A Proposal to Classify and Assess Ecological Status in Mediterranean Temporary Rivers: Research Insights to Solve Management Needs

The biomonitoring methods implemented by water authorities are mostly developed for perennial rivers, and do not apply to temporary rivers (TRs). We propose a new classification for TRs to better assess their ecological status. It arises from the LIFE+ TRivers project, which was conducted in the Catalan and the Júcar Mediterranean river basin districts (RBD). The European Water Framework Directive (WFD) provided two systems to set river types (systems A or B from Annex II), which have been officially used by water authorities across Europe to set “national river types” (NRTs). However, essential hydrological variables for TRs are largely omitted. NRTs established according to the WFD were compared with TR categories obtained by using a rainfall-runoff model, “natural flows prescribed regimes” (NFPRs), and with “aquatic phases regimes” (APRs) calculated by using TREHS software. The biological quality indices currently used in Spain, based on macroinvertebrates and diatoms (IBMWP, IMMI-T, and IPS), were compared with a “general degradation” gradient in order to analyze the two TR river classification procedures (NFPR and APR). The results showed that NRTs did not properly classify TRs, and that the APR classification identified ecologically meaningful categories, especially those related to stagnant phases. Four “management temporary river categories” based on APRs are proposed to be used for water managers to properly assess the ecological status of TRs.

Using stable isotopes to estimate the time since disconnection of pools in temporary rivers

<p>Temporary rivers, characterized by shifts between flowing water, disconnected pools and dry periods, represent over 50% of the world’s river network and future climatic projections suggest their increase. These rivers are understudied, especially when only disconnected pools remain, because gauging stations or hydrological models do not inform of what happens after the cessation of flow. In addition, most of biological indicators for water quality are designed for flowing waters and their adequacy for temporary rivers is uncertain.</p><p>The development of biological metrics adequate for the assessment of disconnected pools is difficult, because the high species replacement during and following flow cessation. For this reason, one hydrological variable of paramount importance for the assessment of ecological quality of disconected pools is the time since disconnection from the river flow.</p><p>The objective of our work is to present a methodology to estimate the time since disconnection of pools from the river flow. This methodology, following the Gonfiantini (1986) model, is based on the sampling of water stable isotopes in disconnected pools. For pools disconnected from the groundwater, knowing the isotopic modification of the water in time due to evaporation, allows to estimate the relative volume of water evaporated since the pool has been disconnected. However, this approach gets complicated when pools have relevant rainfall inputs or exchanges with groundwater.</p><p>Within the Vallcebre research area (42º12’N and 1º49’E), two artificial pools, one covered with a transparent lid to prevent the input of rainfall and another uncovered, were installed to validate this methodology in controlled conditions. From July to November 2020, water volume of these pools were weekly measured and sampled for isotopic analysis. In parallel, meteorological variables were monitored and rainfall was also sampled for water stable isotopes.</p><p>To develop and validate an operational methodology for estimating the time since disconnection, we first calculated the relative amount of evaporated water based on the variations of isotopic composition of the covered pool samples, and estimated the time since disconnection (for a given natural pool) using the potential evaporation calculated from the meteorological data. For the uncovered pool, the information of amount and isotopic composition of rainfall was added in a mass balance model. Additionally, the same estimations were calculated with standard information (i.e. the meteorological data obtained from the National Meteorological Service and precipitation isotopes data from the Global Network of Isotopes in Precipitation (GNIP) of the International Atomic Energy Agency). Finally, measured volumes changes in pools, were used to assess the limitations of the operational methodology and the sensitivity of the results to meteorological conditions.</p><p>Our approach suggests that changes in isotopic composition can be a reliable method to estimate time since disconnection of pools in temporary rivers to better assess their ecological quality.</p>

Conservation and Management of Isolated Pools in Temporary Rivers

Temporary rivers are characterized by shifting habitats between flowing, isolated pools, and dry phases. Despite the fact that temporary rivers are currently receiving increasing attention by researchers and managers, the isolated pools phase has been largely disregarded. However, isolated pools in temporary rivers are transitional habitats of major ecological relevance as they support aquatic ecosystems during no-flow periods, and can act as refugees for maintaining local and regional freshwater biodiversity. Pool characteristics such as surface water permanence and size, presence of predators, local physicochemical conditions, time since disconnection from the river flow, or distance to other freshwater habitats challenge a comprehensive understanding of the ecology of these habitats, and challenge ecological quality assessments and conservation practices in temporary rivers. In this paper, we aim at providing a characterization of isolated pools from a hydrological, geomorphological, physicochemical, biogeochemical, and biological point of view as a framework to better conceptualize, conserve, and manage these habitats.

Statistical modelling of intermittence metrics in temporary rivers of the UK

<p>Temporary rivers (TRs) are important headwater features of river flow networks, varying dynamically in space and time and providing both terrestrial and freshwater habitats.  In parts of the UK, TRs have become a source of tension between the public and regulators against a backdrop of the competing influences of natural variability, climate change and artificial influences.  Despite this importance, such systems have typically been omitted from monitoring endeavours.  Correspondingly, the occurrence, distribution and characteristics of TRs in the UK are poorly understood.  An enhanced understanding of the features of TRs has the potential to underpin more robust evidence for the protection of aquatic habitats that are vulnerable to drying.</p><p>In this study, novel approaches to the statistical modelling of TRs in the UK are adopted to enable the simulation of intermittence metrics. Addressing the challenge of limited observational data, models are trained on data from both the UK and France, drawing on their temporal and spatial advantages, respectively, to maximise their robustness and ability to extrapolate spatially. The performance of a range of statistical modelling and machine learning approaches is evaluated, and applied in simulating intermittence metrics in the UK. </p><p>Preliminary validation results suggest that the modelling approaches are able to replicate observed intermittence metrics where data exist.  Hierarchies of modelling approaches are derived which suggest certain families of models are more effective in simulating flow intermittency in TRs.  The best performing models under validation are taken forward to simulate intermittence patterns beyond networks of observations, helping to identify core regions towards which further focus should be directed by the research and operational TR communities.</p><p>Information on the location, prevalence and intermittency of TRs is vital to enhance the efficiency of monitoring strategies with finite resources, and bolster community efforts to engage local stakeholders in gathering additional data.</p>

Paying attention to the isolated pools phase in temporary rivers. A challenge to the ecological quality assessment of temporary rivers.

<p>Temporary rivers are characterized by shifting habitats between flowing, non-flowing and dry phases. Despite the fact that they are currently receiving significant attention by researchers and managers, the non-flowing (standing pools) phase has been largely disregarded. However, isolated pools in temporary rivers are transitional habitats of major ecological relevance as they can act as refuges for maintaining local and regional freshwater biodiversity. Factors such as pool duration and size, local physicochemical conditions, time since disconnection, distance to other freshwater habitats or presence of predators are crucial for a comprehensive understanding of the ecology of these habitats, and compromise to work towards adequate ecological quality assessments and conservation practices in temporary rivers.</p><p>Research is ongoing focused on the development of a method for assessing the ecological status of disconnected pools, based on the relationship between the time elapsed after the pool disconnection and the characteristics of the biological communities taking into account the above-mentioned factors. The prevalence of the pool phase is assessed using the TREHS software tool through interviews with citizens as well as aerial and surface photographs examination. The time since disconnection is assessed with the help of low-cost sensors and water stable isotopes, whereas the local environmental characteristics are assessed using regular metrics. Finally, biological communities of the pools are characterized using both taxonomic and functional metrics, with the support of metabarcoding techniques, applied to diatoms, macrophytes, macroinvertebrates and fishes. This method aims to be used by water managers to improve the monitoring of the ecological status of temporary rivers, which are common around the world, harbor unique biodiversity and provide key ecosystem services.</p>