A novel approach to analysing the regimes of temporary streams in relation to their controls on the composition and structure of aquatic biota

Abstract

Temporary streams are those water courses that undergo the recurrent cessation of flow or the complete drying of their channel. The structure and composition of biological communities in temporary stream reaches are strongly dependent on the temporal changes of the aquatic habitats determined by the hydrological conditions. Therefore, the structural and functional characteristics of aquatic fauna to assess the ecological quality of a temporary stream reach cannot be used without taking into account the controls imposed by the hydrological regime. This paper develops methods for analysing temporary streams' aquatic regimes, based on the definition of six aquatic states that summarize the transient sets of mesohabitats occurring on a given reach at a particular moment, depending on the hydrological conditions: <i>Hyperrheic, Eurheic, Oligorheic, Arheic, Hyporheic</i> and <i>Edaphic</i>. When the hydrological conditions lead to a change in the aquatic state, the structure and composition of the aquatic community changes according to the new set of available habitats. We used the water discharge records from gauging stations or simulations with rainfall-runoff models to infer the temporal patterns of occurrence of these states in the Aquatic States Frequency Graph we developed. The visual analysis of this graph is complemented by the development of two metrics which describe the permanence of flow and the seasonal predictability of zero flow periods. Finally, a classification of temporary streams in four aquatic regimes in terms of their influence over the development of aquatic life is updated from the existing classifications, with stream aquatic regimes defined as <i>Permanent, Temporary-pools, Temporary-dry</i> and <i>Episodic</i>. While aquatic regimes describe the long-term overall variability of the hydrological conditions of the river section and have been used for many years by hydrologists and ecologists, aquatic states describe the availability of mesohabitats in given periods that determine the presence of different biotic assemblages. This novel concept links hydrological and ecological conditions in a unique way. All these methods were implemented with data from eight temporary streams around the Mediterranean within the MIRAGE project. Their application was a precondition to assessing the ecological quality of these streams

An eco-hydrological assessment method for temporary rivers. The Celone and Salsola rivers case study (SE, Italy)

“…In this classification, streams are classified as temporary or permanent river, and temporary rivers are differentiated in their turn in three types on the basis of the control imposed on the biological communities by the absence of flow (Gallart et al, 2012):…”

“…8) where the metric values are incompatible (i.e., if M f takes the value 0.4, as a result of equation (1), Sd 6 cannot assume values higher than 0.8). The red lines used in the plot to differentiate the river types are fixed after analyzing data recorded in several streams in Mediterranean area (Gallart et al, 2012).…”

“…Gallart et al (2012) defined the sets of ecological relevant mesohabitats that can occur in a reach in a certain period depending on the hydrological conditions "Aquatic States" (ASs). They identified six classes of flow (ASs): Hyperrheic (flood), Eurheic (high flow), Oligorheic (continuous low flow or connected pools), Arheic (flow close to zero or disconnected pools), Hyporheic (surface water is absent, there is only hyporheic flow) and Edaphic (surface and hyporheic flow are zero).…”

“…A large number of indices were developed in attempts to characterize different components of hydrological regime for ecological or management purpose (Richter et al, 1996). Gallart et al (2012) identified two metrics as relevant for the qualitative status of a temporary river: the relative annual number of months with flow (M f ) and the 6 months dry season predictability (Sd 6 ) defined by the following equation (1):…”

“…In this paper, we present the hydrological tool aimed at: (1) classifying rivers, (2) characterizing the hydrological regime at reach scale as described by Gallart et al (2012) and (3) evaluating the hydrological alterations due to anthropogenic pressures in a temporary river. Our purpose is to give biologists an overall assessment of regime that can contribute in planning the biological samplings and provide water resource managers an easy tool which could facilitate any investigation into the effects of hydrological modifications within the biotic composition in temporary rivers.…”