The use of fish community structure as a measure of ecological degradation: a case study in two tropical rivers of India

Fish community structure and water chemistry of two tropical rivers of West Bengal, an eastern province of India, were studied for two annual cycles (January 2003-December 2004) and a higher degree of pollution was found in one river (the Churni) than in the other river (the Jalangi). This was reflected in the water quality as well as in fish community structure of the rivers. We observed that 63.6% of fish species appeared to have been eliminated from the polluted Churni river since 1983 in 20 years. For the protection of fish biodiversity and enhancement of fish production, a rational management program should be implemented for the Churni river.     

The status of the freshwater pearl mussel <Emphasis Type="Italic">Margaritifera margaritifera</Emphasis> in Scotland: extent of change since 1990s,threats and management implications

All known rivers in Scotland with recent records of freshwater pearl mussels Margaritifera margaritifera were surveyed in 2013–2015 using a standard methodology. Freshwater pearl mussel populations were classed as: (i) apparently extinct in 11 rivers, (ii) not successfully recruiting in 44 rivers, and (iii) evidence of recent successful recruitment in 71 rivers. On a regional basis, a high proportion of extant populations were located in North and West Scotland. In all regions extant populations were characterised by low pearl mussel densities, with 97 of 115 extant Scottish populations defined as ‘rare’ (0.1–0.9 mussels per 1 m ²) or ‘scarce’ (1.0–9.9 mussels per 1 m ²). Only 18 Scottish rivers now hold pearl mussel populations in densities that are considered to be ‘common’ (10–19.9 mussels per 1 m ²) or ‘abundant’ (>20 mussels per 1 m ²). Based on survey evidence, the number of apparently extinct pearl mussel populations in Scottish rivers is now 73. The decline is particularly pronounced in the West Highlands and Western Isles strongholds. The key threats are: (i) pearl fishing, (ii) low host fish densities, (iii) pollution/water quality, (iv) climate change and habitat loss, (v) hydrological management/river engineering and (vi) ‘other factors’, such as non-native invasive species. Over the last 100 years this endangered species has been lost from much of its former Holarctic range. Scotland’s extant M. margaritifera populations continue to be of international importance, but their continued decline since the first national survey in 1998 is of great concern.     

The ‘wet–dry’ in the wet–dry tropics drives river ecosystem structure and processes in northern Australia

1. Northern Australia is characterised by a tropical wet–dry climate that regulates the distinctive character of river flow regimes across the region. There is marked hydrological seasonality, with most flow occurring over only a few months of the year during the wet season. Flow is also characterised by high variability between years, and in the degree of flow cessation, or intermittency, over the dry season. 2. At present, the relatively low human population density and demand for water in the region means that most rivers have largely unmodified flow regimes. These rivers therefore provide a good opportunity to understand the role of natural flow variability in river ecosystem structure and processes. 3. This review describes the major flow regime classes characterising northern Australian rivers, from perennial to seasonally intermittent to extremely intermittent, and how these regimes give rise to marked differences in the ecological character of these tropical rivers, particularly their floodplains. 4. We describe the key features of these flow regimes, namely the wet and dry seasons and the transitions between these seasons, and how they regulate the biophysical heterogeneity, primary productivity and movement of biota in Australia’s wet–dry tropical rivers. 5. We develop a conceptual model that predicts the likely hydrological and ecological consequences of future increases in water abstraction (e.g. for agriculture), and suggest how such impacts can be managed so that the distinctive ecological character of these rivers is maintained.     

Amphidromy links a newly documented fish community of continental Australian streams, to oceanic islands of the west Pacific

Background

Indo-Pacific high island streams experience extreme hydrological variation, and are characterised by freshwater fish species with an amphidromous life history. Amphidromy is a likely adaptation for colonisation of island streams following stochastic events that lead to local extirpation. In the Wet Tropics of north-eastern Australia, steep coastal mountain streams share similar physical characteristics to island systems. These streams are poorly surveyed, but may provide suitable habitat for amphidromous species. However, due to their ephemeral nature, common non-diadromous freshwater species of continental Australia are unlikely to persist. Consequently, we hypothesise that coastal Wet Tropics streams are faunally more similar, to distant Pacific island communities, than to nearby faunas of large continental rivers.

Methods/Principal Findings

Surveys of coastal Wet Tropics streams recorded 26 species, 10 of which are first records for Australia, with three species undescribed. This fish community is unique in an Australian context in that it contains mostly amphidromous species, including sicydiine gobies of the genera Sicyopterus, Sicyopus, Smilosicyopus and Stiphodon. Species presence/absence data of coastal Wet Tropics streams were compared to both Wet Tropics river networks and Pacific island faunas. ANOSIM indicated the fish fauna of north-eastern Australian coastal streams were more similar to distant Pacific islands (R = 0.76), than to nearby continental rivers (R = 0.98).

Main Conclusions/Significance

Coastal Wet Tropics streams are faunally more similar to distant Pacific islands (79% of species shared), than to nearby continental fauna due to two factors. First, coastal Wet Tropics streams lack many non-diadromous freshwater fish which are common in nearby large rivers. Second, many amphidromous species found in coastal Wet Tropics streams and Indo-Pacific islands remain absent from large rivers of the Wet Tropics. The evolutionary and conservation significance of this newly discovered Australian fauna requires clarification in the context of the wider amphidromous fish community of the Pacific.     

Streamflow-induced variations in nitrate flux in tributaries to the Atlantic Coastal Zone

Streamflow-related variability in nutrient flux represents an important source of uncertainty in managing nutrient inputs to coastal ecosystems. Quantification of flux variability is of particular interest to coastal resource managers in adopting effective nutrient-reduction goals and monitoring progress towards these goals. We used historical records of streamflow and water-quality measurements for 104 river monitoring stations in an analysis of variability in annual and seasonal flux of nitrate to the Atlantic coastal zone. We present two measures of temporal flux variability: the coefficient of variation (CV) and the exceedence probability (EP) of 1.5 times the median flux. The magnitude of flux variations spans a very wide range and depends importantly upon the season of year and the climatic and land-use characteristics of the tributary watersheds. Year-to-year variations (CV) in annual mean flux range over two orders of magnitude, from 3–200% of the long-term mean flux, although variations more typically range from 20–40% of the long-term mean. The annual probability of exceeding the long-term median flux by more than 50% (EP) is less than 0.10 in most rivers, but is between 0.10 and 0.35 in 40% of the rivers. Year-to-year variability in seasonal mean flux commonly exceeds that in annual flux by a factor of 1.5 to 4. In western Gulf of Mexico coastal rivers, the year-to-year variablity in the seasonal mean flux is larger than in other regions, and is of a similar magnitude in all seasons. By contrast, in Atlantic coastal rivers, the winter and spring seasons, which account for about 70% of the annual flux, display the smallest relative variability in seasonal mean flux. We quantify the elasticity of nutrient flux to hypothetical changes in Streamflow (i.e., the percent increase in flux per percentage increase in mean discharge) to allow the approximation of flux variability from streamflow records and the estimation of the effects of future climatically-induced changes in Streamflow on nutrient flux. Flux elasticities are less than unity (median = 0.93%) at most stations, but vary widely from 0.05% to 1.59%. Elasticities above unity occur most frequently in the largest rivers and in rivers draining the arid portions of the western Gulf of Mexico Basin. Historical flux variability and elasticity generally increase with the extent of arid conditions and the quantity of nonurban land use in the watershed. We extend the analysis of flux variability to examine several case studies of highly unusual meteorological events capable of significantly elevating nitrate flux and degrading estuarine ecology.     

Environmental determinism of year-to-year recruitment variability of European eel Anguilla anguilla in a small coastal catchment, the Frémur River, north-west France

The influence of environmental factors (mainly the river flow) on the year-to-year variability of European eel Anguilla anguilla fluvial recruitment in a small coastal catchment, the Frémur River (north-west France) was examined. A comprehensive survey of catches from fixed traps at two weirs located at 4·5 km (Pont es Omnes Dam) and 6·0 km (Bois Joli Dam) above the river mouth was carried out between 1997 and 2004. Young pigmented elvers (mean ± s.d. total length, 133·7 ± 29·6 mm) were recruited in eel-passes from February to October, but the main runs followed a modal curve from April to September with a peak centred in May to June. Catches varied greatly between years, from 381 to 26 765 elvers. For each trap, a positive linear relationship between monthly mean river flow that preceded the maximal intensity of captures and annual total catches was observed. These relationships explained 73·1% ( P < 0.01) and 89·0% ( P < 0.001) of the year-to-year variability of the recruitment observed in the Pont es Omnes and Bois Joli traps respectively. A significant increase in river flow at the beginning of the migration peak would thus trigger a greater proportion of A. anguilla settled in the estuary and in the downstream zone of the Fremur River to begin their freshwater colonization. The physicochemical roles of changes in river discharge in stimulating upstream migration are discussed. It is concluded that fluvial recruitment in the Fremur River is mainly determined by environmental factors.     

Differentiation of <Emphasis Type="Italic">Harmonia axyridis</Emphasis> Pall. according to polymorphic morphological traits and variability of the mitochondrial COI gene

The geographic variability of Harmonia axyridis Pall. has been studied with regard to three morphological features. A study of the cytochrome C oxidase subunit I (COI) polymorphism was begun. Two geographically separated zones inside the Russian part of the geographic range of the species are recognized. Interpopulational differences, as judged using polymorphic traits, are very small within each zone. In contrast, the differences between the populations of the western and eastern zones reach the subspecies level of. A zone of clinal variability of morphological traits is noted in Transbaikalia. It obviously resulted from secondary contact between the western and eastern groups of populations, which were separated during the last glaciation and then rejoined.     

Geographical segregation in Dunlin Calidris alpina populations wintering along the East Atlantic migratory flyway – evidence from mitochondrial DNA analysis

Dunlin Calidris alpina is one of the most abundant shorebirds using coastal habitats in the East Atlantic migratory flyway, that links arctic breeding locations (Greenland to Siberia) with wintering grounds (West Europe to West Africa). Differential migration and winter segregation between populations have been indicated by morphometrics and ringing recoveries. Here, we analyse the potential of genetic markers (mitochondrial DNA – mtDNA) to validate and enhance such findings. We compared mtDNA haplotypes frequencies at different wintering sites (from north-west Europe to West Africa). All birds from West Africa had western (European) haplotypes, while the eastern (Siberian) haplotypes were only present in European winter samples, reaching higher frequencies further north in Europe. Compilation of published results from migrating birds also confirmed these differences, with the sole presence of European haplotypes in Iberia and West Africa and increasingly higher frequencies of Siberian haplotypes from south-west to north-west Europe. Comparison with published haplotype frequencies of breeding populations shows that birds from Greenland, Iceland, and North Europe were predominant in wintering grounds in West Africa, while populations wintering in West Europe originated from more eastern breeding grounds (e.g. North Russia). These results show that genetic markers can be used to enhance the integrative monitoring of wintering and breeding populations, by providing biogeographical evidence that validate the winter segregation of breeding populations.     

Nutrients distribution in the coastal water of East Asia relative to the Kuroshio

Nutrients in the coastal water of East Asia were surveyed from the point of contribution of the diluted water to the open water relative to the Kuroshio, the western boundary current. The Kuroshio runs along eastern Asia, sensing changes in environment in the coastal region. The nutrients-depleted water from the Kuroshio, close to the coastal water, provides heat energy and high saline water. The fresh water from the river contributes to the coastal water as diluted water and exhibits the entrainments of nutrient water into the Kuroshio. The nutrients are important parameters to monitor the interactions among the Kuroshio and inland waters. Nutrients exhibited a wide range of ratio between nitrate to phosphate, nitrate to silicate, and silicate to phosphate in East Asia as divergences from the Redfield ratios. The distribution of nutrients could also be monitored as the distribution of chlorophyll-a concentration using the ocean color sensors. Contrasts in chlorophyll-a concentration between the oligotrophic water of the Kuroshio and the trophic water of the coastal region exhibit the mixing process of two water masses. Although the chlorophyll-a concentrations estimated by the current ocean color algorithm over the coastal water are of insufficient accuracy, the distribution of chlorophyll-a concentration exhibits a variation of seasonal consumption of nutrients and response to the Kuroshio and river runoff.     

Different dispersal histories of lineages of the earthworm <Emphasis Type="Italic">Aporrectodea caliginosa</Emphasis> (Lumbricidae,Annelida) in the Palearctic

Earthworms are among the most abundant and ecologically important invasive species, and are therefore a good object for studying genetic processes in invasive populations. Aporrectodea caliginosa is one of the most widespread invasive earthworms in the temperate zone. It is believed to have dispersed from Europe to all continents except Antarctica. It is known that A. caliginosa consists of three genetic lineages, and genetic diversity is high both among and between them. We attempted to use that high genetic diversity to study A. caliginosa dispersal in the Palearctic based on a sample of 40 localities ranging from eastern Europe to the Russian Far East, and to compare our data to other studies on this species in western Europe and North America. Two genetic lineages were found in the studied sample. Only negligible decrease in genetic diversity was observed for the lineage 2 of A. caliginosa from West Europe to the Far East, suggesting multiple human-mediated introductions. In contrast, lineage 3 is abundant in West Europe and Belarus, but is absent from the East European Plain, the Urals, and the Far East. However, it is present in West Siberia, where it has greatly reduced genetic diversity, indicating long-distance dispersal accompanied by a bottleneck event. Thus, although these two lineages of A. caliginosa are morphologically indistinguishable, they have dramatic differences in their distributions and dispersal histories.     

Molecular and morphological analysis of secondary contact zones of Cottus gobio in Fennoscandia: geographical discordance of character transitions

Northern Europe was postglacially colonized from different directions by distinct phylogeographical lineages of the bullhead Cottus gobio L. (Pisces: Scorpaeniformes). These lineages have then come into contact in coastal habitats of the currently brackish Baltic Sea and in the freshwaters north of it. We studied the patterns of intergradation in the contact zones in four morphometric and six molecular characters. In the north, intergradation between the western (W) and eastern (E) bullhead lineages is found both among rivers (west‐to‐east) and along individual rivers (south‐to‐north). The locations of the transition zones probably relate to the timing of the initial contact, subsequent Baltic shoreline displacement (i.e. emergence of the lower river reaches), and dispersal barriers caused by variations of coastal salinity. The transitions (clines) in different characters are, however, not geographically coincident. Mitochondrial DNA clines are generally found upstream and to the east of the other transitions, and GPI‐1 allozyme clines are mostly shifted downstream in the rivers, and west of the other transitions on the broader scale of the Baltic Sea. The location of the mtDNA clines may best reflect the initial contact between lineages, and the displacement of the other clines could result from dispersal being overall asymmetric (predominantly downstream) and sex‐biased (stronger in males). Alternatively, the non‐coincidence might reflect selection against deleterious cytonuclear character combinations. No clear evidence of reproductive incompatibility between the lineages was seen in local population structures; no remaining genetic correlations were observed locally among traits. In another transition area, a coastal transect in southern Finland, clinal patterns similar to those in the northern contact zone were recorded, but the population compositions could not be explained by simple in situ mixing of any of the putatively pure, invading refugial lineages. Probably, the bullhead stocks that initially came into contact in this southern study area already represented mixtures of the invading lineages. © 2004 The Linnean Society of London, Biological Journal of the Linnean Society, 2004, 81 , 535–552.     

Molecular and morphometric variation in two sibling species of the genus Praomys (Rodentia: Muridae): implications for biogeography

The rodent genus Praomys is widely distributed in the African tropics. The species are cryptic, rendering the species taxonomy unclear. There are differences of opinion concerning the specific status of Praomys misonnei and Praomys tullbergi, and their geographical distribution. We sequenced the cytochrome b and/or the 16S gene of 221 specimens from 12 countries in order to evaluate the genetic variability within these two species, and to precisely determine their geographical distribution. Morphological and morphometrical analyses on the sequenced specimens were also performed to find criteria useful for the identification of museum specimens. Our results confirm that P. misonnei and P. tullbergi are two valid species that can be separated by molecular data. However, no single discrete morphological character or simple metric measurement can be used to discriminate them. The percentage of misclassified individuals in multivariate discriminant analysis is relatively high (10%). The two species have allopatric distributions: P. tullbergi occurs in West Africa, from eastern Guinea to western Ghana, and P. misonnei is widely distributed from eastern Ghana to western Kenya. Within P. misonnei we identified three or four major geographical clades: a West Central African clade, an East African clade, a Nigerian clade, and a possible West African clade. Within P. misonnei, high geographical morphometrical variability was also identified. The role of both rivers and Pleistocene forest refugia in promoting speciation within the genus Praomys is discussed. © 2010 The Linnean Society of London, Zoological Journal of the Linnean Society, 2010, 160 , 397–419.     

The decapod researcher’s guide to the galaxy of sex determination

The objective of this study was to assess the frequency of use of marine resources in recruitment of Southern Hemisphere native riverine fish Galaxias maculatus from rivers across a latitudinal gradient. To do this, we analysed the concentrations of δ³⁴S in vertebral column tissues from fish collected in ten Chilean river systems across latitudes 36°–47°S. The analyses of δ³⁴S signatures in these rivers suggest that the use of marine resources by riverine populations of G. maculatus in large river systems in Chile is variable, with marine resources playing a limited role in more northern large rivers, characterised by warmer temperatures and predictable flow regimes and floodplain inundations. This is in contrast to life histories described for G. maculatus in rivers from New Zealand and Australia, where riverine populations are believed to be characterised by an obligatory recruitment phase in marine environments. Recruitment of G. maculatus in Chilean large rivers appears to depend on their freshwater productivity driven by climate as well as both longitudinal (headwaters lakes-estuary) and lateral (main channel-floodplains) hydrologic connectivities.     

A population genetic characterization of Estonians

This paper discusses the genetic characterization of Estonians on the basis of eight blood group systems, and the traits of PTC tasting and colour blindness in 40 Estonian population samples from various parts of the country. The allele frequencies for the total Estonian population and for the four most different regions are presented. The survey shows genetic heterogeneity within the Estonians; the greatest genetic differences were observed in West-East direction. The West-Islands, West, and North Estonia differ from the other regions (East, South-East, also South-West and Central Estonia--which form a compact cluster). The mean allele frequencies of the Estonians are comparable to those typical for populations from North and East Europe, but the allele frequencies of Estonians are characterized by tendencies in two opposite (western and eastern) directions, like in other Finno-Ugric populations and concerning other anthropological traits. Estonians reveal closest similarities to the nearest neighbouring populations, regardless of their language group. The genetic heterogeneity and antagonistic traits in Estonians seem to be traces of the original genetic structure of Finno-Ugric ancestor populations which were neither Mongoloid nor Caucasoid.     

Fish fauna and environments of the Pilcomayo-Paraguay basins in Formosa,Argentina

The ichthyofauna from subtropical (East) and tropical (West) areas in the Formosa province (Northern Argentina) is analyzed. An up-to-date list of fishes is provided, including 18 new reports for environments associated with the Pilcomayo and Paraguay rivers, considering the detailed distribution of the fishes. Dominant families in terms of number of species are Characidae, Pimelodidae, Loricariidae and Curimatidae. Percentage of individuals (about 5000 examined) were 66% for characoids and 25% for siluriforms, with less than 10% for other groups. Comparisons were made with other South American environments. Most frequent species were Psellogrammus kennedyi, Astyanax bimaculatus, Gymnocorymbus ternetzi, Hoplosternum thoracatum and Cichlasoma portalegrense. The predominant type of environment sampled, of small size and shallow depth, with extensive plant cover, and temporal level variations, explains the dominance of the above groups. This interpretation is supported by independent studies. Diversity values ranged between 0.71 and 3.92. A marked reduction in number of species from East to West was observed (79 and 41 species respectively, 31 shared). No species of Gymnotidae, Hemiodidae, Characidiidae, Trichomycteridae, Lebiasinidae nor Aspredinidae were captured in the West. The higher number of species in eastern environments is considered to be due to the influence of the Paraguay river and the complex hydrology of the area. Other factors, both historical and ecological are considered. A correlation between total phosphorous content in the water and fish richness is also suggested. Water chemistry is compared with near by environments. Sites studied showed pH values from 6.14 to 7.5. The dissolved solid contents ranged from 51.8 to 474.3 mg 1^–1, within the hypohaline level. Ionic composition differed somewhat between East and West, water in the East being mainly hypocalcic, as in the Paraguay river. Conductivity was rather variable and Secchi disk values similar to those in the Parana river.     

PATTERNS OF ORIENTATION IN UNIONIDS AS A FUNCTION OF RIVERS WITH DIFFERING HYDROLOGICAL VARIABILITY

As unionids can become dislodged with high flows, it may provebeneficial for an individual to minimize its exposure to theflow. This can be accomplished by either burrowing as deep aspossible or orienting in a way that effectively reduces thedrag exerted on the mussel by the flow. The patterns of orientationwere examined in unionids with respect to hydrological variability.The orientation of mussels to flow was measured at four sitesalong an event river (hydro-logically variable) and a stableriver (hydrologically stable). Burrowing depth was measuredat a reference site in each river. Most individuals in bothriver types were oriented with their siphons pointing upstream.Mussel orientation differed significantly between the two rivertypes with mussels in the event river orienting more parallelto the flow than those in the stable river. Mussels in the eventriver were significantly larger than those in the stable riverbut the size of a mussel did not determine its orientation withina river. Burrowing depth did not differ for mussels betweenevent and stable rivers. The observed differences in orientationsamong river types are likely a function of differences in thepattern of orientation of the mussel community as a whole, withineach drainage. This burrowing behaviour may be an attributethat enhances the adaptations mussels have for remaining burrowedin the sediment. (Received 2 December 1996; accepted 24 February 1997)     

Length–weight and length–length relationships of 16 fish species from Amapá, Brazilian Amazon

The present study provides length‐weight (LWR) and total length‐standard length (LLR) relationships for 16 freshwater fish species collected in coastal rivers of the northern Brazilian, state of Amapá in 2015. Fishes were caught in heterogenic environments like ponds, stream and river channel with sieve and trawl. As result, we present novel information of LWR and LLR for 11 and 15 species, respectively. Maximum values (first values or new maximum values) of TL, SL and WT are presented for 14, 12 and 15 species, respectively.     

Mitochondrial DNA variability of West New Guinea populations.

This paper reports human mitochondrial DNA variability in West New Guinea (the least known, western side of the island of New Guinea), not yet described from a molecular perspective. The study was carried out on 202 subjects from 12 ethnic groups, belonging to six different Papuan language families, representative of both mountain and coastal plain areas. Mitochondrial DNA hypervariable region 1 (HVS 1) and the presence of the 9-bp deletion (intergenic region COII-tRNA(Lys)) were investigated. HVS 1 sequencing identified 73 polymorphic sites defining 89 haplotypes; the 9-bp deletion, which is considered a marker of Austronesian migration in the Pacific, was found to be absent in the whole West New Guinea study sample. Statistical analysis applied to the resulting haplotypes reveal high heterogeneity and an intersecting distribution of genetic variability in these populations, despite their cultural and geographic diversity. The results of subsequent phylogenetic approaches subdivide mtDNA diversity in West New Guinea into three main clusters (groups I-III), defined by sets of polymorphisms which are also shared by some individuals from Papua New Guinea. Comparisons with worldwide HVS 1 sequences stored in the MitBASE database show the absence of these patterns outside Oceania and a few Indonesian subjects, who also lack the 9-bp deletion. This finding, which is consistent with the effects of genetic drift and prolonged isolation of West New Guinea populations, lead us to regard these patterns as New Guinea population markers, which may harbor the genetic memory of the earliest human migrations to the island.     

Microfloristic provincialism in the Upper Triassic Circum-Mediterranean area and palaeogeographic implication

This study represents a contribution to the developing knowledge about the microfloristic provincialism affecting Upper Triassic palynoflora. The compositional differences existing between the Onslow and the Ipswich microfloras are mainly based on the presence, in the Onslow microflora, of a diverse and more varied group of gymnosperm pollen grains, including typical European elements. In this study, the palynological assemblages recovered from Carnian successions of the western Tethyan margin (Sicily, Tunisia, Albany, Libya and Israel) are compared with those of West Timor microfloral assemblages, which have been referred to the Onslow microflora of southern hemisphere. They contain several common taxa, mainly consisting of conifer miospores widely recorded in Carnian European successions and less frequently recovered in the Carnian of western and eastern Australia. The number of Circum-Mediterranean sporomorphs in the Onslow microflora assemblages is wider than previously thought, providing new evidences to extend the distribution of the Onslow microflora to include Carnian associations formerly assigned to the Circum-Mediterranean assemblages. These broad microfloral affinities seem to indicate the existence of a homogeneous microflora that maintains, with minor variations, its composition from western Tethys coasts to the northern Australian margin (West Timor). The parent plant community grew in a coastal environment, along the continental margins; the establishment of an equable climatic regime influenced by warm equatorial currents and suitable humid conditions probably conditioned its diffusion. It has long been recognised that the strong floral provinciality which characterised the Late Triassic world gave way to a more homogeneous flora in the Early Jurassic. The decrease in macrofloral diversity is associated with a less pronounced microfloristic provincialism, which in turn coincides with the rise, to strong dominance, of cheirolepidiaceous conifers (Circumpolles producers). This important microfloristic event occurs in the southern hemisphere during the Early Jurassic, however this study reveals their incipient diffusion during the Norian.     

Distributions of tree species along point bars of 10 rivers in the south‐eastern US Coastal Plain

Aim To determine the degree to which rivers within the south‐eastern US Coastal Plain show a predictable spatial distribution of floodplain tree species along each point bar of river bends in relation to elevation and/or soil texture, as seen on the Bogue Chitto River, Louisiana, USA. Also, to understand spatial patterns of tree species on land created during river‐bend migration, and to interpret which physical characteristics of rivers predict this pattern of vegetation. Location The south‐eastern US Coastal Plain. Methods Ten randomly selected rivers within a portion of the region were studied. At each of 10 river bends per river, a census of trees and shrubs was taken and elevation and soil texture were measured at upstream, mid‐ and downstream locations along the forest–point bar margin. To identify physical characteristics of rivers that are predictive of patterns of tree species along point bars, aerial photographs, hydrographs and field data were analysed. Results Tree species composition varied predictably among the three point bar locations, corresponding to an elevation gradient on each bar, on seven of 10 rivers. Species occupying a given point bar location on one river usually occupied the same location on other rivers, in accordance with species‐elevation associations identified in past studies of floodplain forests. Multivariate analysis of river characteristics suggested that rivers failing to show the expected pattern were those with relatively low stream energy and geomorphic dynamics and/or those with hydrological regimes altered by upstream dams. Main conclusions A distinct pattern of streamside forest community structure is related to fluvial geomorphic processes characterizing many rivers within the south‐eastern US Coastal Plain. Characteristics of rivers required to promote the predicted pattern of tree species include a single, meandering channel with point bars; an intermediate level of stream energy; a natural hydrological regime; and location in a biome where a large number of tree species are capable of colonizing point bars.