Response of benthic invertebrate assemblages to varying drought conditions in the Po river (NW Italy)

Meta-analysis reveals less sensitivity of non-native animals than natives to extreme weather worldwide

Extreme weather events (EWEs; for example, heatwaves, cold spells, storms, floods and droughts) and non-native species invasions are two major threats to global biodiversity and are increasing in both frequency and consequences. Here we synthesize 443 studies and apply multilevel mixed-effects metaregression analyses to compare the responses of 187 non-native and 1,852 native animal species across terrestrial, freshwater and marine ecosystems to different types of EWE. Our results show that marine animals, regardless of whether they are non-native or native, are overall insensitive to EWEs, except for negative effects of heatwaves on native mollusks, corals and anemone. By contrast, terrestrial and freshwater non-native animals are only adversely affected by heatwaves and storms, respectively, whereas native animals negatively respond to heatwaves, cold spells and droughts in terrestrial ecosystems and are vulnerable to most EWEs except cold spells in freshwater ecosystems. On average, non-native animals displayed low abundance in terrestrial ecosystems, and decreased body condition and life history traits in freshwater ecosystems, whereas native animals displayed declines in body condition, life history traits, abundance, distribution and recovery in terrestrial ecosystems, and community structure in freshwater ecosystems. By identifying areas with high overlap between EWEs and EWE-tolerant non-native species, we also provide locations where native biodiversity might be adversely affected by their joint effects and where EWEs might facilitate the establishment and/or spread of non-native species under continuing global change.

Extreme weather events threaten biodiversity and functions of river ecosystems: evidence from a meta-analysis

Both gradual and extreme weather changes trigger complex ecological responses in river ecosystems. It is still unclear to what extent trend or event effects alter biodiversity and functioning in river ecosystems, adding considerable uncertainty to predictions of their future dynamics. Using a comprehensive database of 71 published studies, we show that event - but not trend - effects associated with extreme changes in water flow and temperature substantially reduce species richness. Furthermore, event effects - particularly those affecting hydrological dynamics - on biodiversity and primary productivity were twice as high as impacts due to gradual changes. The synthesis of the available evidence reveals that event effects induce regime shifts in river ecosystems, particularly affecting organisms such as invertebrates. Among extreme weather events, dryness associated with flow interruption caused the largest effects on biota and ecosystem functions in rivers. Effects on ecosystem functions (primary production, organic matter decomposition and respiration) were asymmetric, with only primary production exhibiting a negative response to extreme weather events. Our meta-analysis highlights the disproportionate impact of event effects on river biodiversity and ecosystem functions, with implications for the long-term conservation and management of river ecosystems. However, few studies were available from tropical areas, and our conclusions therefore remain largely limited to temperate river systems. Further efforts need to be directed to assemble evidence of extreme events on river biodiversity and functioning.

Fish community responses to antecedent hydrological conditions based on long-term data in Mediterranean river basins (Iberian Peninsula)

In recent decades many studies have proven the paramount impact of flow regimes on the structure of lotic ecosystems, both through extreme events (i.e. floods and droughts) but also during intermediate flows, which temporarily and spatially regulate the habitat availability. Human demand for water is steadily increasing and scientists are challenged to define ecosystem needs clearly enough to guide policies and management strategies. However, field studies demonstrated that a variety of interacting factors, such as, presence of barriers (e.g. dams) and temporal changes in habitat structure affect the abundance, composition and distribution of fish assemblages. This work based on quantile regression tested hypotheses to elucidate the effect of antecedent hydrological conditions on fish communities. A large monitoring database collecting and homogenizing the existing information on fish fauna in the Júcar River Basin District (Eastern Iberian Peninsula) was gathered and used to evaluate biological metrics (species richness, Capture Per Unit Effort-CPUE, and CPUE ratio over the total CPUE) related to life history strategies (i.e. periodic, opportunistic or equilibrium) and species origin (i.e. native, translocated or alien). The resulting dataset was complemented with diverse indicators of the measured daily discharge at the nearest gauging site. Most of the significant relationships confirmed the role of antecedent hydrological conditions as limiting factors, although other environmental factors likely play additional roles. In general, richness and abundance of alien species showed the higher proportion of significant associations, particularly spring flows and annual minima and maxima. These flow-ecology relationships shall be particularly useful to manage ecological responses to hydrological alteration. They also provide with clear ecological foundations for developing environmental flows assessments in Mediterranean river basins worldwide, using holistic approaches which can harmonise eco-hydrological approaches with smaller-scale and habitat-based ecohydraulics methods, especially under the current climate trends.

Role of the Hyporheic Zone in Increasing the Resilience of Mountain Streams Facing Intermittency

We investigated the impact of intermittence in previously-perennial Alpine stream reaches, targeting the role of the hyporheic zone in increasing the resilience of these aquatic systems. We selected a perennial and an intermittent site in a reach of the Po River (North-Western Italy). We installed piezometers reaching −1 m (permanent and intermittent site), and −3 m (intermittent site) and monitored three supraseasonal droughts over a period of three years. We classified the hyporheic fauna into three categories of increasing affinity to life in the hyporheic (stygoxene, stygophile, stygobite), and used communities composition, abundance, beta-diversity and functional groups: (1) to compare assemblages at the same depth but with different hydrological characteristics, as well as assemblages from two depths at the intermittent site, and (2) to assess how the connection with surface water and the direction of the vertical aquifer flow determined the faunistic assemblages. Different taxonomic groups responded differently to intermittence, the hyporheic zone acted as a refuge increasing the resilience of the system, but resilience decreased with increasing degree of affinity to hyporheic life. Disentangling the effects of intermittence on the different faunistic component in the hyporheic zone can help guiding effective protection and restoration measures of river systems with temporary reaches.

Numerical Modelling as a Support Tool for River Habitat Studies: An Italian Case Study

Numerical modelling is becoming a major tool for supporting environmental studies at different scales, thanks to the ability of up-to-date codes to reproduce the complex mechanisms of the natural environment in quite a reliable manner. In evaluating the habitat diversity of anthropized rivers, however, many issues are rising because of the intrinsic complexity of the physical processes involved and the limitations associated with numerical models. Using a reach of the Po River in Italy as a case study, the present works aims to provide a qualitative description of the changes of the Eco-Environmental Diversity index as a response to different constant flow discharges typically observed along this reach. The goals are achieved by means of two solvers of the freeware iRIC suite, applied in cascade to first simulate the 2D fluvial hydrodynamics and subsequently provide a qualitative estimate of the habitat conditions. Despite the several simplifications intrinsically present in the modelling cascade and the ones introduced for practical purposes, the results show that an extremely strong and long-lasting reduction of the flow discharge, like the one very recently observed, can ultimately threaten the overall biological status of the river. Because of the modelling uncertainties, these preliminary outcomes are only qualitative and show the need for more research, both in terms of data acquisition and numerical schematization, to adequately and quantitatively evaluate the effects of transient hydrology on the river ecosystems. Moreover, additional field surveys are necessary to calibrate and validate the used biological parameters, aiming to obtain sufficiently reliable estimates.

Effects of human-driven water stress on river ecosystems: a meta-analysis

Human appropriation of water resources may induce water stress in freshwater ecosystems when ecosystem needs are not met. Intensive abstraction and regulation cause river ecosystems to shift towards non-natural flow regimes, which might have implications for their water quality, biological structure and functioning. We performed a meta-analysis of published studies to assess the potential effects of water stress on nutrients, microcontaminants, biological communities (bacteria, algae, invertebrates and fish), and ecosystem functions (organic matter breakdown, gross primary production and respiration). Despite the different nature of the flow regime changes, our meta-analysis showed significant effects of human-driven water stress, such as significant increases in algal biomass and metabolism and reduced invertebrate richness, abundance and density and organic matter decomposition. Water stress also significantly decreased phosphate concentration and increased the concentration of pharmaceutical compounds. The magnitude of significant effects was dependent on climate, rainfall regime, period of the year, river size and type of water stress. Among the different causes of water stress, flow regulation by dams produced the strongest effects, followed by water abstraction and channelization.

Assessing long-term effects of multiple, potentially confounded drivers in ecosystems from species traits

Although species traits have the potential to disentangle long-term effects of multiple, potentially confounded drivers in ecosystems, this issue has received very little attention in the literature. We aimed at filling this gap by assessing the relative effects of hydroclimatic and water quality factors on the trait composition of invertebrate assemblages over 30 years in the Middle Loire River (France). Using a priori predictions on the long-term variation of trait-based adaptations over the three decades, we evaluated the ability of invertebrate traits to indicate the effects of warming, discharge reduction and water quality improvement. Hydroclimatic and water quality factors contributed to up to 65% of the variation in trait composition. More than 70% of the initial trait response predictions made according to observed long-term hydroclimatic changes were confirmed. They supported a general climate-induced trend involving adapted resistance and resilience strategies. A partial confounding effect of water quality improvement acting on trophic processes was also highlighted, indicating that improved water quality management can significantly help to reduce some adverse effects of climate change. This trait-based approach can have wider implications for investigating long-term changes driven by multiple, potentially confounded factors, as frequently encountered in the context of global change.

The running waters macroinvertebrates community: sampling techniques

The community of running water macroinvertebrates has proved to be one of key subjects for fluvial ecology and bioindication studies, thanks both to the different trophic roles within the range of taxa and to the ease with which they may be collected and identified. However, the complex nature of this community creates problems concerning the complete identification of the full range of taxa, even when restricting the taxonomic classification to families and genera. Even so, the need to use the community for the implementation of indexes of Ecological Status of freshwaters and for the detection of reference conditions, necessarily means a deeper knowledge of this structure. Hence, a standard methodology of the capture effort is required to identify not only the ecological quality but also a reference community for each selected fluvial typology and for each section examined. Starting from the processing of data collected during intercalibration exercises of the IBE method, the authors analyse the results underlining the share given by the size of the sample collected (catchment effort), and by the distribution models of different taxa within the community, in order to give a contribution to the evaluation of the reliability level of standard samples. The results confirm the models already described in previous publications and lead us to accept the presence of marginal degrees of uncertainty in standard samples.