Species-rich freshwater invertebrate communities recover quickly when flowing water returns, with intrepid recolonists arriving from nearby wet refuges, while invertebrate eggs, juveniles and adults can survive in dry riverbed sediments.
A river without water may seem like a contradiction in terms, but temporary streams - which may lose all surface water during dry seasons - are the dominant river type in arid and mediterranean regions.
In addition, temporary streams occur in temperate climates - and they are becoming increasingly common in the UK.
Temporary flow may be natural, reflecting interactions between factors such as catchment geology and rainfall inputs. For example, in England's Peak District, the limestone bedrock - which gives us the undulating White Peak landscape - contains cracks and fissures into which surface water is lost.
As a result, temporary streams including Staffordshire's River Manifold (pictured) may experience long, dry summers, interrupted only by unpredictable flow pulses if the heavens open.
Elsewhere, previously permanent streams have shifted to a temporary flow regime as natural catchment features interact with human activities. In particular, over-abstraction for public water supply and irrigation of agricultural land can lead to greater drying of freshwater systems, including iconic chalk streams such as Hertfordshire's River Mimram, as reported by The Ecologist in 2010.
In addition, climate change is resulting in extreme climatic events becoming more common in rivers, including changes in the frequency, intensity, and duration of droughts. Predictions of future changes in drought occurrence are uncertain and vary regionally. However, in densely populated countries such as the UK, droughts typically collide with peaks in abstraction to reduce instream water levels and increase drying events.
Temporary streams support biodiverse aquatic communities - even during dry phases
Temporary streams support high levels of biodiversity, with aquatic and terrestrial communities taking turns to thrive in the ever-changing instream habitats.
Freshwater invertebrate communities may be species-rich, in particular during flowing phases, and these small, spineless animals support food webs that include national treasures such as otters, kingfishers and brown trout. As flows decline, a series of isolated pools may form which - if the timing is right - could provide spawning habitats for amphibians.
Species-rich freshwater invertebrate communities recover quickly when flowing water returns, with intrepid recolonists arriving from nearby wet refuges. Invertebrate eggs, juveniles and adults can also survive in dry riverbed sediments, forming a drying-tolerant 'seedbank'.
Seedbank inhabitants include caddisfly larvae, stonefly nymphs, beetles and snails, some of which are temporary stream specialists. These hardy souls are poised to contribute to rapid community recovery, but survival is not guaranteed, and sediment moisture levels are one crucial determinant of persistence within a seedbank.
Droughts and human land use are threats to temporary stream biodiversity
Recognition of the seedbank as a mechanism by which invertebrates survive in temporary streams comes with a warning: seedbanks are at risk. Research recently published in the journal Freshwater Biology indicates that droughts reduce seedbank survival.
During a multi-season drought, the abundance and richness of seedbank assemblages fell as the dry period length increased in the Peak District's River Lathkill, and only the hardiest worms, fly larvae, and pea mussels survived in sediments that remained dry for eight months.
Human land use poses an additional threat to seedbank diversity. Specifically, clearance and management of riverbank vegetation in agricultural and urban landscapes has exposed riverbeds to direct sunlight.
For example, sheep roam the riverbanks in the temporary River Lathkill headwaters, where a carefully managed grazing regime supporting valuable limestone grassland habitat. The solar radiation increases evaporation and reduces sediment moisture, potentially reducing invertebrate survival in the seedbank - especially during droughts.
Professor Paul Wood from Loughborough University, who worked on the Peak District study, has emphasized the effects multi-season drought have on aquatic invertebrates, noting that "we need to examine how different species respond to streambed drying, and to determine species' ability to survive as drying-tolerant life stages. We will then better understand the likely effects of future changes in drying regimes on these enigmatic communities."
Restoration is needed to safeguard temporary stream communities
How can we help biodiverse communities, in particular invertebrate seedbanks, persist in drying riverbed sediments? Various management and restoration activities can promote moisture retention and therefore invertebrate survival.
Tall bankside vegetation casts shade that reduces evaporation from the channel, and so restricting mowing, cutting, and grazing along some riverbank stretches can create moist refuges that support diverse seedbanks. More ambitious schemes may revegetate riverbanks with a mix of plants including shrubs and trees, to cast dense shade along some reaches.
Such riparian revegetation projects can be simple to implement and have multiple benefits for ecosystem health, due to improved bank stabilization, and interception of fine sediment and nutrient-laden runoff. In addition, seasonal leaf litter inputs provide energy resources for invertebrates, including those emerging from seedbanks. And, as highlighted in The Ecologist in 2014, trees improve water infiltration and so regulate water delivery to rivers - reducing peak flood flows, and also delaying the onset of drying events.
Other restoration activities can provide additional, localized moist refuges even in unshaded channels. For example, sediments beneath woody debris lose less moisture by evaporation, and so fallen dead wood should be left instream - or even intentionally introduced. In addition, as shown in the Peak District study, sediments supplied by upwelling groundwater can be biodiversity hotspots for invertebrate seedbanks, highlighting the need for careful management of groundwater abstraction.
We need to work together to protect temporary stream communities
The Water Framework Directive (WFD) is an ambitious piece of EU legislation that has resulted in major improvements to the ecological health of European freshwaters. However, intensive efforts to achieve the WFD goal of ecological and chemical 'Good Status' in all freshwaters have, until recently, overlooked temporary streams.
Regardless of whether the UK does actually leave the EU, scientists at the Environment Agency (the Competent Authority for the WFD in England) now recognize the challenges of achieving Good Status in temporary streams. Environment Agency Research Scientist Dr. Judy England, has stressed that "we need to better understand temporary streams so that we can assess their environmental quality, diagnose the pressures they face, then develop measures to restore their quality."
So, challenges have been identified in relation to temporary stream protection. Firstly, we need to determine the current ecological status of temporary streams: how can we tell a 'healthy' ecosystem from an impacted one? In particular, we need to learn to distinguish community responses to the natural stressor of intermittent flow from responses to the impacts of human activities.
Ultimately, suitable 'biomonitors' need to be identified, to allow effective programmes for ecological quality assessment to be developed. Opportunities have been identified too - opportunities to protect and manage freshwater ecosystems that experience natural flow intermittence.
As climate change and water resource pressures interact to make temporary streams an increasingly common feature of our landscape, it is vital that regulatory agencies, landowners and other interested parties work together to maintain and enhance the ecological integrity of these unique ecosystems and the biodiverse communities they support.
The paper: 'Macroinvertebrate seedbank composition in relation to antecedent duration of drying and multiple wet-dry cycles in a temporary stream' is by Rachel Stubbington, John Gunn, Sally Little, Thomas P. Worrall & Paul J. Wood, and published in Freshwater Biology as an open access paper.
Rachel Stubbington is a Senior Lecturer in Ecology and Environmental Sciences in the School of Science & Technology at Nottingham Trent University. She is a research-active freshwater ecologist who regularly publishes articles in journals including Freshwater Biology. She is a core member of an EU-funded COST Action on the Science and Management of Intermittent Rivers and Ephemeral Streams. The views expressed in this paper are those of the authors and do not represent the views of the organizations they work for. She tweets @rstubbington.