Biotic Assemblages Research Articles

Effects of riparian forest buffers and agricultural land use on macroinvertebrate and fish community structure

Although forested riparian buffers can improve stream habitat, there remains debate as to whether biotic integrity can be maintained by buffers in highly degraded watersheds. To determine if forest buffers can ameliorate cropland impacts, we examined macroinvertebrate and fish assemblages in headwater streams categorized into three land use groups (High Buffer-Low Agriculture, High Buffer-High Agriculture, Low Buffer-High Agriculture). Sampling was conducted seasonally over 3 years to explore temporal patterns in biotic assemblages related to land use, instream habitat, and water quality parameters. Macroinvertebrate and fish abundance were higher in low buffer streams due to large numbers of pollution tolerant taxa and herbivores that proliferated with increased periphyton. In contrast, High Buffer-Low Agriculture streams had the best biotic integrity likely due to increased depth and better habitat quality. Non-metric multidimensional scaling ordinations of macroinvertebrate taxa showed no separations, but fish communities were significantly different among all three land use groups. Findings suggest fish community structure is more sensitive than macroinvertebrates to agricultural influences at both the riparian and watershed scales. Our study provides information on the effects of riparian vegetation on agricultural stream communities and highlights the importance of managing riparian forests to conserve stream biotic integrity.

Integrated marine monitoring for drilling discharges under mesotidal forcing

There is a range of hydrocarbon exploration and extraction industrial project activities taking place on the North West Shelf of the Australian Continent. The shelf is a diverse and important marine ecosystem, with a number of globally significant shallow coral reefs and marine protected areas, which are vital for survival of threatened and protected species. Some of the mentioned exploration and extraction activities, such as offshore drilling, may be permitted within few kilometres away from sensitive habitats extant near a project resource extraction area. The habitats may in some cases be presented by submerged shoal ecosystems of high biodiversity and thus high environmental/ecological value, due to the presence of either or both benthic biota and fish assemblages. As part of an environmental assessment program for the industrial activities, some of the offshore exploration operators examine the effects of drilling discharges on the marine ambience. This study describes such an assessment. For the assessment, a novel integrated marine monitoring approach was conceptually designed; the program was then executed in the field using bottom-mounted equipment and a remotely operated underwater vehicle with sensors mounted on it. This study focuses on assessing the wind conditions in the project area, sea level and current velocities, in situ water temperature, salinity, turbidity and acoustic echo intensity, over several stages of the drilling operations while the drilling cuttings and muds were discharged into the water column. The collected during the austral autumn months of April and May winds showed prevailing east-southeaserlies, which is aligned well with the general understanding of wind climatology of the area. Importantly, stronger than usual winds may have a defining impact on current speeds and directions, with the current speeds enhanced by such winds. On these occasions the water transporting a discharge material may pushed towards the sensitive habitat present in the area. Normally though the hydrodynamics in the project area are dominated by semi-diurnal tides. Both vertical current velocity and water temperature distributions indicated the presence of three layers within the water column over the project site. The vertical distribution of water salinity was rather homogenous throughout the entire water column. The monitored by an optical instrument turbidity and acoustic echo intensity from an ADCP (acoustic Doppler current profiler) showed that tracing discharge plumes with a remotely operated underwater vehicle is a viable technique, which could be implemented at other locations. Such independent parameters though need a thorough cross-calibration and inter-validation for the results to be fully understood.

Small dam impairs invertebrate and microbial assemblages as well as leaf breakdown: a study case from a tropical savanna stream

Globally, dams severely affect the hydrology of lotic ecosystems, impacting biotic assemblages and ecological processes. Although their consequences are not fully explored, especially in the tropics, hundreds of small dams have been constructed worldwide in the last decade for hydropower development and water supply. Here, we assessed the effects of a small dam (ca. 0.14 km2) on water quality, leaf breakdown and the associated assemblages (microbes and invertebrates) in three reaches of a tropical savanna stream (upstream and downstream and in the reservoir). Water quality measured as pH, dissolved oxygen, and electrical conductivity were similar in all reaches, but turbidity and temperature were higher and water velocity lower in the downstream reach and in the reservoir. The upstream reach had higher leaf breakdown (as leaf mass loss, 72, 41 and 53%), microbial biomass (98, 74 and 73 nmol ATP g-1 AFDM), fungal biomass (347, 240 and 260 μg ergosterol g-1 AFDM) and invertebrate richness (4, 2 and 3 taxa sample-1) than the reservoir and downstream reach. These results suggest that damming a stream can reduce water quality, impoverish invertebrate assemblages, and reduce microbial biomass and carbon processing.

Facies architecture and depositional evolution of Palaeocene–Eocene, Subathu Formation, Garhwal Himalaya, Uttarakhand, India

The Late Palaeocene–Middle Eocene marine sedimentary sequence of the Himalayan foreland basin, represented by the rocks of Subathu Formation, is enriched with the presence of foraminifera, bryozoa, corals, gastropods, bivalves and so on within both carbonate and clastic dominant lithologies. The carbonate and the shale consist mainly of larger benthic foraminifera (LBF) and the assemblage is composed of Nummulites and Assilina along with the other skeletal and non-skeletal components. The presence of a palaeosol at the base of Subathu indicates a gap in sedimentation. The lithological and biotic assemblages along with the bed form, bed geometry and primary sedimentary features helped us to establish four facies associations, A, B, C and D, and have been corroborated with the shoreline migration (transgression/regression) history. The facies association-A, representing the basal horizons of Subathu Formation, indicates the onset of transgression and deposition in lagoonal condition with carbonaceous shale and oolitic ironstone, followed by the facies association-B and -C deposited in shallow marine shoreface. The uppermost unit, i.e., the medium- to coarse-grained sandstone (facies association-D) of Subathu Formation, represents a fall in relative sea level (progradational stacking pattern), whereas the underlying contact between the facies association-B and -C is represented by an aggradational stacking pattern between the siltstone and the shale, but certainly without exposing the shelf. Petrographic studies based on characteristic features such as framework constituent, the percentage of matrix and grains, nature of cementing material, textural features and fossil content help to deduce a distinct change in depositional setting from an open marine (shelf) to shoal, a lagoon that gradually grades to foreshore/beach environment. The study reveals that the basin has gone through a transgressive (facies association-A and -B), regressive (facies association-C) as well as distinct forced regressive (facies association-D) phase of shoreline migration history.

Spatial distribution patterns of benthic macroinvertebrate functional feeding groups by stream size and gradient in Republic of Korea

Benthic macroinvertebrate functional feeding groups (FFGs) are influenced by the availability of food sources, which can be affected by stream size and gradient. We investigated the spatial distribution patterns of FFGs by stream size (stream order, catchment area and stream width) and gradient (altitude and slope). In addition, we attempted stream classification based on the distribution patterns of FFGs. To achieve these goals, a total of 996 sampling sites of national scale located in the Republic of Korea were investigated from 2008 to 2016. Among the five physical variables, stream width and slope showed linear relationships with the indicator values of FFGs. These two physical variables had larger influences on the distribution of FFGs compared to other variables. Geology (the type of rocks) showed little influences on the distribution of benthic macroinvertebrate communities. We classified streams focusing on catchment (stream size) area and altitude (stream gradient) because they are constants that do not change with disturbances. The FFGs and benthic macroinvertebrate assemblages were distinctly changed by stream types, indicating that the stream classification criterion adequately represented changes in physical variables in biotic terms. In addition, the distribution of FFGs and biotic assemblages showed that benthic macroinvertebrates in the Republic of Korea were more influenced by the altitude (stream gradient) than the catchment area (stream size). In overall, this study found that the stream size and gradient markedly influence the distribution of FFGs in national regions of the Republic of Korea. Further research is needed focusing on the level of microhabitats and other parameters to identify the promising variables that influence biotic distribution.

Aquatic health indicators: holistic evaluation tools for sustainable management of a tropical oxbow lake ecosystem

Quantitative relationship of primary productivity and fish productivity and estimating aquatic health indices in tropical oxbow lake ecosystems remains to be addressed as predictors of fisheries yield in sustainable management. The aquatic health indicators and aquatic health status assessed with biological properties of phytoplankton, zooplankton, macroinvertebrates, macrophytes and fish in a semi-closed tropical oxbow lake ecosystem in the eastern India were studied along with socioecological and socio-economical aspects. Five ecological classes namely phytoplankton, zooplankton, macroinvertebrate, macrophyte and fish have been championed as effective biological indicators supported by physicochemical attributes. The established aquatic health indices registered the health status as moderate to poor. Organic loading and consequent nutrient enrichment and water quality deterioration during monsoon led to more homogeneous benthic biotic assemblage. With observed diverse nature of each trophic level of the ecosystem, there occurred a downstream ‘spillover’ of diversity along the food chain. The ‘structure affecting structure’ argument goes in tune with the theory of consumers controlling species diversity. Total 38 fish productivity regression equation models explained clearly that fish abundance and productivity were declining due to high anthropogenic activities of jute retting and indiscriminate uses of fishing gears of various mesh sizes during monsoon and thereafter, which need to be regulated.

Effects of urban multi-stressors on three stream biotic assemblages

During 2014, the U.S. Geological Survey (USGS) National Water-Quality Assessment (NAWQA) project assessed stream quality in 75 streams across an urban disturbance gradient within the Piedmont ecoregion of southeastern United States. Our objectives were to identify primary instream stressors affecting algal, macroinvertebrate and fish assemblages in wadeable streams. Biotic communities were surveyed once at each site, and various instream stressors were measured during a 4-week index period preceding the ecological sampling. The measured stressors included nutrients; contaminants in water, passive samplers, and sediment; instream habitat; and flow variability. All nine boosted regression tree models – three for each of algae, invertebrates, and fish – had cross-validation R2 (CV R2) values of 0.41 or above, and an invertebrate model had the highest CV R2 of 0.65. At least one contaminant metric was important in every model, and minimum daytime dissolved oxygen (DO), nutrients, and flow alteration were important explanatory variables in many of the models. Physical habitat metrics such as sediment substrate were only moderately important. Flow alteration metrics were useful factors in eight of the nine models. Total phosphorus, acetanilide herbicides and flow (time since last peak) were important in all three algal models, whereas insecticide metrics (especially those representing fipronil and imidacloprid) were dominant in the invertebrate models. DO values below approximately 7 mg/L corresponded to a strong decrease in sensitive taxa or an increase in tolerant taxa. DO also showed strong interactions with other variables, particularly contaminants and sediment, where the combined effect of low DO and elevated contaminants increased the impact on the biota more than each variable individually. Contaminants and flow alteration were strongly correlated to urbanization, indicating the importance of urbanization to ecological stream condition in the region.

High frequency monitoring reveals fine scale spatial and temporal dynamics of the deep chlorophyll maximum of a stratified coastal lagoon

Coastal lagoons are a dynamic habitat, with varying marine and freshwater inputs determining the presence and extent of stratification, and the physical and chemical environment of the epi- and hypolimnion. As a result, the biotic assemblages that thrive in such environments are a diverse mix of species, with wide ranges of tolerances. While annual succession and vertical distribution of phytoplankton assemblages in marine and freshwater ecosystems are well documented, few data are available which describe the spatial and temporal variability of phytoplankton in coastal lagoons, even though these are a protected habitat under the European Union's Habitat Directive. In this study, high frequency monitoring (HFM) of chlorophyll fluorescence (ChlF) using a vertical profiling sonde was used to describe the variation in algal biomass over six annual cycles and through the water column of Lough Furnace, a perennially stratified coastal lagoon on the west coast of Ireland. Spot sampling, and species enumeration of phytoplankton were used to confirm the patterns observed in the fluorescence data, which indicated a general pattern of increasing biomass starting in spring, and a deepening of the deep chlorophyll maximum (DCM) over the summer. Typical of a stratified system, the DCM was often situated below the surface mixed layer, in saline hypoxic water. The best predictors of daily maximum ChlF were the depth of the surface mixed layer and daily solar radiation. We conclude that HFM allows an unprecedented glimpse into the substantial variability and complexity of phytoplankton distributions, a key biotic variable.

Concordance in biological condition and biodiversity between diatom and macroinvertebrate assemblages in Chinese arid-zone streams

Understanding the concordance between aquatic assemblages in ecological assessments and their responses to human-induced disturbances are fundamental steps toward achieving sustainable stream and catchment management. Using diatom, macroinvertebrate, and environmental data collected from northwest Chinese arid-land streams, we tested the concordance between the two assemblages in (1) the assessment results using multimetric indices (MMIs), (2) the stressors affecting the MMIs and beta-diversity, and (3) the response trajectories of MMI and beta-diversity to disturbances. Random Forest analyses revealed that diatom and macroinvertebrate metrics responded most sensitively to climatic and geomorphic variables, respectively. Diatom MMI scores had greater precision and responsiveness than macroinvertebrate MMI scores. Diatom MMI scores were negatively related to gradients in observed–expected conductivity, chemical oxygen demand, and vegetated riparian zone width. Macroinvertebrate MMI scores responded strongly to observed–expected mean substrate composition, conductivity, and phosphate. Diatom beta-diversity decreased, as nitrate, channel alternation, and phosphate increased beyond expected natural background levels. Macroinvertebrate beta-diversity was the lowest when both nitrite and % cobble neared their natural background expectations. Our results indicate that protecting aquatic systems from anthropogenic pressures depends not only on revealing causes of impairment, but also on anticipating and understanding the responses to various stressors of multiple stream biotic assemblages.

Limited congruence exhibited across microbial, meiofaunal and macrofaunal benthic assemblages in a heterogeneous coastal environment

One of the most common approaches for investigating the ecology of spatially complex environments is to examine a single biotic assemblage present, such as macroinvertebrates. Underlying this approach are assumptions that sampled and unsampled taxa respond similarly to environmental gradients and exhibit congruence across different sites. These assumptions were tested for five benthic groups of various sizes (archaea, bacteria, microbial eukaryotes/protists, meiofauna and macrofauna) in Plymouth Sound, a harbour with many different pollution sources. Sediments varied in granulometry, hydrocarbon and trace metal concentrations. Following variable reduction, canonical correspondence analysis did not identify any associations between sediment characteristics and assemblage composition of archaea or macrofauna. In contrast, variation in bacteria was associated with granulometry, trace metal variations and bioturbation (e.g. community bioturbation potential). Protists varied with granulometry, hydrocarbon and trace metal predictors. Meiofaunal variation was associated with hydrocarbon and bioturbation predictors. Taxon turnover between sites varied with only three out of 10 group pairs showing congruence (meiofauna-protists, meiofauna-macrofauna and protists-macrofauna). While our results support using eukaryotic taxa as proxies for others, the lack of congruence suggests caution should be applied to inferring wider indicator or functional interpretations from studies of a single biotic assemblage.

Allopatric diversification and evolutionary melting pot in a North African Palearctic relict: The biogeographic history of Salamandra algira

North Africa is a climatically and topographically complex region with unique biotic assemblages resulting from the combination of multiple biogeographic realms. Here, we assess the role of climate in promoting intra-specific diversification in a Palearctic relict, the North African fire salamander, Salamandra algira, using a combination of phylogenetic and population genetic analyses, paleoclimatic modelling and niche overlap tests. We used mitochondrial DNA (Cyt-b), 9838 ddRADseq loci, and 14 microsatellite loci to characterize patterns of genetic diversity and population structure. Phylogenetic analyses recover two major clades, each including several lineages with mito-nuclear discordances suggesting introgressive patterns between lineages in the Middle Atlas, associated with a melting pot of genetic diversity. Paleoclimatic modelling identified putative climatic refugia, largely matching areas of high genetic diversity, and supports the role of aridity in promoting allopatric diversification associated with ecological niche conservatism. Overall, our results highlight the role of climatic microrefugia as drivers of populations’ persistence and diversification in the face of climatic oscillations in North Africa, and stress the importance of accounting for different genomic regions when reconstructing biogeographic processes from molecular markers.

Taxa, turnover and taphofacies: a preliminary analysis of facies-assemblage relationships in the Ediacara Member (Flinders Ranges, South Australia)

The Ediacara Member of the Flinders Ranges (South Australia) preserves body and trace fossils of the Ediacara biota. Fossils span five lithofacies representative of a range of shallow-marine environments and are preserved as in situ and transported material. Previous work has demonstrated a relationship between paleoenvironment and taxa at the Nilpena fossil site. We expand the analysis to include facies-taxa data from a further nine localities across the Flinders Ranges to assess if the taxa–paleoenvironment relationship is site specific or valid at a regional scale. The new analysis demonstrates that the distribution of taxa within the lithofacies, as a proxy for paleoenvironment, is non-random. This preliminary analysis presents a beta diversity-like spatial turnover across the range of shallow marine Ediacaran environments, and demonstrates taxonomic assemblages are specific to given paleoenvironmental zones. These assemblages are consistent over a broad spatial extent and also a presumed temporal distribution. This specificity indicates that a marked sensitivity to environmental parameters was present in these communities, as demonstrated by the non-random distribution of taxa and spatial turnover of biotic assemblage throughout the gradational environments of the Ediacara Member. This study highlights the variability and heterogeneity that is a characteristic of shallow marine settings, and offers a novel approach to the future investigation of the relationship between Ediacaran environments and taxa assemblages.

Higher-taxon and functional group responses of ant and bird assemblages to livestock grazing: A test of an explicit surrogate concept

Biodiversity monitoring programs are routinely established to quantify changes in biotic communities in response to land management. Surrogacy is implicitly used in many such monitoring programs whereby the measurement of a component of biodiversity is used to infer responses of broader biodiversity. Yet rarely is this surrogacy validated by demonstrating that measured variables and the target variable of interest have matching responses to management treatments. Here we examined the responses of higher-taxon and functional groupings of ants and birds (our surrogate variables) two years after the implementation of experimental livestock grazing treatments, and compared these with the responses of total ant and bird species richness (our target variables) to the same treatments. We found significant and strong correlations between surrogate and target variables, but this did not predict corresponding similar response to treatments. For ants, we found that the genus Monomorium had a negative response to the grazing exclusion treatment, but there was no matching response of species richness, and so no surrogacy was identified. For birds, total species richness had a weak positive response to spring/summer grazing exclusion, and the abundance of honeyeaters (Meliphagidae) showed a similar positive response, suggesting surrogacy. Our study highlights that correlations among variables do not necessarily lead to surrogacy, and indeed that different sub-components of biotic assemblages can respond in ways that contrast with overall species richness. Careful assessment of the matched responses of surrogate and target variables to management can provide a simple and robust way to critically assess biodiversity surrogacy.

Ecosystem Responses to Climate-Related Changes in a Mediterranean Alpine Environment Over the Last ~ 180 Years

The effect of recent climatic warming is significant in the Mediterranean region, especially in high-mountain areas. This study uses multiple sedimentary proxies from Rio Seco Lake, a remote alpine lake in the Sierra Nevada, southeastern Spain, to reconstruct recent environmental and ecological changes in the lake and catchment. Two main climatic periods can be distinguished during the past 180 years: Period One (1820 to ~ 1920s) characterized by colder and wetter conditions than the more recent Period Two (~ 1920s to the present), characterized by warmer and drier conditions. Independent proxies such as subfossil chironomid assemblages, n-alkane indices, pollen data and/or spectrally inferred chlorophyll-a concentrations indicate a longer ice-cover period, colder water temperature and more pronounced accumulation of snow in the catchment during Period One than in Period Two, likely producing water stress for catchment plant growth because of the low rate of ice melting in Period One. As temperature increases and precipitation decreases from the 1920s onwards, a wider development of wetland plants is observed, which is associated with the longer warm season that contributed to snow and ice melting in the catchment. This continuing temperature rise and precipitation decrease over the past 60-years by ~ 0.24°C per decade and –0.92 mm/y, respectively, lead to an important increase in chlorophyll-a and changes in lake biotic assemblages. Major chironomid community structure changes to warmer water taxa were recorded, resulting in a 2°C increase in mean July air temperature inferred by chironomids from ~ 1950 onwards. An inferred increase in primary production for the past few decades is consistent with higher temperatures, while wider development of wetland plants is associated with longer warm season. The coherence between independent environmental proxies, each associated with distinct mechanistic linkages to climatic shifts, strengthens our interpretations of a recent warming trend and an intensification of summer drought in this high-mountain area leading to distinct changes in the lake and its catchment. The impact of this climate change on the summits of Sierra Nevada and its influence transcends its geographical limits because these systems provide ecosystem services to a vast area.

Impact of artificial coastal protection structures on Ascidians settlement along the Tamil Nadu coast, India

Summary Ascidians are one of the dominant marine sedentary filter feeders recorded more frequently as introduced species than other taxa. It is renowned that artificial structures offer novel niches to the non-native species. A yearlong investigation was carried out to understand the role of ascidian colonization on various artificial structures located along 84 stations stretched on the 1076 km long Tamil Nadu coast of South India. It revealed the occurrence of 26 ascidian species, among these18 specimens were identified to species level, 8 were identified to genus level based on morphological characters. As on origin and nativity, out of the total 26, 3 species were classified as introduced, 8 species were classified as native and 15 as cryptogenic species. Interestingly, Polyclinum isipingense and Diplosoma variostigmatum were reported first time in Indian waters. The cryptogenic and colonial forms of ascidians are dominant in the artificial structures. There were significant differences observed between artificial structure type, geographic locations (p = 0.0071) and between ascidians forms as well as geographic areas (p = 0.00375). This study also confirms the artificial structures offer new niches for non-native ascidian colonization. The influence of the substrate (structure type) as well as geographic locations on the biotic assemblage was also observed.

Environmental correlates of food-chain length, mean trophic level and trophic level variance in invaded riverine fish assemblages

Examining how the trophic structure of biotic assemblages is affected by human impacts, such as habitat degradation and the introduction of alien species, is important for understanding the consequences of such impacts on ecosystem functioning. We used general linear mixed models and hierarchical partitioning analyses of variance to examine for the first time the applicability of three hypotheses (ecosystem-size, productivity and disturbance) for explaining food-chain length (FCL) in invaded fish assemblages. We used Fishbase trophic level (TL) estimates for 16 native and 18 alien fish species in an extensive riverine system in north-eastern Spain (99,700 km2, 15 catchments, 530 sites). The FCL of assemblages ranged from 2.7 to 4.42. Ecosystem size-related variables (Strahler stream order, physical habitat diversity) and human-disturbance (conductivity) made the largest contribution to the explained variance in the FCL model after accounting for spatial confounding factors and collinearity among predictors. Within-assemblage TL also was positively associated with Strahler stream order, suggesting that large rivers have the highest trophic diversity. High conductivity was negatively associated with FCL, as did with the mean TL of fish assemblages. However, an inverse association was found between mean TL and Strahler stream order, possibly because the presence of fish species of high TL may be offset by larger numbers of alien species of lower TL in large rivers. Given that there may be trophic replacements among native and alien species, this inference needs to be addressed with detailed trophic studies. However, reducing water conductivity by improved wastewater treatment and better agricultural practices probably would help to conserve the fish species on the apices of aquatic food-webs.

Patterns of long-term dynamics of aquatic communities and water quality parameters in the Great Lakes: Are they synchronized?

Long time series data can provide insights into dynamics of large lakes. We use the USEPA-GLNPO biological monitoring dataset (phytoplankton, benthos, zooplankton and water quality), collected from 1996 through 2016, to identify whether there is evidence of concordant linear or non-linear trends in community composition, density/biomass/biovolume and major environmental parameters. We show changes in biotic assemblages and water quality variables, particularly in Lakes Michigan and Huron. These include changes in phytoplankton biovolume and zooplankton biomass, increasing invasive Dreissena abundance and decreasing densities of other benthos. Biotic changes are accompanied by pronounced changes in water quality and nutrient ratios. Species change-points, identified using threshold indicator taxon analysis, are often less abrupt, but there are clear shifts in a large proportion of species in each assemblage. The concordance of breakpoints among assemblages or lack thereof provides valuable insight into potential drivers of ecosystem change.

Unusual microbial mat-related structural diversity 2.1 billion years ago and implications for the Francevillian biota.

The 2.1-billion-year-old (Ga) Francevillian series in Gabon hosts some of the oldest reported macroscopic fossils of various sizes and shapes, stimulating new debates on the origin, evolution and organization of early complex life. Here, we document ten representative types of exceptionally well-preserved mat-related structures, comprising "elephant-skin" textures, putative macro-tufted microbial mats, domal buildups, flat pyritized structures, discoidal microbial colonies, horizontal mat growth patterns, wrinkle structures, "kinneyia" structures, linear patterns and nodule-like structures. A combination of petrographic analyses, scanning electron microscopy, Raman spectroscopy and organic elemental analyses of carbon-rich laminae and microtexture, indicate a biological origin for these structures. The observed microtextures encompass oriented grains, floating silt-sized quartz grains, concentrated heavy minerals, randomly oriented clays, wavy-crinkly laminae and pyritized structures. Based on comparisons with modern analogues, as well as an average δ13 C organic matter (Corg ) composition of -32.94±1.17‰ (1 standard deviation, SD) with an outlier of -41.26‰, we argue that the mat-related structures contain relicts of multiple carbon pathways including heterotrophic recycling of photosynthetically derived Corg . Moreover, the relatively close association of the macroscopic fossil assemblages to the microbial mats may imply that microbial communities acted as potential benthic O2 oases linked to oxyphototrophic cyanobacterial mats and grazing grounds. In addition, the mat's presence likely improved the preservation of the oldest large colonial organisms, as they are known to strongly biostabilize sediments. Our findings highlight the oldest community assemblage of microscopic and macroscopic biota in the aftermath of the "Great Oxidation Event," widening our understanding of biological organization during Earth's middle age.

Processes structuring amphibian assemblages along a subtropical arid gradient

Biotic assemblages are organized according to multiple factors, including environmental filtering and species interactions. However, the roles of abiotic and biotic factors in the organization of amphibian assemblages in arid regions are still not completely understood. Here, I examined the distribution patterns of seven species of amphibians that occur in Tunisia (north-western Sahara Desert) and the composition of 328 amphibian assemblages, including their meta-structural organization. The species showed different responses to environmental gradients, predominantly the aridity and vegetation gradients. The assemblages in xeric regions were composed of nested subsets of the species present in mesic regions. Most of the significant species associations were explained by shared environmental responses, indicating that they can be largely attributed to environmental filtering. The only exception was between two species of generalist bufonids, Bufotes boulengeri and Sclerophrys mauritanica, which showed a residual correlation greater than their shared association with the gradient. In this case, phylogenetic and morphological similarities could favour patterns of mutual exclusion in habitats such as desert oases, where the resources are limited.

Imaging analysis of biological soil crusts to understand surface heating properties in the Mojave Desert of California

Biological soil crusts (biocrusts) are composed of a combination of cyanobacteria, green algae, lichens, and mosses that can protect arid soil surfaces from erosive forces. The presence of biocrusts in desert regions is influenced by a complex interaction of climatic, edaphic, and topographic variables, but their distribution and coverage densities in southern California deserts are not precisely mapped. Visible satellite bands (red-green-blue; RGB) and infra-red image resolution are generally too coarse to distinguish between biocrusts and their interstitial sand substrates, limiting most remote sensing analysis of these soil biotic assemblages. The purpose of this study was to better understand the relationships between biocrust cover, surface coloration and roughness, and the thermal infra-red (TIR) emission properties of biocrusts to determine if new types of imaging of desert surfaces could be used to more accurately detect biocrust presence. Based on the results from megapixel digital photos and TIR imaging for nearly 130 different, well-developed biocrust surfaces in the Mojave and Lower Colorado Deserts of California, we designed laboratory heating experiments to test the effect of biocrust cover on the heating properties of sandy wash habitats. Biocrust samples collected from the field were subjected to artificial heating and cooling to simulate the Mojave Desert diurnal cycle. We used image segmentation algorithms to separate true-color biocrust images from their sandy substrates, and to map the heating and cooling patterns of these surfaces with different levels of biocrust coloration and roughness. The major findings of this study showed that biocrust patch images in the field had Red texture mean values between digital numbers of 95 and 131 with moderate positive tail shewness levels, indicative of a highly rugose surface of the biocrust cover. Such patches were also well-compacted overall, with moderate skewness toward negative tails. In controlled laboratory temperature experiments to simulate the desert diurnal cycle, surface wetting with a fine mist of water resulted in significantly lower peak surface temperatures of biocrust samples compared to dry biocrust heating results of the same crust and sand samples. The findings of this study can improve future mapping for lichen-dominated biocrust surfaces in the Mojave Desert.