Distinct protistan assemblages characterize the euphotic zone and deep sea (2500 m) of the western North Atlantic (Sargasso Sea and Gulf Stream)

Protistan diversity was characterized at three locations in the western North Atlantic (Sargasso Sea and Gulf Stream) by sequencing 18S rRNA genes in samples from euphotic (< or = 125 m) and bathypelagic depths (2500 m). A total of 923 partial-length protistan sequences were analysed, revealing 324 distinct operational taxonomic units (OTUs) determined by an automated OTU-calling program set to 95% sequence similarity. Most OTUs were comprised of only one or two sequences suggesting a large but rare pool of protistan diversity. Many OTUs from both depth strata were associated with recently described novel alveolate and stramenopile lineages while many OTUs from the bathypelagic were affiliated with Acantharea, Polycystinea and Euglenozoa and were not observed in euphotic zone libraries. Protistan assemblages from the euphotic zone and the deep sea were largely composed of distinct OTUs; only 28 of the 324 protistan OTUs were detected in both shallow and deep sea clone libraries. The diversity of protistan assemblages in the deep sea was distinctly lower than the diversity of euphotic zone assemblages. Protistan assemblages from the Gulf Stream were the most diverse for either depth strata. Overall, protistan assemblages from different stations but comparable depths were more similar than the assemblages from different depths at the same station. These data suggest that particular groups of protistan OTUs formed distinct 'shallow' and 'deep-sea' assemblages across widely spaced oceanic locales.     

Are land bird assemblages functionally saturated? An empirical test in Mexico

According to the model of ideal free distribution, few or no assemblages will contain unused resources as long as species are free to colonize areas where resources are available. Consequently, because several food resources for birds are available throughout Mexico (nectar, seeds and fruits, vertebrates, carrion and different types of invertebrates), we predicted that all assemblages from Mexico contain birds from the corresponding foraging guilds. However, severe climates may prevent colonization by certain birds. Thus, a second prediction was that, if assemblages with unused resources were found, these would be from localities with climates that pose extreme stress (either extremely high temperature and low rainfall, or extremely low temperature). We tested these predictions on bird assemblages from 77 localities that represent the entire range of climatic conditions found in Mexico. Only two assemblages containing unused resources were found. These were from the localities with lowest temperature and lowest rainfall, respectively. We observed that land bird assemblages in Mexico do not contain unused resources, except when climate selectively restricts colonization by species from certain foraging guilds. Therefore, we conclude that even assemblages with almost no species in common have a common guild structure based on food resources. Null models of competition would be more realistic if they were to incorporate such patterns in their regional species pools.     

Large-scale spatial distribution patterns of echinoderms in nearshore rocky habitats

This study examined echinoderm assemblages from nearshore rocky habitats for large-scale distribution patterns with specific emphasis on identifying latitudinal trends and large regional hotspots. Echinoderms were sampled from 76 globally-distributed sites within 12 ecoregions, following the standardized sampling protocol of the Census of Marine Life NaGISA project (www.nagisa.coml.org). Sample-based species richness was overall low (<1-5 species per site), with a total of 32 asteroid, 18 echinoid, 21 ophiuroid, and 15 holothuroid species. Abundance and species richness in intertidal assemblages sampled with visual methods (organisms >2 cm in 1 m(2) quadrats) was highest in the Caribbean ecoregions and echinoids dominated these assemblages with an average of 5 ind m(-2). In contrast, intertidal echinoderm assemblages collected from clearings of 0.0625 m(2) quadrats had the highest abundance and richness in the Northeast Pacific ecoregions where asteroids and holothurians dominated with an average of 14 ind 0.0625 m(-2). Distinct latitudinal trends existed for abundance and richness in intertidal assemblages with declines from peaks at high northern latitudes. No latitudinal trends were found for subtidal echinoderm assemblages with either sampling technique. Latitudinal gradients appear to be superseded by regional diversity hotspots. In these hotspots echinoderm assemblages may be driven by local and regional processes, such as overall productivity and evolutionary history. We also tested a set of 14 environmental variables (six natural and eight anthropogenic) as potential drivers of echinoderm assemblages by ecoregions. The natural variables of salinity, sea-surface temperature, chlorophyll a, and primary productivity were strongly correlated with echinoderm assemblages; the anthropogenic variables of inorganic pollution and nutrient contamination also contributed to correlations. Our results indicate that nearshore echinoderm assemblages appear to be shaped by a network of environmental and ecological processes, and by the differing responses of various echinoderm taxa, making generalizations about the patterns of nearshore rocky habitat echinoderm assemblages difficult.     

Autochthonous death assemblages from chemoautotrophic communities at petroleum seeps: Palaeoproduction,energy flow,and implications for the fossil record

Death assemblages produced by chemoautotrophic communities at cold seeps represent a type of autochthonous accumulation that is difficult to differentiate from other heterotrophic autochthonous communities using taphonomic characteristics. We test the hypothesis that cold‐seep assemblages can be discriminated by unique biological or community attributes rather than taphonomic attributes. To test this hypothesis, we compared several cold seeps on the Louisiana upper continental slope to heterotrophic sites on the Louisiana slope and to a putative seep site in the middle‐late Campanian Pierre Shale near Pueblo, Colorado. Seep assemblages are characterized by a unique tier and guild structure, size‐frequency composition, and animal density that together identify the palaeoenergetics structure of these communities and distinguish them from the other assemblages of the shelf and slope. All seep assemblages were dominated by primary consumers, whereas the heterotrophic assemblage was dominated by carnivores. Carnivore dominance seems to be typical of shelf (or euhaline) death assemblages. Seep assemblages, in contrast, retain the theoretically‐expected rarity of predaceous forms in fossil assemblages. Epifauna and semi‐infauna dominate the tier structure of the heterotrophic assemblage as is typical for continental shelf and slope assemblages. The infaunal tier was unusually well represented in most petroleum seep assemblages. Local enrichment of food resources and the dominance of shelled primary consumers explain the guild and tier structure of seep assemblages. Hindcasting of energy demand (palaeoingestion) and an estimate of sedimentation rate confirms that energy demand by the community exceeds the supply from planktonic rain in seep communities. Thus, seep assemblages can be recognized using biological attributes where taphonomic analysis is ambiguous.     

MODERN LAND MOLLUSCA FROM MAIDEN CASTLE, DORSET, AND THEIR RELEVANCE TO THE INTERPRETATION OF SUBFOSSIL ARCHAEOLOGICAL ASSEMBLAGES

To aid in the interpretation of molluscan assemblages from archaeologicalsites, modern land mollusc faunas were analysed from 43 turfsamples on a transect of the ramparts of Maiden Castle, Dorset,in chalk grassland. Distinct faunas in terms of species composition,molluscan abundance and species diversity as defined by theShannon and Brillouin indices were present. They were relatedto habitat factors, especially lime, structural diversity andwarmth. The results allow the identification of different grasslandtypes and show that distinct assemblages can occur in closeproximity, delineated by sharp boundaries. However, interpretationof subfossil assemblages from archaeological contexts is bestmade in terms of general relations of relevance between assemblagesand environment because matching by species composition betweenmodern faunas and subfossil assemblages is generally impossible. (Received 17 November 1993; accepted 1 February 1994)     

Postmortem transport and resedimentation of foraminiferal tests: relations to cyclical changes of foraminiferal assemblages

Cyclical changes in microfossil (mainly foraminiferal) assemblages were analysed for sixteen boreholes from three stratigraphical levels in the Lower and Middle Miocene of the Central Paratethys. The following characteristics of assemblages were quantified and used for interpretation of cyclical changes in assemblages: (1) abundance of foraminifers, calcareous nannoplankton, sponge spicules and diatoms; (2) similarity, diversity and epifauna/infauna ratio of benthonic foraminiferal assemblages; (3) planktic/benthic ratio of foraminiferal assemblages. The palaeoecology (mainly palaeobathymetry) fluctuations were interpreted from the species composition of assemblages. Values of the mentioned quantitative characteristics as well as palaeoecological interpretations may be influenced by postmortem transport and resedimentation of foraminiferal tests. Therefore, prior to the interpretation of cyclical changes of quantitative characteristics, the studied sections were classified on the basis of the intensity of taphonomical changes in foraminiferal assemblages. Three different categories of sections were obtained. For every category, those quantitative characteristics of foraminiferal assemblages were chosen which reflect cyclical changes most efficiently: (1) cyclical changes of abundance of foraminifers, calcareous nannoplankton and sponge spicules, as well as the fluctuations in palaeobathymetry for sections dominated by indigenous foraminifers; (2) percentage of indigenous, suspension-transported, bedload-transported and reworked foraminiferal tests and changes in the abundance of indigenous tests for sections dominated by transported tests; (3) different modes of test preservation used for the identification of a cycle boundary for sections with only transported or reworked tests. The distinguished cycles were interpreted predominantly as manifestations of relative sea-level changes. If comparable data exist from other Paratethys regions, the determined cycles can be correlated with the other basins.     

Recolonization of infauna on a tidal flat: An experimental analysis of modes of dispersal

Animals can colonize intertidal sediments by lateral movement through (by burrowing) and on (by crawling) the sediment or by settling (vertically) from the water-column (during larval or bed-load transport above the surface of the sediment). To determine the relative importance of these in recolonization of cleared areas, animals were experimentally prevented from lateral movement through sediments (using fences) and/or by settlement from the water-column (using lids). Necessary controls required nine experimental treatments and an untouched control. After 16 days, assemblages in experimental treatments differed from natural assemblages: recovery was not yet complete. There were differences among assemblages in plots with lids, but no differences among assemblages in plots with fences. After 33 days, treatments with lids still showed little recolonization. Multivariate analyses of assemblage showed the relevance of colonization from the water-column. Analysis of individual taxa revealed complex patterns of recolonization. In general, juvenile bivalves used the water-column, but numbers of large polychaetes were reduced by the fences, indicating that lateral movement was important. These experiments illustrate the complex dynamics of soft-bottom benthic assemblages and great importance of small-scale dispersal.     

崇明东滩DT孔有孔虫组合特征及其环境意义

通过对崇明东滩DT孔的微体古生物学研究,结合沉积物粒度特征、环境磁学特征及14C测年分析,详细阐述了崇明东滩13.0kaBP年来的有孔虫组合特征。自下而上将该孔有孔虫划分出5个组合带,探讨了DT孔5个有孔虫组合带潮流作用变化特点,自组合带1(13.0—8.0kaBP)至组合带4(1.0—0.36kaBP)潮流作用依次增强;组合带4(1.0—0.36kaBP)至组合带5(0.36—0kaBP)潮流作用呈减弱趋势变化;沉积速率自组合带1(13.0—8.0kaBP)至组合带5(0.36—0kaBP)变化特点为较低→高→较高→低→较低;自13.0kaBP年来,按浅海→浅海-河口湾→三角洲前缘-前三角洲→河口沙坝→潮汐水道-潮滩的规律演变。     

Effect of disturbance on assemblages: an example using porifera

Extensive sponge assemblages are found in a number of habitats at Lough Hyne Marine Nature Reserve. These habitats are unusual in experiencing a range of environmental conditions, even though they are only separated by small geographic distances (1-500 m), reducing the possibility of confounding effects between study sites (e.g., silica concentrations and temperature). Sponge assemblages were examined on ephemeral (rocks), stable (cliffs), and artificial (slate panels) hard substrata from high- and low-energy environments that were used to represent two measures of disturbance (flow rate and habitat stability). Sponge assemblages varied considerably between habitat types such that only 26% (25 species) of species reported were common to both rock and cliff habitats. Seven species (of a total of 96 species) were found in the least-developed assemblages (slate panels) and were common to all habitats. Sponge assemblages on rocks and panels varied little between high- and low-energy environments, whereas assemblages inhabiting cliffs varied considerably. Assemblage composition was visualized using Bray-Curtis similarity analysis and Multi-Dimensional Scaling, which enabled differences and similarities between sponge assemblages to be visualized. Cliffs from high- and low-energy sites had different assemblage compositions compared to large rocks, small rocks, and panels, all of which had similar assemblages irrespective of environmental conditions. Differences in assemblages were partially attributed to sponge morphology (shape), as certain morphologies (e.g., arborescent species) were excluded from 2-D rock habitats. Other mechanisms were also considered responsible for the sponge assemblages associated with different habitats.     

Uppermost Pliensbachian-Lowermost Toarcian ostracod assemblages from the Western Tethys: Comparison between Traras Mountains (Algeria), Subbetic (Spain), and Algarve (Portugal)

Major changes in ostracod assemblages have been recorded in the Lower Jurassic of Western Tethys related to biotic crises of the Pliensbachian-Toarcian boundary and the Polymorphum-Levisoni boundary (Toarcian Oceanic Anoxic Event). The study is focused on the ostracods assemblages from Benzerka section (Traras Mountains, Algeria) and their comparison with sections from External Subbetic and Algarve Basin (South Iberian Palaeomargin). Three stratigraphic intervals coincident with the ammonite zones have been differentiated with important fluctuations in the composition of ostracod assemblages. In the ostracod assemblages from the Traras Mountains the turnover related to this benthic biotic crisis is shown by assemblages previous to the biotic crisis (uppermost Pliensbachian) mainly composed by families Healdiidae and Cytherellidae to assemblages just after the T-OAE (upper part of the Levisoni Zone) with assemblages dominated by families Protocytheridae and Polycopidae and the extinction of family Healdiidae (suborder Metacopina). The Toarcian Oceanic Anoxic Event (T-OAE) probably resulted in restricted conditions but a barren interval due to oxygen-depleted conditions in this setting is not recorded. In this context, the change of components of ostracod assemblages looks gradual. In the Southiberian Palaeomargin, represented by the External Subbetic the turnover of ostracods assemblages is also recorded but the families involved are different than in Traras Mountains. Therefore, different phases have been differentiated in the ostracod turnover occurring from Upper Pliensbachian (assemblages dominated by families Pontocyprididae, Healdiidae and Bairdiidae) to end of Lower Toarcian (assemblages dominated by family Cytherellidae and the extinction of family Healdiidae). In the Algarve Basin the families Healdiidae and Bairdiidae were dominant previously to the Toarcian Oceanic Anoxic Event. Differences in the composition of ostracod assemblages between Traras Mountains and External Subbetic may be related to the fragmentation of these palaeomargins related to early stages of rifting in the Western Tethys. Moreover, the incidence of oxygen-depleted conditions was higher in External Subbetic than in Traras Mountains where some ostracod genera survive during the biotic crisis.     

Ground beetle (Coleoptera, Carabidae) assemblages inhabiting Scots pine stands of Puszcza Piska Forest: six-year responses to a tornado impact

Ground beetle assemblages were studied during 2003-08 in the Pisz Forest by comparing stands disturbed by a tornado to undisturbed control stands. The following exploratory questions were put forward. (1) How do the carabid assemblages change during six years following the tornado impact? (2) Does the carabid assemblage recovery begin during the six first post-tornado years? To assess the state of carabid assemblages we used two indices: the MIB (Mean Individual Biomass) and the SPC (Sum of Progressive Characteristics). Carabid assemblages in the disturbed and in the control stands, as expressed by these two indices, were compared using the length of a regression distance (sample distance in a MIB:SPC coordinate system). A cluster analysis revealed that the assemblages of the disturbed and the control stands were different. The tornado-impacted stands produced lower carabid catch rates, but species richness was significantly higher there than in the control stands. They hosted lower proportions of individuals of European species, of large zoophages, and of forest and brachypterous species, than the control stands. The observed reduction in SPC and MIB, and an increase in the regression distances may indicate that the carabid assemblages had not started to recover from the tornado-caused disturbance. Carabid assemblages apparently responded to the tornado in two steps. Firstly, the first three years were characterized by moderate decreases of index values. Secondly, from the fourth to the sixth year after the tornado, many observed changes became magnified. We did not observe clear signals of the recovery of forest carabid assemblages during the six follow-up years.     

Nested bird and micro-habitat assemblages in a peatland archipelago

Biotic assemblages of insular habitats are nested when poor assemblages are subsets of richer ones. Nestedness of species assemblages is frequent and may result from selective extinction or frequent colonization in insular habitats. It may also be created by a nested distribution of habitats among islands or by sampling bias. We sampled 67 isolated peatlands (7–843 ha) in southern Quebec, Canada, to measure nestedness of bird species assemblages among peatlands and assess the habitat nestedness hypothesis. Species and microhabitat assemblages were both strongly nested among peatlands. Whether sites were ranked by species richness, microhabitat richness or peatland area had no effect on nestedness. However, microhabitat nestedness was significantly reduced when sites were sorted by area rather than by microhabitat richness. As expected, if bird-microhabitat associations are responsible for the nested pattern of distribution, we found a positive correlation between the contributions of bird species and microhabitats to individual site nestedness. Nevertheless, microhabitat assemblages were significantly less nested than bird species assemblages, possibly because of frequent recolonization by birds or uneven sampling among sites. Received: 12 June 1998 / Accepted: 20 September 1998     

Between-site comparison of freshwater bacterioplankton by DNA hybridization

Natural assemblages of freshwater bacterioplankton in enriched lowland rivers had greater cell-specific metabolic activity than those in gravel-pit ponds. Similarly, cell-specific activity and mean cell Size in calcareous headstreams tended to be greater than in intermittently-acid headstreams on millstone grit. DNA was extracted and purified from bacterioplankton assemblages, and between-site comparisons were made in terms of percentage similarity as indicated by DNA hybridization. Cluster analysis, using percentage-similarity matrices, placed bacterioplankton assemblages from different site types into distinct groups. This suggested that between-site physiological differences were related to intrinsically different bacterial composition rather than to different physiological response to different environmental conditions by essentially similar bacterial assemblages. Offprint requests to: R. Goulder.     

Habitat variation in species composition of flea assemblages on small mammals in central Europe

We studied patterns of variation in species composition of flea assemblages on small mammals across different habitats of Slovakia and compared flea species composition within and across host species among habitats. We asked (1) how variable the composition of flea assemblages is among different populations of the same host occurring in different habitats and (2) whether the composition of flea assemblages in a habitat is affected either by species composition of hosts or by environmental affinities of this habitat. Between-habitat similarity in flea species composition increased with an increase in the similarity in host species composition. Species richness of flea assemblages of a host species correlated positively with mean number of cohabitating host species but not with the number of habitats occupied by a host species. Results of the ordination of flea collections from each individual host demonstrated that the first five principal components explained most of the variance in species composition of flea assemblages. The segregation between rodent and insectivore flea assemblages was easily discerned from the ordination diagram when flea assemblages were plotted according to their hosts. When flea assemblages were plotted according to their habitat affinities, the distinction of habitats based on variation in flea composition was not as clear. The results of ANOVA of each principal component showed the significant effect of both host species and habitat type. The variation in each principal component was explained better by the factor of host species compared with the factor of habitat type. Multidimensional scaling of flea assemblages within host species across habitats demonstrated that among-habitat variation in flea composition was manifested differently in different hosts.     

Fish assemblage changes over half a century in the Yellow River,China

Riverine environments have been threatened by anthropogenic perturbations worldwide, whereby their fish assemblages have been modified by habitat changes and nonendemic species invasions. We assessed changes in fish assemblages by comparing the species presence in historical and contemporary fish data in the Yellow River from 1965 to 2015. The temporal change in species assemblages was found with increased nonendemic species and fewer natives. Fish species richness of the river declined 35.4% over the past fifty years. Moreover, the decreased mean Bray–Curtis dissimilarity among reaches suggested that the fish assemblages of different reaches in the Yellow River were becoming more similar over time. However, temporal patterns of fish assemblages varied among reaches. In the upper Yellow River, higher species richness and more invasive species were found than those in the historical record, while the lower reaches experienced significant species loss. Dam constructions, exotic fish invasions, and flow reductions played the vital role in structuring the temporal fish assemblages in the Yellow River. It is suggested that river basins which experienced different types and levels of stressors by anthropogenic perturbations can produce varied effects on their temporal trends of species assemblages.     

Spatiotemporal variation in functional and taxonomic organization of stream-fish assemblages in central Texas

Stream ecosystems across the world are dynamic and complex. Biogeographic and historical constraints cause many of these systems to be unique and create difficulty in predicting compositional changes in fish assemblages. However, each system appears to comprise similar functional groupings despite phylogenetic differences. I hypothesized that assemblages within a river would be more similar to one another, regardless of season, than assemblages from other localities based on taxonomic organization; however, assemblages from different rivers but within the same season would be more similar to one another based on functional organization. I assessed functional and taxonomic organizations of fish assemblages from three tributaries of the Colorado River in Texas (Pedernales River, San Saba River, and South Llano River) across all four seasons. Direct ordination methods resulted in assemblages from the same river, regardless of season, clustering together. More specifically, the functional and taxonomic organizations of assemblages from the South Llano River, regardless of season, were noticeably different than that from both the Pedernales River and the San Saba River, at least with respect to the first ordination axis. Current velocity, habitat availability, and temperature were influential in structuring the functional organization of assemblages in central Texas, while current velocity, substrate availability, and stream width were influential in structuring the taxonomic organization.     

Phylogenetic compositions of bacterioplankton from two California estuaries compared by denaturing gradient gel electrophoresis of 16S rDNA fragments.

The phylogenetic compositions of bacterioplankton assemblages from San Francisco Bay and Tomales Bay, Calif., differed substantially when analyzed by PCR-denaturing gradient gel electrophoresis; these differences are consistent with the results of previous studies demonstrating differences in their metabolic capabilities. PCR-denaturing gradient gel electrophoresis analysis of complex microbial assemblages was sensitive and reliable, and the results were reproducible as shown by experiments with constructed and naturally occurring assemblages.     

The Role of Substrate Type on Benthic Diatom Assemblages in the Daly and Roper Rivers of the Australian Wet/Dry Tropics

The selection of one or more river substrata for the collection of benthic diatoms is fundamental to any monitoring or research programme because it can potentially bias the diatom data set. In the wet/dry tropics of Australia, where the use of benthic diatoms for river health assessment is in its infancy, the comparability of diatom assemblages on river substrata has been assessed. Benthic diatoms were sampled from seven river sites, with a range of ionic chemistries (conductivities 27–6500 μS cm⁻¹) but low nutrient concentrations. At each site, triplicate samples were collected from 3 to 6 substrata. The diatom assemblages sampled were: epilithon (assemblages on rock), epiphytes on macroalgae and macrophytes, epidendron (assemblages on wood), epipsammon (assemblages on sand), epipelon (assemblages on mud) and bacterial slime. The variability between substrate assemblages, at each site, were assessed according to the following: (1) a multivariate analysis of diatom assemblages, (2) ANOVA tests of species richness, (3) ANOVA tests of the relative abundance of common species (defined by an abundance of at least 10% in any one sample), and (4) a comparison of the number of species unique to a substrate. A total of 198 taxa were identified, with some taxa common to temperate Australia. Common species were found on all substrata, with sometimes statistically significantly different relative abundances. Taxa unique to a substrate had low relative abundances (0.1–2%), were most often found on only one replicate, and are unlikely to be substrate specific because many are known to occur on other substrata. The assemblages on hard substrata, epilithon and epidendron, were found to be most similar. Diatom assemblages on macroalgal and macrophyte substrata, compared to other substrata, were highly variable. This is attributed to the loss of diatoms from grazing and sloughing, followed by recolonisation of newly exposed substrate. Other assemblages, notably epipsammon, were similar to epilithon and epidendron but sometimes differed in their relative abundance of common species. The principal finding of the study was the similarity of the epilithon and epidendron, which are considered to be indistinguishable. Rock and wood hard substrata can be substituted for one another during field surveys, thereby increasing the number of potential sample sites available for monitoring activities that standardise to a hard substrate.     

Biogeographical homogeneity of Caribbean coral reef benthos

Caribbean reef benthic assemblages have been considered biogeographically homogeneous at regional scales, but this concept was recently challenged by Williams et al. (2015, Journal of Biogeography, 42 , 1327–1335). These authors concluded that benthic assemblages exhibit considerable biogeographical variability at regional and smaller scales, that rugosity and wave exposure play key roles in structuring assemblages, and that homogenization has yet to occur at a regional scale. We reassess their conclusions using recently published benthic and fish surveys that targeted sites either protected from fishing or intensively overfished. For sponges, regional variation in assemblages is mostly attributable to the removal of chemically undefended species by sponge‐eating fishes at sites protected from overfishing. We maintain that Caribbean benthic assemblages are remarkably homogeneous when compared to reefs in other tropical regions, and were likely more homogeneous before the localized effects of intensive fishing resulted in top‐down ecosystem alterations in benthic assemblages.