Intermittent rivers and ephemeral streams (IRES) are characterized by hydrological regimes with extreme events (floods, droughts). This includes the alternation of wet and dry phases, and highly variable lateral, vertical, and longitudinal hydrological connections between stream surface waters and surrounding terrestrial and groundwater environments (chapter 2). These hydrological regimes exert a strong influence on key abiotic (e.g., dilution, hydrological connectivity) and biotic (e.g. microbial nutrient uptake and mineralization) processes occurring within the stream and its catchment. These processes control stream water physicochemical characteristics (i.e. pH, salinity, dissolved oxygen, nutrient concentrations) and have relevant effects on stream biological communities (chapter 4); and thus on the ecosystem services provided by IRES (chapter 6). Due to their highly variable hydrological regimes, IRES show acute changes in the magnitude of stream physicochemical parameters, thereby having high spatial and temporal variability (Gómez et al., 2017; von Schiller et al., 2017a). During the drying phase, stream discharge, size of the stream network, and the hydrological connectivity within the catchment are dramatically reduced, ultimately leading to fragmentation and cessation of stream flow. Low discharge is associated with a low dilution capacity of the surface water to the inputs from adjacent compartments (e.g. riparian and hyporheic zones) and an increase in hydrological retention and stream water residence time. Moreover, the loss of hydrological connectivity reduces inputs of solutes from terrestrial ecosystems (at least natural inputs, see section 3.4 below) and increases the relative importance of instream processes on water physicochemical characteristics and their spatial variability (Figure 3-1). During the rewetting phase (i.e. first-flush events), hydrological connectivity is restored. This rewetting boosts the mobilization and transport of solutes accumulated in terrestrial soils and streambed sediments during the dry phase and reduces the spatial variability in stream water physicochemical characteristics (Figure 3.1). Although patterns of variation in these parameters may differ considerably among different types of IRES, their high temporal and spatial variability must be considered to properly assess the ecological status of these ecosystems. Intermittent rivers and ephemeral streams handbook (2020) 47 Environmental policies, such as the European Water Framework Directive (WFD) (EC, 2000), consider water physicochemical characteristics in the assessment of the ecological status of streams. This includes the characterization of thermal, oxygen and acidity conditions, along with salinity, nutrients, priority pollutants and other specific pollutants (Annex II of Directive 2008/105/EC, later amended by the Directive 2013/39/EU) discharged in significant quantities into streams. Therefore, parameters like temperature, dissolved oxygen, pH, conductivity, the concentration of nutrients, insecticides, herbicides and metals are of special interest when monitoring the physicochemical status of any given stream or river, including IRES. Strategies for physicochemical monitoring differ among EU members, and, so far, few adaptations to the particularities of IRES have been undertaken (Prat et al., 2014; Sánchez- Montoya et al., 2012). The use of physicochemical parameters for ecological status assessment entails previously established reference conditions that should encompass the full range of values expected to occur naturally in each stream type. Within this context, the European Commission has published a guide of best practices for establishing nutrient concentrations to assist Member States in determining the levels of phosphorus (P) and nitrogen (N) that are likely to support good ecological status (Phillips et al., 2018). This guidance can be used to check existing boundary values or to develop new ones. Nevertheless, the range of values can lead to misleading interpretations when applied to IRES because of the high variability of physicochemical parameters observed naturally in these freshwater ecosystems. Hence, it is fundamental to consider the natural variability of IRES in order to properly manage and monitor water quality of these ecosystems (Prat et al., 2014). This chapter briefly reviews existing knowledge on the physicochemistry of IRES and how it can be used to monitor and manage these ecosystems. We first describe the temporal and spatial patterns of key physicochemical parameters in IRES. Then we provide recommendations for an effective physicochemical monitoring of these ecosystems. Finally, we highlight critical issues that must be addressed for proper assessment and management of water physicochemical quality in IRES.
Intermittent rivers and ephemeral streams: what water managers need to know. Ch. 3, Water Physicochemistry in IRES.
Zoppini A
2020
Abstract
Intermittent rivers and ephemeral streams (IRES) are characterized by hydrological regimes with extreme events (floods, droughts). This includes the alternation of wet and dry phases, and highly variable lateral, vertical, and longitudinal hydrological connections between stream surface waters and surrounding terrestrial and groundwater environments (chapter 2). These hydrological regimes exert a strong influence on key abiotic (e.g., dilution, hydrological connectivity) and biotic (e.g. microbial nutrient uptake and mineralization) processes occurring within the stream and its catchment. These processes control stream water physicochemical characteristics (i.e. pH, salinity, dissolved oxygen, nutrient concentrations) and have relevant effects on stream biological communities (chapter 4); and thus on the ecosystem services provided by IRES (chapter 6). Due to their highly variable hydrological regimes, IRES show acute changes in the magnitude of stream physicochemical parameters, thereby having high spatial and temporal variability (Gómez et al., 2017; von Schiller et al., 2017a). During the drying phase, stream discharge, size of the stream network, and the hydrological connectivity within the catchment are dramatically reduced, ultimately leading to fragmentation and cessation of stream flow. Low discharge is associated with a low dilution capacity of the surface water to the inputs from adjacent compartments (e.g. riparian and hyporheic zones) and an increase in hydrological retention and stream water residence time. Moreover, the loss of hydrological connectivity reduces inputs of solutes from terrestrial ecosystems (at least natural inputs, see section 3.4 below) and increases the relative importance of instream processes on water physicochemical characteristics and their spatial variability (Figure 3-1). During the rewetting phase (i.e. first-flush events), hydrological connectivity is restored. This rewetting boosts the mobilization and transport of solutes accumulated in terrestrial soils and streambed sediments during the dry phase and reduces the spatial variability in stream water physicochemical characteristics (Figure 3.1). Although patterns of variation in these parameters may differ considerably among different types of IRES, their high temporal and spatial variability must be considered to properly assess the ecological status of these ecosystems. Intermittent rivers and ephemeral streams handbook (2020) 47 Environmental policies, such as the European Water Framework Directive (WFD) (EC, 2000), consider water physicochemical characteristics in the assessment of the ecological status of streams. This includes the characterization of thermal, oxygen and acidity conditions, along with salinity, nutrients, priority pollutants and other specific pollutants (Annex II of Directive 2008/105/EC, later amended by the Directive 2013/39/EU) discharged in significant quantities into streams. Therefore, parameters like temperature, dissolved oxygen, pH, conductivity, the concentration of nutrients, insecticides, herbicides and metals are of special interest when monitoring the physicochemical status of any given stream or river, including IRES. Strategies for physicochemical monitoring differ among EU members, and, so far, few adaptations to the particularities of IRES have been undertaken (Prat et al., 2014; Sánchez- Montoya et al., 2012). The use of physicochemical parameters for ecological status assessment entails previously established reference conditions that should encompass the full range of values expected to occur naturally in each stream type. Within this context, the European Commission has published a guide of best practices for establishing nutrient concentrations to assist Member States in determining the levels of phosphorus (P) and nitrogen (N) that are likely to support good ecological status (Phillips et al., 2018). This guidance can be used to check existing boundary values or to develop new ones. Nevertheless, the range of values can lead to misleading interpretations when applied to IRES because of the high variability of physicochemical parameters observed naturally in these freshwater ecosystems. Hence, it is fundamental to consider the natural variability of IRES in order to properly manage and monitor water quality of these ecosystems (Prat et al., 2014). This chapter briefly reviews existing knowledge on the physicochemistry of IRES and how it can be used to monitor and manage these ecosystems. We first describe the temporal and spatial patterns of key physicochemical parameters in IRES. Then we provide recommendations for an effective physicochemical monitoring of these ecosystems. Finally, we highlight critical issues that must be addressed for proper assessment and management of water physicochemical quality in IRES.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
