We think of rivers as ecosystems characterized by flowing water, but in temporary streams, flow is intermittent: sometimes water stops moving, and a riverbed may dry completely. That temporary streams dominate river networks in arid regions may come as no surprise, but these dynamic ecosystems have also recently been recognized as common, biodiverse, and ecologically valuable in cooler, wetter, temperate regions, as reported in a new opinion article in WIRES Water.
Temporary rivers fluctuate between flowing, pool and dry states, making their instream habitats more dynamic and variable than those in equivalent rivers with year-round flow. In response to these hydrological changes, temporary river residents come and go, with an aquatic fauna including insects, crustaceans, snails and even fish soon arriving from nearby refuges after flow returns. These temporary stream inhabitants include specialist and rare species that are outcompeted in rivers with permanent flow. However, many aquatic organisms are sensitive to drying, and after water is lost, their disappearance makes way for a diverse range of terrestrial recolonists including beetles, ants and spiders. These regular shifts in community composition mean that biodiversity can be higher in temporary streams compared to systems with continuous flow when a full annual cycle is considered.
Temporary rivers are often perceived as a symbol of crisis in temperate landscapes: a problem to be fixed. This may be true, for example where water abstraction to irrigate arable land during drought causes a naturally perennial stream to dry. In contrast, the ecological value of temporary streams that form natural features within a temperate landscape requires greater recognition, and these systems need better representation in the hydrological and biological monitoring work done by regulatory agencies to assess ecological health. Characterizing the communities that represent ‘good health’ creates unique challenges in ecosystems that shift between aquatic and terrestrial states – challenges currently being addressed by a collaborative European research project on the ‘Science and Management of Intermittent Rivers and Ephemeral Streams’. Where ecological health assessments identify streams that fail to reach the quality expected by international legislation, interventions may be required to re-establish a natural flow regime, restore instream habitat mosaics, and allow the biodiverse biota of temporary streams to thrive.
Contributed by Rachel Stubbington