Changes In Water Depth Research Articles

Defining soft bottom habitats and potential indicator species as tools for monitoring coastal systems: A case study in a subtropical bay

The definition of habitats and indicator species is a prerequisite for monitoring and conservation programs. Nonetheless, defining habitats in marine soft-bottom environments is challenging given their spatiotemporal dynamics and apparent homogeneity. The selection of indicator species is also complicated given the large number of occasional species usually presented in benthic communities. This study aims to elaborate a framework based on well-established analytical methodologies to identify soft-bottom habitats and select indicator species to support monitoring and conservation programs. The proposed framework consists of four steps: 1) perform a Redundancy Analysis (RDA) on the community data to identify the community structure response to environmental gradients; 2) conduct a kernel density analysis on the RDA biplot to determine the habitats; 3) use the indicator values analyses (IndVal) to select indicator species of each habitat; 4) run polynomial quantile regression analysis to find the optimum distribution of each indicator species. Such framework allows the determination of habitats based on the association of environmental and biological datasets, instead of relying solely on abiotic surrogates. As a case study, we used data of macro and meiofauna of a biodiverse coastal ecosystem in Southeast Brazil which is under anthropogenic pressure. Three main habitats were identified in the bay, and macro and meiofaunal assemblages were influenced by similar environmental variables. Nevertheless, macrofauna was more sensitive to changes in sediment composition, whereas meiofauna responded strongly to changes in total organic content and water depth. Macro- and meiofauna indicator taxa showed high specificity and fidelity values to each habitat, supporting their use in monitoring and conservation programs. The spatio-temporal organization of each habitat and the optimum distribution of each indicator species provide baseline knowledge to be used to monitor environmental changes in the study area and help in its conservation.

Dynamic Manning's roughness coefficients for hydrological modelling in basins

Abstract Manning's roughness coefficient (n) has a significant impact on routing in hydrological models. However, computational methods for dynamic roughness coefficients are of little concern in current research. Few studies have produced spatial-temporal distributions of the roughness coefficients in basins. In this study, a formula to calculate the n value was established based on a statistical analysis of estimated n values by Manning's formula. The routing model of a distributed hydrological model was then improved using the new formula to calculate n. The roughness coefficient is not a constant; instead, it changes dynamically with changes in water depth and vegetation in the improved hydrological model. The improved model was applied to the Yellow River basin. The results show that using dynamic n can improve the streamflow simulation of hydrological models, especially on slopes. The dynamic spatial-temporal distribution of n can now be used in other models.

Functional traits mediated cascading effects of water depth and light availability on temporal stability of a macrophyte species

Interspecific trait variability (e.g., functional diversity) is often correlated with community productivity and thus promotes ecosystem stability. However, we know little about how intraspecific trait variability mediated the effects of environmental gradients on population stability. We applied generalized multilevel path models to investigate multivariate causal hypotheses that environmental effects on population stability of a macrophyte species - Potamogeton maackianus - are mediated by physiological and morphological traits variability. Using a hierarchical nested design, we measured quarterly nine physiological traits, and monthly ten morphological traits from 9720 individuals at 27 plots along water depth gradients in Lake Erhai, China. Results showed that changes in water depth and light availability caused significant shifts in intraspecific trait variability, with cascading direct and indirect effects on temporal stability of P. maackianus. Water depth exerted major control over population stability, both directly and indirectly via shifts in morphological and physiological traits. Light availability affected population stability indirectly by altering morphological traits. Size traits (shoot morphology and root/shoot ratio) and metabolic traits captured the effect of water depth and light availability on population stability with a relative high accuracy. Our study provide a strong support to the hypothesis that intraspecific trait variability mediate the combined effects of variations on water depth and light conditions on the temporal stability of a dominant macrophtye species in Lake Erhai. Maintaining and enhancing trait variability within plant populations may help to buffer negative effects of anthropogenic environmental changes on population stability.

Numerical and Physical Investigation of the Performance of Turbulence Modeling Schemes around a Scour Hole Downstream of a Fixed Bed Protection

Local scour occurs around hydraulic structures such as piers, bed protections, and dikes. In this study, the turbulent flow around a scour hole downstream of a fixed bed protection was investigated. Numerical modeling with OpenFOAM was applied to compute the flow velocity and turbulent kinetic energy with respect to flow conditions by changing water depth. A proper computational grid size and time step for simulations are suggested. Three typical turbulent models, k − ε , k − ω , and k − ω S S T , were considered for simulating the flow around a scour hole. The performances of the three models were evaluated by comparing them with numerical and laboratory experimental results. Mean flow velocity profiles computed by the three turbulent schemes are generally in good agreement with laboratory measurements. However, k − ω has a limitation in simulating reversal flow in the scour hole, and the k − ε model does not predict turbulent kinetic energy well near the bottom. Thus, this study found that the most suitable turbulent model for simulating flow around a scour hole downstream of a fixed bed protection is the k − ω S S T model.

The influence of water depth on kinematic and spatiotemporal gait parameters during aquatic treadmill walking

The purpose of this study was to investigate kinematic and spatiotemporal variables of aquatic treadmill walking at three different water depths. A total of 15 healthy individuals completed three two-minute walking trials at three different water depths. The aquatic treadmill walking was conducted at waist-depth, chest-depth and neck-depth, while a customised 3-D underwater motion analysis system captured their walking. Each participant’s self-selected walking speed at the waist level was used as a reference speed, which was applied to the remaining two test conditions. A repeated measures ANOVA showed statistically significant differences among the three walking conditions in stride length, cadence, peak hip extension, hip range of motion (ROM), peak ankle plantar flexion and ankle ROM (All p values < 0.05). The participants walked with increased stride length and decreased cadence during neck level as compared to waist and chest level. They also showed increased ankle ROM and decreased hip ROM as the water depth rose from waist and chest to the neck level. However, our study found no significant difference between waist and chest level water in all variables. Hydrodynamics, such as buoyancy and drag force, in response to changes in water depths, can affect gait patterns during aquatic treadmill walking.

Spatiotemporal variation in wetland fish assemblages in the Western Ghats region of India

The Western Ghats region of peninsular India contains high diversity of freshwater fishes that support artisanal fisheries, but no studies to date have investigated fish assemblages of the region's extensive wetlands. This study examined fish population densities and the structure of local species assemblages in pools of the Periyakulam wetland of the Western Ghats. From 2000 to 2001, fishes and local abiotic environmental parameters were surveyed during three periods with contrasting rainfall (dry, major wet, minor wet). We hypothesized that fish density would be higher during the dry season when aquatic habitat is reduced, and that local assemblage structure would be strongly associated with habitat conditions. Total fish densities were higher during the dry season, but the magnitude of seasonal change was relatively low, and this apparently was because changes in water depth were minimal due to operation of sluice gates that control wetland hydrology. Chanda nama, Pseudetroplus maculatus, Rasbora daniconius, and Danio aequipinnatus were dominant species in most habitats during all three seasons. Multivariate ordinations revealed strong associations between assemblage structure and habitats based on vegetation cover. Local assemblages in shallow-vegetated habitats varied seasonally in association with gradients of rainfall and water quality parameters. Spatial variation of local fish assemblages in the Periyakulam wetland appears to be maintained by species-specific habitat selection. Although hydrologic regulation probably has reduced seasonal variation in the structure of local assemblages, seasonal differences still occur and appear to be caused by species differences with regard to periods of reproduction, recruitment, dispersal and habitat selection.

Study of dam break flow interaction with urban settlements over a sloping channel

This paper describes a dam break experiment on a sloped channel, carried out in a hydraulic flume at UCL for the purpose of computer model validations of extreme events, such as flash floods. An elevated reservoir was situated upstream followed by a 1/20 slope leading up to a flat floodplain. Plexiglas blocks were positioned on the floodplain constituting different urban settlements and creating different obstructions to the flow. The flume was instrumented along its length measuring the change in water depth in the reservoir; the water depth time histories in various locations; the flow patterns and flood front velocity; and lastly the pressure and load on the buildings. The experiments were repeated for different urban settlements, flood intensities (two different initial water depths in the reservoir) and roughness layers along the slope, representative of a vegetated and a non-vegetated hill. In the present study, the experimental results were described qualitatively and compared with theoretical processes and 2D numerical results obtained using OpenFOAM's RAS turbulent model. Water depth, velocity and load measurements were analysed for different cases and it was found that while the 2D model provided a good fit on the slope, the flows generated around the building were more complex 3D formations which lead to inaccuracies. All experiments were repeated multiple times to ensure repeatability and thus the procedure was validated successfully providing a complete dataset that can be used for the validation of computational models for extreme events.

Effects of water regime on the growth of the submerged macrophyte Ceratophyllum demersum at different densities

ABSTRACTTo successfully restore deteriorated lake ecosystems, it is vital to identify influencing environmental factors that impact submerged macrophytes. Planting density and water regime are important factors for submerged macrophytes’ growth. While many experimental studies have examined effects of water regime on the growth of some aquatic plant species, very few have tested both planting density and water regime on population, individual, and internode growth of a submerged population. We constructed Ceratophyllum demersum populations at two density levels (four and 16 shoot fragments per pot, responding to 96 and 384 plants m−2), subjected to two static water depths (30 and 150 cm) and to low, medium, and high water level fluctuation frequencies (24, 12, and 6 days per fluctuation cycle of water depth change between 30 and 150 cm). Initial density had no significant effect on individuals of C. demersum; however, it had a positive effect on population performance. Fluctuation frequency did not affect the growth of C. demersum, whereas increasing water depth significantly decreased both individual and internode biomass, and also increased shoot length regardless of comparison level. We therefore conclude that managing water depth and establishing populations with higher plant density may be helpful for the restoration of submerged macrophytes in degraded wetlands.

Nonlinear wave dynamics in self-consistent water channels

We study long-wave dynamics in a self-consistent water channel of variable cross-section, taking into account the effects of weak nonlinearity and dispersion. The self-consistency of the water channel is considered within the linear shallow water theory, which implies that the channel depth and width are interrelated, so the wave propagates in such a channel without inner reflection from the bottom even if the water depth changes significantly. In the case of small-amplitude weakly dispersive waves, the reflection from the bottom is also small, which allows the use of a unidirectional approximation. A modified equation for Riemann waves is derived for the nondispersive case. The wave-breaking criterion (gradient catastrophe) for self-consistent channels is defined. If both weak nonlinearity and dispersion are accounted for, the variable-coefficient Korteweg–de Vries (KdV) equation for waves in self-consistent channels is derived. Note that this is the first time that a KdV equation has been derived for waves in strongly inhomogeneous media. Soliton transformation in a channel with an abrupt change in depth is also studied.

Solid-phase speciation and post-depositional mobility of arsenic in lake sediments impacted by ore roasting at legacy gold mines in the Yellowknife area, Northwest Territories, Canada

Arsenic (As) concentrations in lake sediments in the Yellowknife region, Northwest Territories, Canada, are elevated from the weathering of mineralized bedrock and/or from the aerial deposition of arsenic trioxide (As2O3) generated via the roasting of arsenical gold ores at the now-defunct Giant Mine and other historical roasting operations in the area. Sediment cores were collected from shallow- and deep-water sites in two lakes located downwind of Giant Mine to determine the origin of As in sediments and understand how As concentrations and solid-phase speciation vary with sediment depth and spatially with changes in water depth and sediment texture. Deep-water cores were dated using 210Pb and 137Cs methods. Select sediment intervals were chosen for polished section preparation and analyzed using scanning electron microscopy (SEM)-based automated mineralogy, electron microprobe analysis (EMPA), and synchrotron-based microanalyses to characterize As-hosting solid phases. Sediment redox conditions and the post-depositional mobility of As at the shallow-water sites were assessed by sampling sediment porewaters using dialysis arrays (peepers). At the shallow-water sites, maximum sediment As concentrations occur at the sediment-water interface. In contrast, maximum As concentrations in the deep-water cores occur at sediment depths >15 cm. Dating using 210Pb and 137Cs methods indicates that the As-rich horizons at both deep-water sites are coincident with the period of maximum emissions from the Giant Mine roaster. Four predominant As-hosting solid phases were identified: anthropogenic As2O3 from stack emissions, and authigenic realgar, As-bearing Fe-oxyhydroxide, and As-bearing framboidal pyrite. The contribution of arsenopyrite to total sediment As concentrations was negligible, suggesting that elevated As concentrations are largely derived from ore roasting. In near-surface sediments at the shallow-water sites, the dominant host of As is Fe-oxyhydroxide. Congruent porewater profiles for As and Fe indicate that the post-depositional mobilization of As is governed by the reductive dissolution of As-bearing Fe-oxyhydroxide. Deep-water sediments are enriched in As2O3 that has persisted for more than 60 years. The presence of authigenic As-bearing sulphides suggests that the partial dissolution of As2O3 leads to the formation of less bioaccessible phases where reduced sulphur (S) is available. The distributions of As-hosting solid phases at the shallow- and deep-water sites are interpreted to be both directly and indirectly influenced by water depth. Fine-grained As2O3 particles are less likely to persist in shallow-water areas and typically accumulate in deep-water zones as a result of sediment-focusing processes. Fine-grained organic matter also preferentially accumulates in deeper areas, which influences redox gradients in porewater, the stability field for As-bearing Fe-oxyhydroxide, and the depth of authigenic As-bearing sulphide precipitation.

A new and simple method for measuring in situ field-saturated hydraulic conductivity using a falling-head single cylinder

Hydraulic properties of soil play important roles in water and temperature regimes. Measuring hydraulic properties has been studied for decades in the laboratory and in the fields. In 1989 the Guelph Permeameter was introduced to measure in situ field-saturated hydraulic conductivity, K fs, but it required an empirical constant. Until recently, no procedure had been introduced to in situ measure K fs without an empirical constant. In this article, we proposed a new simple method to measure K fs. Field and laboratory measurements for volcanic ash origin Kanto loam, loess, and Toyoura sand were taken using a metallic cylinder (30 cm long and 4.5 cm inner diameter) or a PVC cylinder (30 cm long and 5.0 cm inner diameter) installed into soil down to a 5 cm depth. Temporal changes in water depth or hydraulic head inside the cylinder were measured with a laser measure. Values of K fs measured with this proposed method agreed well with saturated hydraulic conductivity measured in the laboratory for undisturbed soil cores. New analytical solution was derived for a future automated device for this purpose.

Spatiotemporal heterogeneity of the palaeoecological record in a large temperate palaeolake, Šúr, southwest Slovakia: Comparison of pollen, macrofossil and geochemical data

Aquatic macrophytes living within shallow lakes are particularly sensitive to climate change and fluctuations in water depth. In the Dabubian Lowland of southwestern Slovakia, sediments from the large, shallow palaeolake, named Šúr, provide a unique opportunity to reconstruct changes in water depth to analyse succession within local aquatic and terrestrial vegetation. Here, we used a multi-core, multi-proxy approach (macrofossils, pollen, and geochemistry) in order to reconstruct water depth changes and its influence on aquatic and terrestrial vegetation succession during the Holocene. Additionally, we also consider how heterogeneity in proxy data varied across time and space. Using particular macrophyte taxa found in the macrofossil assemblage, which have specific water depth requirements, our results demonstrate changes in water depth created a long-term mosaic of vegetation succession. In the Early Holocene, aquatic macrophytes suggest the water column was high (>100cm). During the Middle Holocene, water depths decreased creating mosaics of aquatic succession. By ca 4200calyrB.P., terrestrialization had occurred in the former lake area. Regional vegetation succession was likely more influenced by the presence of fluvial sands and/or by low precipitation and high evaporation rates, which delayed the expansion of temperate trees and favoured Pinus until 8850calyrB.P. Macrofossils exhibit the lowest temporal variation among all proxies, which suggests a rather stable macrophyte community when water depth was high. Pollen data show very low spatial variation given by the size of the accumulation basin, which reflects regional pollen rain. Higher spatial variation was observed in the geochemistry and macrofossil data compared to the pollen data, which could be the result of different mosaics of aquatic plants, local dispersion of propagules, or changes in sediment types. We conclude that changes in water depth were not homogenous, but rather heterogeneous in space and time.

Palaeoecologic Significance of the Callovian-Oxfordian Trace Fossils of Gangeshwar Dome, Southeast of Bhuj, Mainland Kachchh, India

The shallow marine deposits of the Late-Middle Jurassic (Callovian–Oxfordian) Jumara Formation of the Gangeshwar Dome of Mainland Kachchh, India, comprise a succession of ~247 m thick clastic sediments with few non-clastic bands and contain a diverse group of ichnofauna. The succession is subdivided into seven lithofacies, viz., laminated shale-siltstone facies (LSS), sheet sandstone facies (SS), herringbone sandstone facies (HS), bivalve sandstone facies (BS), bioclastic limestone facies (BL), intraformational conglomerate facies (IC) and oolitic limestone facies (OL). The ichnofaunal study shows 29 ichnospecies of 23 ichnogenera including Arenicolites, Bifungites, Bolonia, Chondrites, Didymaulichnus, Diplocraterion, Gyrochorte, Helminthopsis, Isopodichnus, Laevicyclus, Lockeia, Monocraterion, Taenidium, Ophiomorpha, Palaeophycus, Planolites, Phycodes, Protopalaeodictyon, Rhizocorallium, Skolithos, Thalassinoides, Tisoa, and Zoophycos. These trace fossils are distributed among nine ichnocoenose, characterized by Chondrites, Diplocraterion, Gyrochorte, Ophiomorpha, Rhizocorallium, Skolithos, Taenidium, Thalassinoides and Zoophycos. Their occurrence in the facies corresponds to their trophic and ethological properties. The colonisation of the opportunistic Diplocraterion and the Skolithos ichnocoenose shows a high density and marks foreshore/nearshore environmental conditions. The Gyrochorte, the Rhizocorallium, the Taenidium and the Thalassinoides ichnocoenose indicate the typically lower energy zone of the shoreface-offshore region. The Chondrites ichnocoenosis indicates fluctuation in bottom water oxygen while the Zoophycos ichnocoenosis typically exploited a calm water niche in the offshore region. These ichnocoenose recur throughout the sequence and belong to the Skolithos and the Cruziana ichnofacies which marked changes in energy gradient, substrate stability, water depth and mode of life of invertebrate organisms. The study of trace fossil assemblages with sediment characteristics gives a detailed and accurate picture of foreshore to offshore palaeoenvironmental conditions.

A window into the Cambrian basement and early Carboniferous sedimentation of the Hathern Shelf: the British Geological Survey borehole at Ticknall, South Derbyshire, UK

The Ticknall Borehole was drilled in 1995 to a depth of 209 m in order to investigate the succession on the Hathern Shelf, a fault-bounded structural province that lies on the southern margin of the Widmerpool Half-graben, part of a major early Carboniferous rift system. The borehole demonstrated a ‘basement’ of Upper Cambrian cleaved mudrocks, correlated with the Stockingford Shale Group. Unconformably overlying this is a basal Carboniferous unit, the Calke Abbey Sandstone Formation, of probable Visean (?Holkerian) age, comprising 82.34 m of fluvial sandstones and conglomerates, with interbedded red-grey palaeosols exhibiting highly distinctive ‘pseudogley’ fabrics indicative of emergent episodes. The unit may have been deposited in a localized, possibly fault-controlled basin and was in part sourced from the Precambrian volcanic terrain of Charnwood Forest. As rifting and subsidence proceeded, the encroachment of nearshore/peritidal environments is indicated at the top of the formation by interbedded calcilutites that have yielded marine faunas. Fully marine conditions were established during deposition of the overlying Peak Limestone Group, comprising the early Asbian Cloud Hill Dolostone Formation succeeded by the Ticknall Limestone Formation, of Brigantian age. Comparisons between the Ticknall Borehole and Peak Limestone strata exposed in quarries farther east, around Breedon, show major changes in water depths over a distance of only 4.5 km. Such variations can be reconciled with seismostratigraphical studies in the adjacent Widmerpool Half-graben, which show that sedimentation on the Hathern Shelf was in part controlled by movements along nearby rift-bounding faults.

Modelling of a flap‐type wave energy converter farm in a mild‐slope equation model for a wake effect assessment

It is expected that large farms of wave energy converters (WECs) will be installed and as part of the consenting process it will be necessary to quantify their impact on the local environment. The objective of this study is to assess the impact a WEC farm has on the incoming wave field through the use of a novel methodology. This methodology assesses the changes of the significant wave height surrounding a flap-type WEC farm with a special focus on the lee of the farm. A time-dependent mild-slope equation model is employed to solve the propagation of surface waves and their interaction with the devices. The model represents the devices as obstacle cells with attributed absorption coefficients tuned against near-fields obtained from a boundary element method (BEM) solver. The wake effect of the farm is determined by using a step-by-step approach starting first with an assessment of one device and progressively incrementing to a larger number of flaps. The effect of incident sea states, device separations and water depth changes on the wake effect of the farm is also investigated. This work shows the potential of a WEC farm to reduce significant wave heights on the leeside.

Monitoring water depth, surface area and volume changes in Lake Victoria: integrating the bathymetry map and remote sensing data during 1993–2016

In this study, a framework to monitor the volumetric fluctuation of the inland water body by the combination of a bathymetry map, an optical satellite imagery & multiple satellite altimetry measurements is presented. In spite of the recent studies in monitoring water level changes in lakes using satellite altimetry & optical satellite imagery, it’s still evident that these methods are limited to the water level, surface area and volume changes. However, to effectively study the lakes, it’s important to quantify the total lake volume. This hasn’t been possible as the existing satellite methods cannot estimate the bathymetry depth. The methodology was developed over Lake Victoria during 1993–2016. The results indicate that the water level, area, and volume of Lake Victoria decreased over the past 23 years. The water level shows a slight decrease (−0.005 m/year) of a total of −0.115 m from 1993 to 2016. The changes in water level translates to a reduction in lake area (−100 km2) and volume (−5 km3). Despite the inconsistent changes in area and volume, significant reduction occurred between 1998 and 2006 where (3484 km2) and (122.87 km3) reduction in area and volumes respectively were observed.

Water table drawdown shapes the depth-dependent variations in prokaryotic diversity and structure in Zoige peatlands.

Microbial communities are important to ecosystem function and sensitive to hydrological dynamics. However, we lack predictable knowledge about how soil microorganisms respond to water table drawdown in different depths. This research used a high-throughput sequencing method to determine the responses of prokaryotic communities to the changes of water table and depth on Zoige peatlands. Our results showed that water table drawdown reduced alpha diversity indices (observed species, Shannon diversity and Chao1 richness) of prokaryotic communities. Intriguingly, the reduction of diversity varied in different depths, and was statistically significant in intermediate layers (20-30 cm and 50-60 cm), but not in the surface (0-10 cm) or deep layer (90-100 cm). In deeper layers there was greater relative abundance of most anaerobic microorganisms (e.g. Chloroflexi, Planctomyctes and NC10), but lesser amounts of most aerobes (e.g. Proteobacteria and Acidobacteria). However, the vertical distribution of prokaryotic microbiota along the depth gradient was altered by water table drawdown, mainly by enriching oligotrophs (e.g. Acidobcteria) over copiotrophs (e.g. Bacteriodetes). In addition, we found that the most important soil parameters influencing community structure were soil pH, total organic carbon and total nitrogen. Our study illuminates that the variations of prokaryotic communities caused by water table drawdown are depth-dependent, and that water table drawdown leads to predictive changes of microbiota in peatlands.

Trilobite extinctions, facies changes and the ROECE carbon isotope excursion at the Cambrian Series 2–3 boundary, Great Basin, western USA

The mass extinction of the olenellid trilobites occurred around the Cambrian Series 2–Series 3 boundary. Like many other crises, it coincided with a negative carbon isotope excursion but the associated palaeoenvironmental changes remain unclear. To investigate the causal mechanism for this event, we report facies changes, pyrite framboid petrography and carbon isotope values from Cambrian Series 2–Series 3 (traditionally Early–Middle Cambrian) boundary strata of the Carrara Formation (Death Valley region, California) and Pioche Formation (Nevada). These data reveal regionally changing water depths from high-energy, nearshore facies (oolitic grainstone) to more offshore silty marl and finer-grained carbonate mudstone. In the Carrara Formation, the series boundary occurs within a deepening succession, transitioning from high-energy, nearshore facies (oolitic grainstone and oncolitic packstone) to offshore marl, the latter of which contains pyrite framboid populations indicative of low-oxygen (dysoxic) depositional conditions. Intermittent dysoxia persisted below sub-wave base settings throughout the early and middle Cambrian, but did not intensify at the time of extinction, arguing against anoxia as a primary cause in the olenellid trilobite extinction. Within both field areas, the extinction interval coincided with a minimum in δ13Ccarb values, which we interpret as the regional manifestation of the Redlichiid-Olenellid Extinction Carbon isotope Excursion (ROECE). The Series 2–Series 3 boundary is reported to closely coincide with a large-amplitude sea-level fall that produced the Sauk I/II sequence boundary, but the placement of the Series 2–Series 3 boundary within a transgressive interval of the Carrara Formation shows that this is not the case.The main sequence boundary in the succession occurs much lower in the succession (at the top of the Zabriskie Quartzite) and therefore precedes the extinction of the olenellids and ROECE.

Modern diatoms from a temperate river in South America: the Colorado River (North Patagonia, Argentina)

Diatom assemblage composition and distribution from surface sediment samples of the Colorado River, Patagonia were analysed in relation to environmental variables using multivariate techniques. The aim of this study was to determine the ecological preferences of the taxa and provide analogues for paleoenvironmental reconstructions in estuarine and fluvial environments of southern South America. Cluster analysis identified two main diatom assemblages, one grouping sites with a marine influence, and another group influenced by typical riverine conditions. Canonical correspondence analysis indicated that major ion concentrations contribute significantly to explain the variation in the composition of diatom assemblages. A total of 208 taxa belonging to 56 genera were recorded. In estuarine sites, the assemblages were characterized by marine-brackish tychoplankton, such as Cymatosira belgica, Rhaphoneis amphiceros, Delphineis minutissima, Paralia sulcata and Paralia sulcata var. coronota. Diatom communities in riverine sites were dominated by freshwater taxa such as Staurosira venter, Pseudostaurosira brevistriata, Punctastriata glubokoensis and Punctastriata lancettula. Two Punctastriata species that were difficult to distinguish with LM were examined in detail with SEM, and the value of detailed LM and SEM analysis for distinguishing Punctastriata spp. and other small fragilarioids is discussed. Based on their autoecological affinities, it is likely that the dominance of small fragilarioids is related to the increase in ionic concentration in the basin, as a consequence of both precipitation decrease, and an increase in urban, industrial and agricultural activities. The construction of a flood-control structure in the northern branch of the delta modified the diatom community so that marine taxa were replaced by brackish-freshwater taxa in response to changes in salinity, substrata and water depth.

Entrainment of juvenile horseshoe crab activity to artificial tides

Juvenile American horseshoe crabs, Limulus polyphemus, express both daily and tidal rhythms. To determine if, and how, tidal cues influence the expression of these rhythms, we exposed 25 animals to artificial tides, and 17 to artificial tides with inundation, both with a 12:12 LD cycle. In the first experiment, 24% expressed daily rhythms of activity, 24% tidal rhythms, 12% a combination of the two, and the rest were arrhythmic. Under subsequent atidal conditions some expressed daily rhythms, but more were circatidal. In the second experiment, 6% expressed daily rhythms, 71% tidal, 12% a combination, and 12% were arrhythmic. Those expressing tidal rhythms were more active during flood/high tide, while daily animals tended to be nocturnal. Under subsequent constant conditions, the majority exhibited circatidal activity, with some expressing one activity bout per day. We conclude that juvenile horseshoe crabs entrain to artificial tides, with inundation cycles providing stronger cues than water depth changes.