Variation In Assemblage Structure Research Articles

Spatial variability in the structure of fish assemblages associated with Laminaria hyperborea forests in the NE Atlantic

Understanding fish-habitat associations is critical for ecosystem-based approaches to management and conservation. Kelp species, which are estimated to inhabit around 25% of the world's coastline and underpin highly productive and biodiverse ecosystems, are widely recognised as important nursery and foraging habitats for coastal fish species. However, quantitative assessments of fish assemblages within kelp forests are lacking for many regions. Here, we used Baited Remote Underwater Video (BRUV) and Underwater Visual Census (UVC) to quantify fish assemblages in Laminaria hyperborea forests at eight shallow subtidal rocky reefs that spanned 9° of latitude in the United Kingdom (UK). Fish assemblages were a ubiquitous and conspicuous component of kelp forest communities at all sites. BRUV surveys recorded marked regional-scale variability, with latitudinal shifts in assemblage structure and particularly distinct assemblages recorded in southwest England. These patterns were largely due to greater abundances of Pollachius spp. (i.e. saithe and pollock) in northern regions and higher numbers of Gobiusculus flavescens (two-spotted goby) and labrid species (i.e. rock cook, goldsinny and ballan wrasses) in southern regions. Unlike BRUVs, UVC surveys did not detect significant regional-scale variation in assemblage structure but did detect significant site-level differences, highlighting differences between the two techniques. BRUV surveys also recorded clear latitudinal trends in richness, with more taxa recorded at lower latitudes. Fish assemblages in the NE Atlantic have been, and will continue to be, impacted by ocean warming and fishing activities. Greater spatiotemporal coverage of kelp forest surveys and ongoing robust monitoring is needed to better understand and manage future ecological changes.

Seasonal and Spatial Variations in Fish Assemblage in the Yangtze Estuary and Adjacent Waters and Their Relationship with Environmental Factors

In this work, we identified the seasonal and spatial variations in fish assemblages and their relation to environmental variables in the Yangtze Estuary and adjacent waters. A total of 61 fish species in 32 families were identified over four seasons; of these, 12 species were dominant and Harpadon nehereus was the most dominant species. Fish varied in abundance, biomass, and species composition in different seasons; both biomass and abundance were highest in autumn and lowest in spring. Fish can be spatially divided into high-salinity assemblages and low-salinity assemblages. The spatial variation in fish assemblages was caused by the selectivity and adaptability of species for the environmental conditions of the estuary and the seasonal variation in fish assemblage structure likely resulted from migrations of dominant taxa associated with fish spawning and foraging behavior as well as the environment. The results of CCA analysis showed that temperature, depth, salinity, dissolved oxygen, and chlorophyll were the major factors affecting the fish assemblage differences throughout the seasons. Of these, temperature drove the seasonal variation in assemblage structure, while salinity significantly affected the spatial distribution of assemblages. This paper revealed the relationship between the seasonal and spatial distribution patterns of the fish assemblage and environmental factors and the results could provide a scientific basis for the management and sustainable utilization of fishery resources in the Yangtze Estuary and adjacent waters.

Changes in kelp forest communities off Washington, USA, during and after the 2014-2016 marine heatwave and sea star wasting syndrome

Canopy-forming kelps are foundation species in many coastal ecosystems, but kelp-forest communities are subject to abrupt state changes caused by environmental drivers and trophic dynamics. We examined changes in kelp communities at 5 sites along the Olympic Coast of Washington State, USA, during and following the recent perturbations of anomalous warm-water events and sea star wasting syndrome (SSWS). Anomalously warm water in 2013 and 2014 corresponded with a loss of approximately 50% of Macrocystis pyrifera and Nereocystis luetkeana canopy. However, the canopy quickly recovered, and stipe density increased after 2015. Purple sea urchins Strongylocentrotus purpuratus increased in density 164-fold, largely at one site, but this increase was first observed in 2017 and peaked in 2019, after the warm period. Sea stars did not show recovery from SSWS, with several species including Pycnopodia helianthoides continuing to decline. The majority of variation in assemblage structure occurred at the site level for kelps, macroinvertebrates, and fishes, while year explained most of the variability for juvenile rockfishes Sebastes spp. We did not see strong top-down effects of urchins on kelp, suggesting that top-down impacts were not dominant regionally during this period. In contrast, we found evidence for a bottom-up influence of kelp habitat on juvenile rockfishes, as rockfish recruits occurred with higher probability where kelp stipe density was higher. Our analyses highlight the importance of spatial variation in structuring changes in kelp forest communities associated with disturbance and suggest that it is essential to ensure the protection of a diversity of kelp forests.

Spring Ichthyoplankton Assemblage Structure in the Yangtze Estuary Under Environmental Factors

Estuaries, where fresh and salty water converge, provide abundant nutrients for ichthyoplankton. Ichthyoplankton, including fish eggs, larvae, and juveniles, are important fishery recruitment resources. The Yangtze Estuary and its adjacent waters comprise a typical large-scale estuary and supply many important fish spawning, feeding, and breeding areas. In this study, 1,291 ichthyoplankton individuals were collected in the Yangtze Estuary in spring, from 2013 to 2020. The aims of the study were to provide detailed information on characteristics of the ichthyoplankton assemblage, explore interannual variation, and evaluate the effects of environmental variables on the temporal variation in assemblage structure. Twenty-six species in seventeen families were identified. The dominant species were Coilia mystus, Chelidonichthys spinosus, Engraulis japonicus, Hypoatherina valenciennei, Larimichthys polyactis, Salanx ariakensis, Stolephorus commersonnii, and Trachidermus fasciatus. The ichthyoplankton assemblage changed significantly over time, and Chelidonichthys spinosus became one of the dominant species. Canonical correspondence analysis showed that temperature and chlorophyll a were the key factors affecting the assemblage structure in the Yangtze Estuary in spring.

Natural and anthropogenic factors as possible drivers of variability in rocky shore assemblages at multiple spatial scales

Spatial and temporal variability in rocky shore assemblages at multiple scales can be driven by natural factors and anthropogenic disturbances, operating either separately to each other or in complex combinations. Using the shallow subtidal rocky habitat in Ilha Grande Bay (southeastern Brazil) as a model system, this study analyzed five natural and five anthropogenic variables as possible drivers of variability of cover in rocky shore assemblages, focusing on the following questions: (i) Which are the relevant horizontal scales of variation in assemblage structure?; (ii) Are patterns of horizontal variation consistent across a small depth range?; (iii) Are there significant relationships between spatial patterns of macroalgae and macroinvertebrates, and their potential natural and anthropogenic drivers? These issues were addressed through a hierarchical sampling strategy including five horizontal scales, namely quadrat (0.4–3 m), point (3–10 m), site (100–350 m), island (500–1000 m) and area (2000–6500 m), repeated at two depths (1–2 m and 3.5–4.5 m). The correlations of cover data of 21 groups of organisms and five natural and five anthropogenic variables were analyzed. Assemblages showed relevant variation among quadrats, points and sites, at both depths, while variation at the area scale was relevant only at 3.5–4.5 m depth, indicating that patterns of horizontal variation were not fully consistent between depths. Substrate rugosity, rocky shore extension, water movement, percentage of deforested land area and distance to urban center were the environmental variables that most explained spatial patterns of the assemblages. Our study highlights the potential role of such variables as drivers of variation in rocky subtidal assemblages at multiple spatial scales. For environmental protection programs, relevant scales of variation of target populations and assemblages need to be considered, to effectively capture their dynamics and underlying processes.

Spatial and temporal variation of epigaeic beetle assemblages (Coleoptera, Carabidae, Staphylinidae) in aspen-dominated mixedwood forests across north-central Alberta.

Epigaeic beetle assemblages were surveyed using continuous pitfall trapping during the summers of 1992 and 1993 in six widely geographically distributed locations in Alberta’s aspen-mixedwood forests prior to initial forest harvest. Species composition and turnover (β-diversity) were evaluated on several spatial scales ranging from Natural Regions (distance between samples 120–420 km) to pitfall traps (40–60 m). A total of 19,885 ground beetles (Carabidae) representing 40 species and 12,669 rove beetles (non-AleocharinaeStaphylinidae) representing 78 species was collected. Beetle catch, species richness, and diversity differed significantly among the six locations, as did the identity of dominant species. Beetle species composition differed significantly between the Boreal Forest and Foothills Natural Regions for both taxa. Staphylinidae β-diversity differed significantly between Natural Regions, whereas Carabidae β-diversity differed among locations. Climate variables such as number of frost-free days, dry periods, and mean summer temperatures were identified as significant factors influencing beetle assemblages at coarse spatial scales, whereas over- and understory vegetation cover, litter depth, shade, slope, and stand age influenced beetle assemblages at finer spatial scales. Significant interannual variation in assemblage structure was noted for both taxa. Because composition of epigaeic beetle assemblages differed across spatial scales, forest management strategies based only on generalized understanding of a single location will be ineffective as conservation measures. In addition, site history and geographic variation significantly affect species distributions of these two beetle families across the landscape. Thus, we underscore Terry Erwin’s suggestion that biodiversity assessments focused on species assemblages at different spatial scales provide a sound approach for understanding biodiversity change and enhancing conservation of arthropod biodiversity.

The effects of dispersal ability on metacommunity structure of macroinvertebrates in subtropical Chinese high-mountain streams: seasonal shifts in relative contribution of local environment and spatial processes.

It is generally recognized that dispersal mode can affect the relative role of environmental and spatial factors in structuring biotic communities. Disentangling the effects of dispersal mode on metacommunity structuring is essential to understanding the mechanisms of community assembly. Despite high seasonal variation in assemblage structure and phenological features of lotic macroinvertebrates, few studies examined the seasonal changes in the relative contribution of environmental and spatial processes. Here, we used two-season (spring and autumn) investigation data to link dispersal mode with local environmental and spatial factors that regulate macroinvertebrate metacommunity in a Chinese high-mountain stream network. Our aims were (1) to disentangle the relative role of environmental and spatial processes on structuring macroinvertebrate with different dispersal ability (aquatic passive: AqPa, terrestrial passive: TePa, and terrestrial active: TeAc) and (2) to determine seasonal shifts in metacommunity structuring processes. We found that assemblages of TeAc (with highest dispersal capacity) in both seasons were influenced more by environmental filtering than spatial structuring, whereas assemblages of TePa were mainly determined by spatial processes. Unexpectedly, AqPa group showed low spatial control in both seasons, probably due to their underestimated dispersal capacity via animal vector. The relative role of environmental and spatial factors was broadly stable across two seasons for AqPa and TeAc groups, but the TePa showed seasonal differences in the relative role of spatial factors, because of their seasonally changing dispersal capacity. In summary, our study emphasizes the use of dispersal mode for understanding metacommunity structuring mechanisms.

Trait-based approach reveals how marginal reefs respond to acute and chronic disturbance

Ecosystems worldwide are becoming increasingly altered by environmental stress, yet little is known about how acute disturbances affect ecological communities that already persist under chronically stressful environmental conditions. Here, we use a trait-based approach to understand the effects of repeated flooding on the subtropical reefs of Hervey Bay, Australia (25˚ South) that are considered marginal both in terms of their chronically turbid and high-latitude settings. We quantify variation in taxonomic and functional composition of coral assemblages along spatial gradients of water quality, before and after three flooding events of the Mary River between 2010 and 2013. We demonstrate systematic variation in taxonomic and functional composition along distance from shore and rivers and show remarkable stability of patterns over the course of repeated flooding. This is likely because the reefs of Hervey Bay are regularly exposed to resuspension of sediments, limiting the types of species that are able to persist in these chronically turbid settings, with flood events reinforcing the longer-term stress regime. Greater similarity of co-occurring species on nearshore reefs than expected by chance also supports this hypothesis. Corallite size, coral morphology, the ability to remove sediment, and propagule development rate best explained variation in coral assemblage structure. Specifically, reefs closer to shore and to rivers were characterised by species with large corallites, high sediment removal ability and foliose morphology, strategies considered beneficial in turbid conditions. Among individual traits, symbiont transmission best explained variation in assemblage structure, and it is plausible that the acquisition of locally adapted symbionts from the environment is advantageous in the turbid and light-limited conditions of nearshore reefs. These findings highlight the importance of tolerance to turbidity and sedimentation in driving coral biodiversity patterns in Hervey Bay, with acute flooding reinforcing the long-term stress regime that controls the distribution of species on these marginal reefs.

Multiscale spatial variability in epibiont assemblage structure associated with stipes of kelp Laminaria hyperborea in the northeast Atlantic

Foundation species elevate local biodiversity and underpin critical ecological processes and functions. Kelp species are distributed along ~25% of the world’s coastlines, where they serve as foundation species in intertidal and subtidal habitats. As well as ameliorating environmental conditions and producing organic matter, they provide biogenic habitat for a vast array of associated organisms. Here, we investigated patterns of diversity and structure in assemblages associated with the stipe of the kelp Laminaria hyperborea in the NE Atlantic. Stipes were sampled at 4 study regions (with 3 sites nested within each region) in the UK, spanning ~9° of latitude. Stipe-associated communities were highly diverse (134 species) and abundant (16-4532 ind. stipe-1), with no obvious sequential shift in diversity or overall trends in abundance/biomass of assemblages with latitude. However, we observed high degrees of variability between sites from the same region and individuals within sites, indicating that processes working across smaller spatial scales were more important than those at regional scales. While we observed high between-site variability in assemblage structure, regional differences were also evident. Most notably, sites within our southernmost region (southern England) were largely devoid of amphipods that dominated all other regions. This study highlights the important role of L. hyperborea in elevating biodiversity at local to regional scales through a facilitative interaction. Moreover, given that L. hyperborea forests may be increasingly impacted by ocean warming, changes in coastal water quality and proposed exploitation, our study serves as an important benchmark against which to detect future changes.

Effects of exposure, depth and aspect on sponge communities on a coral reef

Coral reef benthic communities include a wide range of taxa, but most attention has been given to hard coral assemblages, and how their cover and composition vary over strong spatial gradients. Much less is known about the spatial distribution and composition of coral reef sponge communities, which may become increasingly important on reefs with declining coral cover. Here, we examined the effects of exposure, depth, aspect and location on the cover and composition of sponge assemblages on a coral reef in Kimbe Bay, Papua New Guinea. We quantified sponge cover and species composition along replicate line transects on 6 inshore reefs, sampling exposed (seaward) and sheltered (landward) sides of reefs at 5, 10 and 15 m depth, with reef aspect subdivided into slopes or walls along each transect. Although the substratum was generally dominated by corals and algae, sponges ranked 3rd, with an average of 13.1% cover, including 63 recognisable species. Morphologically there were 38 encrusting, 21 erect and 4 massive sponge species, with the encrusting sponges Lamellodysidea cf. chlorea and Dysidea sp1 exhibiting the highest cover. Sponge cover, species richness and species composition all exhibited complex interactions among depth, exposure and location. Sponge cover and species richness increased in transects with higher percentages of wall aspects, and assemblage structure differed between slopes and walls. Sponges are a diverse component of the benthos, with exposure, depth and reef aspect all contributing to explain spatial variation in assemblage structure.

The longest fragment drives fish beta diversity in fragmented river networks: Implications for river management and conservation

Connectivity plays a crucial role in maintaining the structural and functional attributes of river networks. Therefore, the loss of connectivity (fragmentation) alters the functioning and diversity patterns of the biota at local and regional scales. The global hydropower boom is one of the main drivers of river network fragmentation, with significant effects on the diversity of riverine biota. Analyses of beta diversity of fish assemblages in rivers with different degrees of fragmentation can give new insights into mechanisms that contribute to the responses of these assemblages to fragmentation.Here, fish beta diversity within six river networks of central Chile with different levels of fragmentation was studied to assess the responses of fish assemblages to fragmentation. A hypothesis of a significant effect of fragmentation on the beta diversity of native and non-native fish in riffles and pools was tested. This effect is expected to be modulated by both changes in environmental heterogeneity and direct obstruction of natural dispersal routes.Beta diversity based on variation of assemblage structure and environmental heterogeneity showed significant differences among river networks. Fish beta diversity showed a clear response to fragmentation in recently fragmented rivers. Specifically, the beta diversity of native fishes in pools and non-native fishes in riffles decreased with increase of the ratio between the longest non-fragmented sections of the river network to the total length of the network. These effects of fragmentation on fish assemblages were modulated by the biological features of each species, and open-water species were most severely affected.These results have significant implications for planning of the placement of new barriers in river networks subjected to hydropower boom. Planning of the placement of new barriers should consider the maintenance of long, connected sections within river networks in order to minimise the effects of fragmentation on fish biodiversity.

Temporal Variation in a Turtle Assemblage Inhabiting a Florida Spring-Fed River

We conducted a survey of freshwater turtle populations in the spring-fed Ichetucknee River in northern Florida to assess temporal variation in assemblage structure. Following the same protocols as an initial 2007 survey, we hand-captured turtles while snorkeling in 2014 and found changes in species composition, abundance of 3 species, and demographic composition of 2 species. We observed a shift in the most frequently captured species from Sternotherus minor (Loggerhead Musk Turtle) in 2007 to Pseudemys concinna (River Cooter) in 2014. We attribute this shift to an increase in the abundance of adult River Cooters. Long-term studies are needed to evaluate whether the changes we observed represent trends and which environmental factors may be driving them.

The scaling of biomass variance across trophic levels in stream species communities: a macroecological approach

Much has been published about different aspects of body-size distribution resting on the assumptions of metabolic scaling, although a number of studies in aquatic ecosystems have questioned its generality. This study considers the effects of individual body-mass and biomass variability on scaling properties of multi-species communities (protists, meio- and macroinvertebrates), and their intrinsic variations in assemblage structure. We examine how size traits within communities are distributed on local and regional scales and assess the potential sources of variation affecting whole ecosystems. Our results, built upon seven river catchment communities including 1204 species, revealed micro-meiofauna-dominated biomass distributions driven by stochastic hydrophysical processes that induce a fractal fluctuation scaling, irrespective of trophic levels, shaping local and regional scaling relations. Fractal-scaling differences are largely generated by the frequency of high flow events that influence the biomass assemblage configurations, which are significantly better represented by the Power Fraction model compared to single statistical random models. We conclude that environmental random variability contributes to the decoupling of total biomass and body mass per site from assemblage size, resulting in scale-invariant body-size traits among assemblages and systems. Generally, these findings emphasize that ignoring small-sized species and, thus, the wide range of body sizes makes accurate ecological model predictions, impossible.

Environmental Drivers and Distribution Patterns of Carnivoran Assemblages (Mammalia: Carnivora) in the Americas: Past to Present

Understanding species distributions and the variation of assemblage structure in time and space are fundamental goals of biogeography and ecology. Here, we use an ecological niche modeling and macroecological approach in order to assess whether constraints patterns in carnivoran richness and composition structures in replicated assemblages through time and space should reflect environmental filtering through ecological niche constraints from the Last Inter-glacial (LIG), Last Glacial Maximum (LGM) to the present (C) time. Our results suggest a diverse distribution of carnivoran co-occurrence patterns at the continental scale as a result of spatial climatic variation as an important driver constrained by the ecological niches of the species. This influence was an important factor restructuring assemblages (more directly on richness than composition patterns) not only at the continental level, but also from regional and local scales and this influence was geographically different throughout the space in the continent. These climatic restrictions and disruption of the niche during the environmental changes at the LIG-LGM-C transition show a considerable shift in assemblage richness and composition across the Americas, which suggests an environmental filtering mainly during the LGM, explaining between 30 and 75% of these variations through space and time, with more accentuated changes in North than South America. LGM was likely to be critical in species functional adaptation and distribution and therefore on assemblage structuring and rearranging from continental to local scales through time in the continent. Still, extinction processes are the result of many interacting factors, where climate is just one part of the picture.

Hotspots of biotic compositional change in lakes along vast latitudinal transects in northern Canada.

Ecotones mark zones of rapid change in ecological structure at various spatial scales. They are believed to be particularly susceptible to shifts caused by environmental transformation, making them key regions for studying the effects of global change. Here, we explored the variation in assemblage structure of aquatic primary producer and consumer communities across latitudinal transects in northeastern North America (Québec-Labrador) to identify spatial patterns in biodiversity that indicated the location of transition zones across the landscape. We analyzed species richness and the cumulative rate of compositional change (expressed as beta-diversity) of diatoms and chironomids to detect any abrupt shifts in the rate of spatial taxonomic turnover. We used principal coordinates analysis to estimate community turnover with latitude, then applied piecewise linear regression to assess the position of ecotones. Statistically significant changes in assemblage composition occurred at 52 and 55°N, corresponding to the transition between closed- and open-crown forest, and to the southern onset of the forest tundra (i.e., the forest limit), respectively. The spatial distribution of ecotones was most strongly related to air temperature for chironomids and to vegetation- and soil-related chemical attributes of lake water for diatoms, including dissolved organic carbon content and water color. Lakes at mid- to high-latitudes currently face pressures from rapidly rising temperatures, accompanied by large increases in organic carbon inputs from their catchments, often leading to browning and its associated effects. The biota at the base of food webs in lakes located in transition zones are disproportionately affected by the cascading effects of these multi-factorial changes, concurrent with pronounced terrestrial greening observed in these regions. Similar patterns of biotic shifts have been observed along alpine aquatic transects, indicating the potential for widespread restructuring of cold, high-altitude and high-latitude freshwater communities due to global change.

Spatial and temporal variation of late summer ichthyoplankton assemblage structure in the eastern Chukchi Sea: 2010–2015

The survival of the planktonic stages of marine fishes is widely considered to be a bottleneck in recruitment success and is a period when fishes are highly sensitive to changes in their environment. In high latitude areas that are forecasted to experience warming temperatures, it is critical to determine a baseline of ichthyoplankton assemblages to detect how the ecosystem may change in response to shifting conditions. This study used ichthyoplankton data from the eastern Chukchi Sea in the summers of 2010–2015 to assess temporal and spatial variation in assemblage structure. Larval densities were examined in relation to oceanographic conditions at depth and corresponding water masses. Additionally, the standard lengths of polar cod (Boreogadus saida), a key trophic link in Arctic food webs, were assessed for size-at-catch relationships. In 2010 and 2011, years of widespread presence of warm, low salinity bottom water masses such as Alaska Coastal Water, yellowfin sole (Limanda aspera) and longhead dab (Limanda proboscidea) dominated the assemblages. In 2012 and 2013, the increased presence of colder, more saline Winter Water coincided with a shift to polar cod-dominated assemblages. In 2014 and 2015, water masses were less spatially defined and different assemblages, characterized by Arctic sand lance (Ammodytes hexapterus) and saffron cod (Eleginus gracilis), were detected than previous years. The greatest range in sizes of polar cod was observed in 2014, as well as the highest number of juveniles and age-1 individuals were collected. The biophysical patterns identified here support the strong connection between interannual oceanographic conditions and ichthyoplankton assemblages which reflect the importance of continuous, multi-year surveys to document assemblage changes in the Arctic.

How can assemblage structure indices improve monitoring of change in bird communities using ongoing survey data?

Abstract Traditional monitoring of biodiversity has focused on the abundance or distribution of individual species, or on simple summary indices of average trends or diversity or species richness. Supplementing these approaches with an index system that measures subtle changes in assemblage structure is highly desirable and appears not to have been previously attempted. Subject to validation, such indices could inform sensitively about ecosystem health (defined as proximity to a stable or desired state) and the natural capital status or potential ecosystem service delivery. The available approaches for monitoring biological assemblages and the data to which they might be applied are reviewed, including consideration of sources of error. A particularly promising method is the calculation of ‘diversity profiles’ (Leinster & Cobbold, 2012), which present a ‘signature’ describing the range of biological variation, considering both species’ identities and their relative abundances. The diversity profile approach was tested using a long-term bird monitoring data set for woodland birds in the UK, applying species similarities based on phylogeny, habitat selection and ecological function (i.e. diet). Tests were conducted using three nested sets of species with different levels of specialization with respect to woodland. The influence of quantifiable sources of error was evaluated using bootstrapping. Profile shape and temporal changes in diversity varied in magnitude and pattern with the choice of similarity measure and species set, with the pattern of temporal change in diversity showing either an increase or a decline, depending on the choice of analysis. This shows that the profile approach has the potential to reveal subtle variations in assemblage structure, although careful application and interpretation of outputs are required. The bootstrapping results revealed that uncertainty in profile values was high, but that this was due chiefly to uncertainty in interspecific similarities, as opposed to sampled abundance values, but this relates to the specific application to the datasets tested, rather than to the method in general. Different approaches to quantifying differences between species (phylogenetic, habitat preference and dietary) revealed different aspects of assemblage variability. Combined with the calculation of reference levels of diversity for known assemblages or sets of conditions such that observed variation can be interpreted quantitatively, assemblage monitoring via diversity profiles could add valuable, quantitative inference to ongoing monitoring, and hence the assessment of ecosystem health, potentially bridging the gap between conservation and a purely economic valuation of the status of natural capital.

Phytoplankton assemblage composition as an environmental indicator in routine lentic monitoring: Taxonomic versus functional groups

Phytoplankton community assessment using functional groups is a powerful tool for lake and reservoir biomonitoring, but its efficiency has only been tested in intersite, low-frequency datasets. Here, we took advantage of a gradient of spatially close, but environmentally contrasting, meso- to hypereutrophic reservoirs in Southeast Brazil, to compare the efficiency of morphology-based functional classification (MBFC), phylum-level, and species-level classification for high-frequency, i.e. monthly, biomonitoring. Monitoring of water physical and chemical characteristics revealed the presence of two independent environmental gradients, with the first being related to total phosphorus and suspended solids concentrations, as well as Secchi depth, and the second related to dissolved inorganic nitrogen, water temperature and pH. The species approach had the highest power to explain environmental variation in assemblage structure (21% vs 16% and 17%, in the phylum and MBFC approaches). Although both simplified classifications showed similar explanatory power, MBFC allowed a better differentiation of reservoir assemblages. As the MBFC approach is also the least resource intensive, it may be preferable for routine biomonitoring, or to process large sample numbers. Nonetheless, species-level classification provided the most detailed information on assemblage dynamics and is therefore an important option for scientific studies.

Seasonal and longitudinal variation in fish assemblage structure along an unregulated stretch of the Middle Uruguay River

ABSTRACT Diversity patterns and their causes remain important questions, especially for ecosystems that preserve natural conditions. This is the case of fish diversity in large Neotropical rivers. In this context, we investigated fish diversity patterns along an extensive unregulated river section (ca. 450 km) in the Middle Uruguay River Basin. Sampling was conducted seasonally between May 2017 and March 2018, at six sites (patches) with contrasting environmental conditions. We collected 3,008 individuals belonging to 90 species. Nine were migratory, which summed relevant biomass in local assemblages (20 to 60%). We observed spatial variation in assemblage structure, but weak seasonal effects. Overall, biomass was similar among sites, but richness tended to increase downstream. Migratory fishes showed opposed trends, with higher richness and biomass upstream, particularly catfishes (Siluriformes). Ordination analyses separated sites in three groups based on variations in composition and abundance, and revealed associations between these groups and specific hydro-geomorphic conditions (i.e., flow, depth, channel width and the presence of riparian vegetation). Our study revealed that fish diversity distributes heterogeneously along the main channel, where hydro-geomorphic patches select for distinct assemblages along riverine gradients.

Assessing the effectiveness of surrogates for species over time: Evidence from decadal monitoring of a Mediterranean transitional water ecosystem

The use of higher taxa or alternative approach to species surrogacy, such as the BestAgg procedure, could represent cost-effective solutions to avoid expensive species-level identifications in monitoring activities, especially on the long term. However, whether a set of surrogates would be effective in subsequent reiteration of the same assessment remains largely unsolved. We used a long-term dataset on macro-benthic assemblages to test the hypothesis that family-level and BestAgg surrogates which are effective for a limited period of monitoring could be successfully applied to quantify community patterns also in subsequent monitoring programmes. The effectiveness of surrogates in detecting temporal variations in assemblage structure as at species level remained basically unaffected over a decade. Recognizing once and for all if species surrogacy may have a practical value for monitoring will strongly depend on future assessments of the potential of surrogates to reflect community changes and to retain this prerogative over time.