Scale-dependent patterns of macrofaunal distribution in soft-sediment intertidal habitats along a large-scale estuarine gradient

We investigated the pattern of distribution of intertidal soft-bottom fauna in streams and lagoons of the Uruguayan coast at three spatial scales. The Río de la Plata and the Atlantic Ocean produce on this coast a large-scale gradient in salinity, defining a freshwater (west), an estuarine (central) and a marine (east) region. Within each region, there are several streams and coastal lagoons (sites) that define a second scale of variability. A third scale is given by intertidal gradients within each site. Species richness and total abundance was low in the freshwater west region and high in the central and east regions. The community in the west region was characterized by the clam Curbicula fluminea; in the other regions, it was dominated mainly by the polychaete Heteromastus similis. The polychaete Nephtys fluviatilis was more abundant in the east region, while another polychaete, Laeonereis acuta, characterized the central region. Sediment fractions did not vary significantly at this scale. At the scale of the sites, species richness and total macrofaunal abundance were higher in coastal lagoons than in streams. Coarse sands were more common in coastal lagoons, while medium and fine sand characterized the sediment in streams. Within each site, species richness and total abundance increased towards the lower intertidal level; the macrofauna of the upper levels were a subsample of the fauna occurring at the lower levels. There was also a significantly lower proportion of fine sand at the upper level. At regional scales, the observed patterns may be indirectly or directly related to the gradient in salinity, through differential physiological tolerance to osmotic stress. At the scale of the sites, variability may be explained mainly by geomorphological and sedimentological differences between lagoons and streams. Variation among levels may be related to gradients in desiccation, colonization and predation.     

Changes in the community structure and activity of betaproteobacterial ammonia-oxidizing sediment bacteria along a freshwater-marine gradient

To determine whether the distribution of estuarine ammonia-oxidizing bacteria (AOB) was influenced by salinity, the community structure of betaproteobacterial ammonia oxidizers (AOB) was characterized along a salinity gradient in sediments of the Ythan estuary, on the east coast of Scotland, UK, by denaturant gradient gel electrophoresis (DGGE), cloning and sequencing of 16S rRNA gene fragments. Ammonia-oxidizing bacteria communities at sampling sites with strongest marine influence were dominated by Nitrosospira cluster 1-like sequences and those with strongest freshwater influence were dominated by Nitrosomonas oligotropha-like sequences. Nitrosomonas sp. Nm143 was the prevailing sequence type in communities at intermediate brackish sites. Diversity indices of AOB communities were similar at marine- and freshwater-influenced sites and did not indicate lower species diversity at intermediate brackish sites. The presence of sequences highly similar to the halophilic Nitrosomonas marina and the freshwater strain Nitrosomonas oligotropha at identical sampling sites indicates that AOB communities in the estuary are adapted to a range of salinities, while individual strains may be active at different salinities. Ammonia-oxidizing bacteria communities that were dominated by Nitrosospira cluster 1 sequence types, for which no cultured representative exists, were subjected to stable isotope probing (SIP) with 13C-HCO3-, to label the nucleic acids of active autotrophic nitrifiers. Analysis of 13C-associated 16S rRNA gene fragments, following CsCl density centrifugation, by cloning and DGGE indicated sequences highly similar to the AOB Nitrosomonas sp. Nm143 and Nitrosomonas cryotolerans and to the nitrite oxidizer Nitrospira marina. No sequence with similarity to the Nitrosospira cluster 1 clade was recovered during SIP analysis. The potential role of Nitrosospira cluster 1 in autotrophic ammonia oxidation therefore remains uncertain.     

Ecological traits of planktonic viruses and prokaryotes along a full-salinity gradient

Virus-prokaryote interactions were investigated in four natural sites in Senegal (West Africa) covering a salinity gradient ranging from brackish (10‰) to near salt saturation (360‰). Both the viral and the prokaryote communities exhibited remarkable differences in their physiological, ecological and morphological traits along the gradient. Above 240‰ salinity, viral and prokaryotic abundance increased considerably with the emergence of (1) highly active square haloarchaea and of (2) viral particles with pleiomorphic morphologies (predominantly spindle, spherical and linear shaped). Viral life strategies also showed some salinity-driven dependence, switching from a prevalence of lytic to lysogenic modes of infection at the highest salinities. Interestingly, the fraction of lysogenized cells was positively correlated with the proportion of square cells. Overall, the extraordinary abundance of viruses in hypersaline systems (up to 6.8 × 10(8) virus-like particles per milliliter) appears to be partly explained by their high stability and specific ability to persist and proliferate in these apparently restrictive habitats.     

Seasonal changes in caddis larvae assemblages in river-floodplain habitats along a hydrological connectivity gradient

In order to assess the impact of seasonality versus connectivity on the ecological quality of the Lower Rhine river-floodplain habitats, we studied the seasonal variation in diversity and species assemblages of caddis larvae by monthly sampling of the littoral zone of four water bodies over a lateral connectivity gradient. Seasonal variation in diversity showed a general pattern in these floodplain habitats and could be related to caddisfly life history and habitat preferences. Despite this seasonal variation, caddis larvae species assemblages in the water bodies appeared to be rather stable and could be firmly related to the position of their habitats over the connectivity gradient. The main channel (lotic habitat) was clearly separated from the lentic water bodies, and also the sparsely vegetated water bodies from the well-vegetated water bodies. It is concluded that the impact of connectivity, and related parameters such as stream velocity and vegetation coverage, outweighed the impact of season on the caddis larvae assemblages.     

Microbial community structures in anoxic freshwater lake sediment along a metal contamination gradient

Contamination, such as by heavy metals, has frequently been implicated in altering microbial community structure. However, this association has not been extensively studied for anaerobic communities, or in freshwater lake sediments. We investigated microbial community structure in the metal-contaminated anoxic sediments of a eutrophic lake that were impacted over the course of 80 years by nearby zinc-smelting activities. Microbial community structure was inferred for bacterial, archaeal and eukaryotic populations by evaluating terminal restriction fragment length polymorphism (TRFLP) patterns in near-surface sediments collected in triplicate from five areas of the lake that had differing levels of metal contamination. The majority of the fragments in the bacterial and eukaryotic profiles showed no evidence of variation in association with metal contamination levels, and diversity revealed by these profiles remained consistent even as metal concentrations varied from 3000 to 27 000 mg kg⁻¹ total Zn, 0.125 to 11.2 μ pore water Zn and 0.023 to 5.40 μ pore water As. Although most archaeal fragments also showed no evidence of variation, the prevalence of a fragment associated with mesophilic Crenarchaeota showed significant positive correlation with total Zn concentrations. This Crenarchaeota fragment dominated the archaeal TRFLP profiles, representing between 35% and 79% of the total measured peak areas. Lake DePue 16S rRNA gene sequences corresponding to this TRFLP fragment clustered with anaerobic and soil mesophilic Crenarchaeota sequences. Although Crenarchaeota have been associated with metal-contaminated groundwater and soils, this is a first report (to our knowledge) documenting potential increased prevalence of Crenarchaeota associated with elevated levels of metal contamination.     

The distribution of some freshwater planktonic bacteria in two stratified eutrophic lakes

SUMMARY. Changes in bacterial populations and certain physical and chemical variables in Esthwaite Water between June and September 1975 were studied and compared with results obtained from 1972 to 1974 in the hypolimnia of Blelham Tarn and the Lund tubes. The counts of total bacteria ranged between 1 and 7 × 10⁶ml⁻¹ and were highest in the anoxic hypolimnion. The bacterial genera examined in more detail constituted only a small percentage of this count and included Ochrobium (10⁴ml⁻¹), Naumanniella (10³ml⁻¹), Leptothrix (10²ml⁻¹), Planctomyces (10³ml⁻¹), and Metallogenium (10²ml⁻¹). The iron bacteria appear to grow best in the oxycline where there was not only sufficient oxygen for aerobic growth but also a plentiful supply of reduced iron. Planctomyces numbers increased as the thermocline became depressed in September. The results from Blelham Tarn might be interpreted as further evidence of growth by iron bacteria in the absence of dissolved oxygen, but other explanations are possible. Examination of the results by multiple regression analysis showed that it was possible to explain a significant proportion of the bacterial variation (with the notable exception of the Planctomyces counts) in spite of considerable intercorrelation of the regressor variables.     

Biomass and structure of planktonic communities along an air temperature gradient in subarctic Sweden

1. Air temperature will probably have pronounced effects on the composition of plankton communities in northern lake ecosystems, either via indirect effects on the export of essential elements from catchments or through direct effects of water temperature and the ice‐free period on the behaviour of planktonic organisms. 2. We assessed the role of temperature by comparing planktonic communities in 15 lakes along a 6 °C air temperature gradient in subarctic Sweden. 3. We found that the biomass of phytoplankton, bacterioplankton and the total planktonic biomass were positively related to air temperature, probably as a result of climatic controls on the export of nitrogen from the catchment (which affects phytoplankton biomass) and dissolved organic carbon (affecting bacterioplankton biomass). 4. The structure of the zooplankton community, and top down effects on phytoplankton, were apparently not related to temperature but mainly to trophic interactions ultimately dependent on the presence of fish in the lakes. 5. Our results suggest that air temperature regimes and long‐term warming can have strong effects on the planktonic biomass in high latitude lakes. Effects of temperature on the structure of the planktonic community might be less evident unless warming permits the invasion of fish into previous fishless lakes.     

Changes along a disturbance gradient in the density and composition of propagule banks in floodplain aquatic habitats

This study used germination methods to examine the density, species composition and functional composition of propagule banks in a series of riverine wetland aquatic habitats subject to varying degrees of hydrological and management-related disturbance. Under permanent inundation (the conditions prevailing at most sites during the growing season) propagule germination and species richness was low, with floodplain perennials and helophytes particularly affected. Densities of floodplain annuals were largely maintained through continued germination of a few flooding tolerant species. On damp mud (conditions associated with hydrological instability) total seedling number and species richness increased significantly, but species richness of germinating hydrophytes declined. Mean seedling density at 0–0.1m depth was 15450 ± 4400 m–2, reaching a maximum (162 050 m–2) in temporary backwaters. Annual (e.g., Lindernia dubia, Cyperus fuscus) and facultative ruderal species (e.g., Lythrum salicaria and Alisma plantago-aquatica) predominated. Vertical zonation of the propagule bank was weakly developed. The numbers of individuals and species germinating varied significantly between sites. The seasonal, most intensely disturbed sites (temporary backwaters) supported a numerically large, species-rich propagule bank based on floodplain annuals, while the permanent, less disturbed sites (ditches and an oxbow pond) had a small, species-poor propagule bank composed of hydrophytes and helophytes supplemented by allochthonous seed inputs. Sites intermediate on the gradient had a propagule bank dominated by facultative amphibious, ruderal hydrophytes. The composition of the seed bank and the established vegetation was most similar at the heavily disturbed sites where the seed bank was maintained by vigorously fruiting annuals and supplemented by inputs from temporary habitats upstream. At permanent sites much of the propagule bank composition could be accounted for by inputs of floodborne seed from the immediately adjacent floodplain. The established vegetation at such sites appeared to be maintained mainly by vegetative propagation with recruitment from the propagule bank likely only after severe disturbance. The potential contribution of functionally diverse propagule banks to sucessional processes within fluvially dynamic floodplain aquatic habitats is emphasised.     

The detection of non-random patterns of distribution of species along a gradient

Summary A simple method for detecting non-random patterns of distribution of the boundaries of species is described. The method uses transects running across a community, where the number of upper and lower boundaries of species in each quadrat is recorded. The expected number of quadrats containing one or more boundaries can be calculated from the binomial distribution. The mean deviation of observed from expected number of such quadrats, for a set of transects, can be tested for departures from zero. Significant departures greater than zero indicate regular dispersion of boundaries. A mean deviation significantly less than zero indicates clustering of the boundaries. The method is unbiased and thus corrects previously published methods.     

Comparison of soil seed banks of habitats distributed along an altitudinal gradient in northern Iran

In this study we investigated the variations in soil seed banks along an altitudinal gradient in the Alborz mountains, Iran, covering three habitats from lower to upper altitudes: forest, forest-subalpine grassland ecotone and subalpine meadow. In each habitat from 1850 to 2400 m, 20 quadrats were established along four transects, and the above-ground vegetation and the germinable seed banks were determined. Results show that the similarity between seed bank and vegetation was lowest in the ecotone located at intermediate altitudes. Together with the contrasting highest density and species diversity of seeds at these altitudes, the ecotonal role of this habitat was confirmed.We found evidence that lower altitudes could act as storage for seeds of some species growing at higher altitudes; the role of the ecotone was more prominent as a reserve for the meadow plant seeds than the role of the forest as a reserve for seeds of the meadow and ecotone habitats. Soil seed banks, particularly from the ecotone, can be used for restoring vegetation in some degraded sites.     

Bird communities in habitats along a successional gradient: Divergent patterns of species richness,specialization and threat

The long history of human influence on northern temperate landscapes has created a mosaic of successional stages, from closed forest to open grassland. Various species thus adapted to different habitats and it is interesting to explore how these differences in species composition among particular successional stages translate into differences at the community level. For this purpose, we surveyed breeding birds in 233 patches of five different habitats covering a gradient from bare ground to forest in 29 abandoned military training sites scattered throughout the Czech Republic. Linear mixed effects modelling revealed that late-successional habitats (dense scrubland and forest) were the most species-rich, whereas early-successional stages hosted bird communities with the highest habitat specialization and threat level. These results suggest that the habitats of late-successional stages are important for the maintenance of high bird species richness, but that early-successional habitats are essential for highly specialized and threatened bird species. Given the highly adverse impacts of agricultural intensification and land abandonment on open habitats, it is necessary to promote factors creating initial successional stages suitable for specialized and threatened species.     

Contrasting patterns in leaf traits of Mediterranean shrub communities along an elevation gradient: measurements matter

Plant Ecology - We assessed the changes in community-weighted mean (CWM) and variability of specific leaf area (SLA) and leaf area (LA) of different Mediterranean shrub communities along an...     

Extraordinary prolongation of the life cycle in a freshwater planktonic copepod

The limnetic copepod Cyclops scutifer Sars shows an extraordinarily prolonged life cycle in a temperate lake near Oslo, Norway. Developmental sequences become one, two, and three years old, and there are circumstantial evidence of a four year old fraction. Prolongation of the cycle is mainly caused by two consecutive periods of diapause, the first as small copepodids during the second winter, the other as large copepodids during the third winter. There is a marked concentration of diapausing individuals in a small area of the bottom at the deepest part of the lake. There is evidence that this concentration is caused by a horizontal migration to areas with optimal oxygen conditions. The adaptive value of a prolongation of the life cycle is assumed to be an increase in the reproductive potential caused by a spreading out of the reproductive period by three year classes succeeding each other throughout the summer.     

Effects of freshwater shrimp assemblages on benthic communities along an altitudinal gradient of a tropical island stream

1. In tropical island stream ecosystems freshwater shrimps are often the dominant macroconsumers and can play an important role in determining benthic community composition. However, most studies of the ecological role of shrimps are limited to high‐altitude shrimp‐dominated sites where other biota (fishes and snails) are absent or significantly less abundant than at lower altitudes. 2. We examined how effects of different shrimp assemblages on benthic communities changed along an altitudinal gradient in a tropical island stream in Puerto Rico. We used electroshocking and observations to quantify abundance and taxonomic composition of shrimp assemblages at three sites (300, 90 and 10 m a. s. l) along the Río Espíritu Santo. We also experimentally manipulated access of shrimps to the benthic environment simultaneously at each site using electric fences over a 35‐day period. 3. At the high‐altitude site, exclusion of shrimps (predominantly Atya spp. and Xiphocariselongata) resulted in significantly greater accrual of organic and inorganic material, chlorophyll a and algal biovolume. In the absence of shrimps, the algal community was dominated by filamentous green algae (Chlorophyta: Oedogonium and Rhizoclonium). Excluding shrimps did not affect total insect biomass but significantly increased sessile chironomids (Diptera: Chironomidae). We observed similar treatment effects at the mid‐altitude site where shrimps (primarily Macrobrachium spp. and X. elongata) occurred at lower densities. In contrast, at the low‐altitude site there were no treatment differences in organic and inorganic material, chlorophyll a, algal biovolume, algal assemblage composition and insects. 4. The lack of treatment differences at the low‐altitude site was probably because of very high densities of grazing snails (Thiaragranifera and Neritina spp.) which reduced organic and inorganic resources and obscured potential shrimp effects. 5. This study demonstrates that freshwater shrimps can play an important role in determining benthic community composition; however, their effects vary and appear to depend on the presence of other biota. This study suggests that loss of shrimps as a result of anthropogenic disturbances will have different effects on the stream community depending upon location along the altitude gradient.     

Different trends in phylogenetic and functional structure of plant communities along an elevation gradient

The study of diversity gradients due to elevation dates back to the foundation of biogeography and ecology. Although elevation-driven patterns of plant diversity have been reported for centuries, uncertainty still exists about the assembly rules that drive these patterns. In this study, we revealed the causal factor of community assemblies for the diversity of tree and herb species along an elevation. To this end, we applied an integrated method using both functional traits and phylogeny, called the mean pairwise functional-phylogenetic distance, to understand the assembly rules for woody and herbaceous species communities along an elevation gradient. At higher elevation sites, woody and herbaceous communities were comprised of species having similar traits. The phylogenetic trends for woody species were consistent with the functional trends; closely related species co-occurred more frequently than expected at higher elevations. Phylogenetic trends for herb species were opposite to the functional trends; species with similar traits but having a random phylogenetic distribution co-occurred at higher elevations. We suggest that the community assembly rules for woody and herb species vary with elevation; and functional constraints due to environmental filtering at higher elevation act as assembly rules along gradients in both woody and herbaceous communities, even though their phylogenetic backgrounds differ.     

Niche differentiation and distribution of Carex curvula along a bioclimatic gradient in the southwestern Alps

Abstract. Carex curvula is a dominant sedge of European alpine tundra, exhibiting two morphological forms: C. curvula ssp. curvula (Ccc) and C. curvula ssp. rosae (Ccr). In this paper, we attempt to explore whether Ccc and Ccr are ecotypes or vicariant forms and whether between‐ and within‐regional distribution patterns can be explained by variations in the amount of available habitats and/or by changes in niche attributes. The study area covered three bioclimatically distinct regions of the southwestern Alps in which local abundances of Ccc and Ccr strongly differ. The realized niche of both forms was investigated by a direct gradient analysis performed on an extensive floristic and environmental data set. We found no evidence of niche overlap between Ccc and Ccr as their distribution curves differ strongly along disturbance, mesotopography and soil acidity gradients. We investigated the effect of region on the structure of local scale variables tables. Highly concordant patterns among regions were found, except that optimal habitat conditions for Ccc were infrequent in the southernmost regions. We compared the ecological behaviour of range centre and range margin populations of each form. We found a narrower ecological amplitude for Ccr in the northern part of its range, whereas for Ccc the niche breadth of range‐margin populations was not reduced compared to range centre populations. At its southern range limit, Ccc mostly occupies ecologically marginal habitats. We conclude that Ccc and Ccr represent ecotypes, not vicariant forms. Finally, we suggest that habitat availability, resulting from the interplay of regional‐scale and local‐scale variables, satisfactorily explains the distributional patterns of Carex curvula ecotypes in the southwestern Alps.