does not appear to have significantly impacted either the diatom or geochemical records. However, large ecological changes have occurred during the past 400 years, particularly shown by the diatom fluxes and geochemical data. Throughout the core, the diatom record reveals only minor changes in assemblage composition, which may be related to the dominance of Staurosira construens var. venter in the lake’s diatom flora, a tychoplanktonic taxon which is highly adaptive to environmental change. Geochemical analyses, however, reveal a marked change at around the end of the eighteenth century, when C/N ratios suggest an increase in the dominance of algal aquatic sources to lacustrine organic matter, concomitant with a stabilisation of catchment inputs and increased diatom productivity, which may have been caused by reduced glacial inputs. The relationship between these changes at Lower Kitandara Lake and wider regional climate change that occurred at the end of the eighteenth century is not well understood, but this study highlights the need for additional research to link drivers of alpine ecosystem change with those operating at low altitudes.     

Using change-point analysis and weighted averaging approaches to explore the relationships between common benthic diatoms and in-stream environmental variables in mid-atlantic highlands streams,USA

Diatoms are increasingly being used in the bioassessment of aquatic systems. However, autecological information for many common taxa is incomplete. We explored the potential of classification (CT) and regression tree (RT) approaches to identify the hierarchical interaction among water quality variables in predicting the relative abundance of ten common stream diatom taxa in the Mid-Atlantic Highlands ecoregion. RT analysis was also used to identify environmental change points corresponding to major shifts in species abundances. We also used traditional weighted-averaging approaches (WA) to model taxon pH and total phosphorus (TP) optima. RT and WA approaches provided different, yet complementary, information on the complex relationships between common stream diatoms and environmental variables. Both RT and CT highlighted the interaction of stream acidity (pH, acid neutralizing capacity (ANC)), and TP in structuring the stream diatom assemblage. For the RT of taxa, where pH was an important predictor, higher pH predicted higher relative abundances. In contrast, higher TP predicted lower relative abundances for some diatom taxa (e.g., Achnanthidium minutissimum (Kütz.) Czarnecki), while predicting higher relative abundances for other taxa (e.g., Planothidium lanceolatum (Bréb.) Round & Bukht., Gomphonema parvulum (Kütz.) Kütz.). The environmental change point for pH derived from RT analysis was lower than WA optima for all species. We suggest that RT change point analysis can be used to complement traditional WA optima approaches, especially when diatom taxa’s abundances are affected by interactive environmental factors, to provide more refined information on stream diatom environmental preferences. Handling editor: L. Naselli-Flores     

Regional and local drivers of macroinvertebrate assemblages in boreal springs

Aim To identify the most important environmental drivers of benthic macroinvertebrate assemblages in boreal springs at different spatial scales, and to assess how well benthic assemblages correspond to terrestrially derived ecoregions. Location Finland. Methods Benthic invertebrates were sampled from 153 springs across four boreal ecoregions of Finland, and these data were used to analyse patterns in assemblage variation in relation to environmental factors. Species data were classified using hierarchical divisive clustering (twinspan ) and ordinated using non‐metric multidimensional scaling. The prediction success of the species and environmental data into a priori (ecoregions) and a posteriori (twinspan ) groups was compared using discriminant function analysis. Indicator species analysis was used to identify indicator taxa for both a priori and a posteriori assemblage types. Results The main patterns in assemblage clusters were related to large‐scale geographical variation in temperature. A secondary gradient in species data reflected variation in local habitat structure, particularly abundance of minerogenic spring brooks. Water chemistry variables were only weakly related to assemblage variation. Several indicator species representing southern faunistic elements in boreal springs were identified. Discriminant function analysis showed poorer success in classifying sites into ecoregions based on environmental than on species data. Similarly, when classifying springs into the twinspan groups, classification based on species data vastly outperformed that based on environmental data. Main conclusions A latitudinal zonation pattern of spring assemblages driven by regional thermal conditions is documented, closely paralleling corresponding latitudinal patterns in both terrestrial and freshwater assemblages in Fennoscandia. The importance of local‐scale environmental variables increased with decreasing spatial extent. Ecoregions provide an initial stratification scheme for the bioassessment of benthic macroinvertebrates of North European springs. Our results imply that climate warming, landscape disturbance and degradation of spring habitat pose serious threats to spring biodiversity in northern Europe, especially to its already threatened southern faunistic elements.     

Hemispheric‐scale patterns of climate‐related shifts in planktonic diatoms from North American and European lakes

A synthesis of over 200 diatom‐based paleolimnological records from nonacidified/nonenriched lakes reveals remarkably similar taxon‐specific shifts across the Northern Hemisphere since the 19th century. Our data indicate that these diatom shifts occurred in conjunction with changes in freshwater habitat structure and quality, which, in turn, we link to hemispheric warming trends. Significant increases in the relative abundances of planktonic Cyclotella taxa (P<0.01) were concurrent with sharp declines in both heavily silicified Aulacoseira taxa (P<0.01) and benthic Fragilaria taxa (P<0.01). We demonstrate that this trend is not limited to Arctic and alpine environments, but that lakes at temperate latitudes are now showing similar ecological changes. As expected, the onset of biological responses to warming occurred significantly earlier (P<0.05) in climatically sensitive Arctic regions (median age=ad 1870) compared with temperate regions (median age=ad 1970). In a detailed paleolimnological case study, we report strong relationships (P<0.005) between sedimentary diatom data from Whitefish Bay, Lake of the Woods (Ontario, Canada), and long‐term changes in air temperature and ice‐out records. Other potential environmental factors, such as atmospheric nitrogen deposition, could not explain our observations. These data provide clear evidence that unparalleled warming over the last few decades resulted in substantial increases in the length of the ice‐free period that, similar to 19th century changes in high‐latitude lakes, likely triggered a reorganization of diatom community composition. We show that many nonacidified, nutrient‐poor, freshwater ecosystems throughout the Northern Hemisphere have crossed important climatically induced ecological thresholds. These findings are worrisome, as the ecological changes that we report at both mid‐ and high‐latitude sites have occurred with increases in mean annual air temperature that are less than half of what is projected for these regions over the next half century.     

Validation of ecological status concepts in UK rivers using historic diatom samples

We assessed the feasibility of using herbarium specimens to validate reference conditions in the UK by comparing diatom community composition of river sites with both recent and historic diatom samples. The question of substrate specificity was addressed by comparing epilithon (stone-derived) and epiphyton (plant-derived) samples from a number of rivers. No significant differences were found between the Trophic Diatom Index (TDI), species richness, species diversity, and percentage of motile valves between paired diatom samples (epilithic and epiphytic) from contemporary samples. Significant differences were recorded between a number of indices derived from analysis of the historic diatom samples on plant material sampled pre-1930 compared with diatoms from stones collected post-1990 from the same river location. The TDI, mean species richness, and species diversity and percentage of motile valves and nutrient tolerant valves were all significantly greater in the contemporary samples (p ≤ 0.05). The percentage of nutrient sensitive valves was significantly lower in the contemporary samples (p ≤ 0.05).The relative abundance of Achnanthidium minutissimum and Cocconeis placentula var. lineata was significantly greater on the herbarium material compared to matched contemporary samples. Calculated values for the TDI (43 ± 3) expected at reference conditions were similar to the observed TDI values derived from herbarium material (44 ± 12) showing no significant deviation in ecological status.     

Impact of Urban Pollution from the Hanoi Area on Benthic Diatom Communities Collected from the Red, Nhue and Tolich Rivers (Vietnam)

The effects of urban pollution from Hanoi city on the benthic diatom communities of the Nhue–Tolich river system were studied during the 2003 dry season. Benthic diatoms were allowed to grow on glass slides suspended in the water flow for 4 weeks. To reveal the relationship between water quality and diatom communities, Canonical correspondence analysis (CCA) was used on data concerning relative abundances of diatom species and environmental variables. Two diatom indices, IPS and DAIpo, were applied to evaluate water quality in the three rivers. A total of 291 diatom taxa were found in the Red, Nhue and Tolich Rivers. These were mainly cosmopolitan taxa, with some tropical, subtropical and endemic taxa. The most abundant taxa at the Red site were Aulacoseira granulata, Achnanthidium minutissimum, Encyonema minutum, Navicula recens and other halophilous taxa such as Nitzschia kurzii, Seminavis strigosa, Entomoneis paludosa, Bacillaria paradoxa. Diatom assemblages at the Tolich site consisted mainly of Nitzschia umbonata, Nitzschia palea and Eolimna minima. Diatom density ranged from 660 to 30,000 cells/cm². Environmental variables and diatom assemblage composition at all sites were significantly correlated. Two diatom indices gave similar results and indicate the Tolich River with the lowest values as a highly polluted site.     

The influence of substrates and physicochemical factors on the composition of diatom assemblages in karst springs and their applicability in water-quality assessment

The methodological standards of monitoring programs recommend collecting material from stones, but this community is often absent in slow-flowing waters, whereas a great number of localities do offer bryophytic material for sampling. In order to investigate whether the kind of substrate (epilithic or bryophytic) influences the diatom-based biomonitoring results the structure of diatom assemblages from 13 anthropogenically altered springs were investigated. To examine differences between diatom assemblages, the species richness of diatom assemblages, H′ (Shannon-Wiener diversity index), Trophic Diatom Index (TDI) and Specific Pollution sensitivity Index (SPI) were compared. The TDI and SPI differed significantly (P = 0.002 and P = 0.004) between springs located in villages and springs away from villages, and did not differ significantly between epilithic and bryophytic samples (P > 0.05). No significant differences in species richness or the diversity index (H′) (P > 0.05) were found between the epilithic and bryophytic diatom assemblages. This study suggests that both the kinds of substrates can be successfully used for diatom-based water-quality biomonitoring in mesotrophic and eutrophic environments.     

Comparison of Freshwater Diatom Assemblages from a High Arctic Oasis to Nearby Polar Desert Sites and Their Application to Environmental Inference Models

Arctic oases are regions of atypical warmth and relatively high biological production and diversity. They are small in area (<5 km²) and uncommon in occurrence, yet they are relatively well studied due to the abundance of plant and animal life contained within them. A notable exception is the lack of research on freshwater ecosystems within polar oases. Here, we aim to increase our understanding of freshwater diatom ecology in polar oases. Diatoms were identified and enumerated from modern sediments collected in 23 lakes and ponds contained within the Lake Hazen oasis on Ellesmere Island, and compared with diatom assemblages from 29 sites located outside of the oasis across the northern portion of the island. There were significant differences in water chemistry variables between oasis and northern sites, with oasis sites having higher conductivity and greater concentrations of nutrients and related variables such as dissolved organic carbon (DOC). Taxa across all sites were typical of those recorded in Arctic freshwaters, with species from the genera Achnanthes sensu lato, Fragilaria sensu lato, and Nitzschia dominating the assemblages. A correspondence analysis (CA) ordination showed that oasis sites generally plotted separately from the northern sites, although the sites also appear to plot separately based on whether they were lakes or ponds. Canonical correspondence analysis (CCA) identified specific conductivity, DOC, and SiO₂ as explaining significant (P < 0.05) and additional amounts of variation in the diatom data set. The most robust diatom‐based inference model was generated for DOC, which will provide useful reconstructions on long‐term changes in paleo‐optics of high Arctic lakes.     

Long‐term trends in the diatom composition of the spring bloom of a German reservoir: is Aulacoseira subarctica favoured by warm winters?

1. Long‐term data on the meteorology, hydrology, physicochemistry and plankton of a reservoir and its tributaries in SE Germany run from 1976 until now. This dimictic reservoir changed from mesotrophic to eutrophic in the 1970s, remained eutrophic in the 1980s and returned to the mesotrophic state after a sharp reduction in P loading in 1990. 2. Phytoplankton biomass reaches an annual maximum in spring and consists almost entirely of diatoms. While Asterionella formosa was dominant until 1990, Aulacoseira subarctica became more frequent at the end of the 1990s and was particularly abundant in years with short winters. 3. Statistical analyses suggested that these changes were triggered primarily by the mild winters that were frequent after 1988. Climate‐related hydrophysical variables and the initial biomass of the diatoms at the beginning of the year, considered as an ‘inoculum’, were identified as most important. These variables explained 39% of the total variance of the relative abundance, whereas the change in trophic conditions was responsible for about 20%. 4. The absolute and relative abundance of A. subarctica was positively related to short ice cover, early ice‐out and a long‐lasting spring circulation. Owing to its physiological traits, and particularly its ability to survive under low‐light conditions, A. subarctica benefitted from short, mild winters. Under such conditions, it could sustain or establish a high initial biomass, whereas the concentrations of the other diatoms decreased over winter. However, this advantage may be lost if further warming causes an early onset of summer stratification. Because of its low population growth rate and requirement for high turbulence, A. subarctica needs long, cold springs to exploit the improved starting conditions and to become abundant. 5. In contrast to A. subarctica, A. formosa required a substantial soluble reactive phosphorus supply to compete successfully. The eutrophic conditions until 1990 were the prerequisite for its mass growth under low‐light and low‐temperature conditions during the spring. After reduction in P concentration from 1990, A. formosa declined and other diatom species became more abundant. 6. These other diatoms may be viewed as ‘stopgaps’ when conditions were not favourable for A. subarctica or A. formosa. Diatoma elongatum exploited brief circulation periods in years with low P loading. Synedra acus and Fragilaria crotonensis, because of their poor competitive ability at low light intensity, reached high density in the upper water column in the transitional period between spring circulation and summer stratification. 7. Our study suggests that climate‐related variables have crucial impacts on the spring phytoplankton dynamics of deep stratified waterbodies. They can mask the consequences of changes in the trophic conditions and, corresponding to the functional traits of the different phytoplankton species, also decisively control their relative abundances. In this reservoir, the warmer winters and prolonged spring circulations did not only lead to high phytoplankton biomass (despite considerably reduced nutrient loads) but also cause a marked shift in the diatom assemblage during the spring bloom.     

Weather-driven ecology of planktonic diatoms in Lake Tovel (Trentino,Italy)

The seasonal development of planktonic diatoms in Lake Tovel has been studied from 2002 to 2004 within the SALTO research project. The objective was to investigate the role of weather conditions and lake hydrology in regulating the development of Cyclotella and Fragilaria functional species groups. The different responses of the two species to the environmental variability and their reciprocal relationships were considered. Cyclotella showed a weak relationship with the selected environmental variables and its seasonal development was difficult to interpret, also in relation to its opportunistic behaviour. On the contrary, Fragilaria complex showed a prompt response to environmental variability and its seasonal development appeared to be mainly regulated by hydrological conditions, through nitrate nitrogen and, secondarily, by silica concentrations. Water temperature and thermal stability affected only Fragilaria species, while different nutrient requirements allowed Cyclotella species to grow under conditions of limited resources for Fragilaria. Since Lake Tovel has scarce internal nutrient reserves and its hydrology is strongly affected by precipitation both in winter and summer, diatom phenology seems to be indirectly regulated by weather conditions.     

Cadmium and lead toxicity on tropical freshwater periphyton communities under laboratory-based mesocosm experiments

Periphyton constitutes an important community that is useful for assessment of ecological conditions in lotic systems. The objective of this study was to assess the effects of different mixtures of Cd and Pb on periphyton growth as well as Cd and Pb mixtures toxicity to diatom assemblages in laboratory mesocosm experiments. A natural periphyton community sampled from the Monjolinho River (South of Brazil) was inoculated into five experimental systems containing clean glass substrates for periphyton colonization. The communities were exposed to mixtures of dissolved Cd and Pb concentrations of 0.01 and 0.1 mg l⁻¹ Cd and 0.033 and 0.1 mg l⁻¹ Pb. Periphyton ash-free dry weight, growth rate, diatom cell density and diatom community composition were analyzed on samples collected after 1, 2 and 3 weeks of colonization. High Cd concentration (0.1 mg l⁻¹) has negative effects on periphyton growth while high concentration of Pb (0.1 mg l⁻¹) decreased the toxic effects of Cd on periphyton growth. Shifts in species composition (development of more resistant species like Achnanthidium minutissimum and reduction of sensitive ones like Cymbopleura naviculiformis, Fragilaria capucina, Navicula cryptocephala, Encyonema silesiacum, Eunotia bilunaris, and Gomphonema parvulum), decreases in species diversity of diatom communities with increasing Cd and Pb concentrations and exposure duration have been demonstrated in this study making diatom communities appropriate monitors of metal mixtures in aquatic systems.     

Effects of small‐scale environmental variation on metaphyton condition and community composition

1. Metaphyton mats typically originate as benthic algal biofilms that experience higher solar radiation and temperatures, and reduced access to nutrients, once they reach the water surface, but the impacts of these physicochemical changes on metaphyton condition and community composition have received little attention. 2. Using microprobes positioned at 0, 2, 4 and 6 cm depth, we recorded small‐scale differences in water chemistry within metaphyton mats constrained in floating nets during an 8‐week period. Concurrent weekly samples of filamentous algae and their diatom epiphytes were collected from the top, middle and bottom of the mats and were analysed for changes in ash‐free dry mass (AFDM) and chlorophyll‐a, nutrient (N, P, C, Si) content and taxonomic composition. 3. Light intensity, temperature and dissolved oxygen declined both with increasing depth in the mat and over the study period. The autotrophic index (=AFDM/chlorophyll‐a) was greatest at the top of the mats and increased over time; samples also had higher C/P and C/N ratios than samples deeper within the mat. 4. Pithophora was consistently the dominant algal filament throughout the study (representing 85% of all filaments averaged over time and depth); epiphytic diatom cover on Pithophora (calculated as epiphyte area index) declined over time, particularly at the top of the mat. 5. Densities of the diatom epiphytes Gomphonema, Cocconeis and Fragilaria increased with increasing depth within the mat, whereas Cymbella/Encyonema was more common in surface samples. 6. Our results indicate that metaphyton mats are highly dynamic communities, spatially organised in part by small‐scale environmental variation and subject to changes in taxonomic composition following their arrival at the water surface.     

Species composition,habitat configuration and seasonal changes of coral reef fish assemblages in western Mexico

In spite of their ecological and economic importance, reef fishes from the coast of Oaxaca, Mexico are rarely studied, therefore precluding their management and conservation. In order to identify the set of habitat characteristics/environmental conditions that predict major shifts in fish assemblages in space and time, a stationary census (5′, φ = 5 m) was conducted on a semi‐monthly basis from 2006 to 2009 at patch reefs along the coast. Habitat configuration was gathered using 25 m long point‐intersect transects (data every 25 cm), recording all underlying coral species and substrate characteristics (rocks, sand, algal mats, rubble or dead corals). Recorded were 65 452 fishes grouped in 11 orders, 36 families, 65 genera and 89 species. Labridae (nine species), Pomacentridae (eight species) and Serranidae (seven species) were the most frequent families. Abundance is severely skewed among species; four species Thalassoma lucasanum, Chromis atrilobata, Apogon pacificus and Stegastes acapulcoensis comprise nearly 59% of the fish abundance, 11 species contribute 30%, whereas most of the species (75) can be considered as rare since they contribute <1% each to the total. Species richness and family‐level assemblage composition are similar to those recorded elsewhere in the eastern Pacific. Non‐parametric multivariate analysis of variance demonstrated that changes of diversity metrics might be associated with environmental differences on the scale of hundreds of meters to kilometers, as well as coupled with major changes on oceanographic variables throughout time, exerting meaningful changes on reef‐related fish assemblages.     

Seasonal and decadal patterns in Discostella (Bacillariophyceae) species from bi‐weekly records of two boreal lakes (Experimental Lakes Area,Ontario, Canada)

A recent rise in the relative abundance of Discostella species (D. stelligera and D. pseudostelligera) has been well documented from sedimentary diatom assemblages across the Northern Hemisphere. This unprecedented change over the last ~150 years has been linked to rises in atmospheric temperatures, changes in ice cover, and/or increases in thermal stability, among other factors. The bi‐weekly monitoring data from two boreal lakes at the Experimental Lakes Area (ELA) in northwestern Ontario were analyzed across seasons (spring, summer, and fall) and decades (1970s–2000s). We found that Discostella species are primarily spring/early summer bloomers (i.e., late April to June) in these lakes and changes in concentrations of Discostella over time were most pronounced in the spring or early summer months. Increases in Discostella abundance over time may be linked to earlier ice‐off and a longer period of spring turnover, resulting from increased winter and spring temperatures. It is also possible that a trophic mismatch between the spring diatom bloom and zooplankton is occurring, thus reducing diatom loss rates, and resulting in greater overall abundance. Moreover, the spring dominance of Discostella in our study lakes occurred at a time of the year when nutrient concentrations were at their highest seasonally, suggesting that these taxa are neither limited directly by nutrients, nor responding to enhanced stratification during the summer months in these lakes.