Benthic and interstitial habitats of a lentic spring as invertebrate refuges during supra-seasonal drought

Instream refuges are places where invertebrates persist during disturbances due to reduced adverse impacts. During droughts, low flows may be accompanied by elevated temperatures, and potential refuges including subsurface sediments and spring-fed headwaters are therefore characterized by hydrological and thermal stability. This study examined invertebrate use of benthic and interstitial habitats (analogous to the hyporheic zone) in a groundwater-dominated, perennial limnocrene spring during a supra-seasonal drought. Although exceptionally high air temperatures occurred as flow declined, environmental conditions in the spring were relatively stable, and refuge-seeking vertical migrations into interstitial habitats did not coincide with peak temperatures. However, maximum benthic abundance of two amphipods (Gammarus pulex and Crangonyx pseudogracilis) occurred shortly after the period of elevated temperatures. It is suggested that this temporary increase in the abundance of these mobile taxa reflected upstream migrations triggered by a combination of refuge-seeking behaviour and thermally-stimulated activity. In addition, the spring provided a passive refuge for many lotic invertebrate taxa. A conceptual model is presented, which illustrates the potential contribution of multiple refuges to invertebrate persistence in drought-impacted ecosystems

The macroinvertebrate seedbank promotes community persistence in temporary rivers across climate zones

1. Aquatic macroinvertebrates inhabiting temporary rivers are typically described as having low resistance to riverbed drying. However, little research has examined the ‘seedbank’ within dry riverbed sediments, which comprises aquatic life stages that survive in dewatered sediments and from which active organisms may develop only after surface water returns. 2. We synthesised published and unpublished data from studies that had experimentally rehydrated sediments collected from dry riverbeds, to establish the importance of the seedbank in promoting macroinvertebrate community resistance. Studies from across climate zones were included, to examine seedbank importance in relation to environmental harshness and, in particular, sediment moisture. We also assessed the importance of the seedbank relative to alternative habitats promoting persistence of the flowing river (FR) assemblage. We predicted that the proportion of the FR assemblage present in rehydrated sediments (RS) would decrease with environmental harshness, due to conditions within the sediments becoming less conducive to the survival of biota. 3. A negative relationship between the proportion of FR taxa present in RS and harshness was observed, and this contributed to a reduction in the compositional similarity of FR and RS assemblages as harshness increased. Significant positive correlations were identified between sediment moisture content and macroinvertebrate community metrics (density and taxon richness) in some systems. 4. Habitats external to the dry reach, which contribute to community resilience, were invariably inhabited by a greater number of FR taxa than rehydrated sediments. However, rehydrated sediments included several FR taxa that were not found in any other habitats during the dry phase, including families of Coleoptera and Diptera. 5. Our results indicate the importance of the seedbank as a resistance mechanism for temporary river macroinvertebrates. With climate change scenarios predicting an increase in riverbed drying, maintaining habitats that facilitate the persistence of instream communities during dry phases is an increasing priority. We identified strong relationships between sediment moisture and taxon richness, and river management and rehabilitation activities should therefore aim to retain moisture in drying sediments, by manipulating parameters such as riparian shading

Spatial variability in the hyporheic zone refugium of temporary streams

A key ecological role hypothesized for the hyporheic zone is as a refugium that promotes survival of benthic invertebrates during adverse conditions in the surface stream. Many studies have investigated use of the hyporheic refugium during hydrological extremes (spates and streambed drying), and recent research has linked an increase in the abundance of benthic invertebrates within hyporheic sediments to increasing biotic interactions during flow recession in a temporary stream. This study examined spatial variability in the refugial capacity of the hyporheic zone in two groundwater-dominated streams which flow permanence varied over small areas. Two non- insect taxa, Gammarus pulex and Polycelis spp. Were common to both streams and were investigated in detail. Hydrological conditions in both streams comprised a four- month period of flow recession and low flows, accompanied by reductions in water depth and wetted width. Consequent declines in submerged benthic habitat availability were associated with increases in population densities of mobile benthic taxa, in particular G. pulex

Occurrence of 'Antrobathynella stammeri' (Jakobi, 1954) (Crustacea: Syncarida: Bathynellidae) in the hyporheic zones of two English karst rivers

This paper reports the first records of Antrobathynella stammeri in the British Isles since 1985. Numerous individuals were observed in hyporheic water extracted from the River Skirfare (Yorkshire, UK), whilst, in an independent study, a single confirmed specimen was recorded from hyporheic water pumped from the River Lathkill (Derbyshire, UK). The latter observation is the first record of the superorder Syncarida in the Peak District and provides an important geographical link between previously recorded individuals in Scotland and north-west England and records from the south of England. These records suggest that bathynellids may be more abundant in the UK than previously thought, highlighting the need for more effective sampling of subterranean habitats to determine the conservation status of such rarely seen species

The first occurrence of the Ponto-Caspian invader, Hemimysis anomala G.O. sars, 1907 (Mysidacea) in the UK

An invasive Ponto-Caspian mysid, Hemimysis anomala G.O. Sars, 1907, was recorded in England for the first time in 2004. Usually a deep water species, in England H. anomala has been observed in shallow waters, in which it shelters under or within anthropogenic structures during daylight. This behaviour renders traditional, net-based survey methods ineffective. Therefore, a distribution survey of the English East Midlands was conducted by searching for individuals by torchlight after dark. H. anomala was found to be widespread within the study area, occurring at 24 out of 51 sites surveyed. However, the geographical limits of its distribution were not determined. The species occurred at low densities in canals and in backwaters of the River Trent, whilst dense swarms were observed in September 2005 in a regatta lake connected to the River Trent. H. anomala has the potential to spread through England's canal network and could colonize the lower reaches and estuaries of rivers including the River Thames and River Severn. Habitat preference analysis indicated that flowing water and absence of shelter prevented population establishment, although the species' U.K. distribution suggests that it can migrate through such areas of unsuitable habitat

Changes in invertebrate assemblage composition in benthic and hyporheic zones during a severe supraseasonal drought

Droughts are unpredictable disturbances characterized in streams by declining flow, reduced habitat availability, and deteriorating abiotic conditions. Such events typically reduce benthic invertebrate taxon richness and modify assemblage composition, but little is known about how hyporheic invertebrate assemblages respond to drought or how these responses relate to changes in benthic assemblages. We hypothesized that taxon richness (diversity) and variability (as within-site diversity) in benthic assemblage composition would decline as drought proceeded, whereas concurrent changes in hyporheic assemblages would be lower in this more stable environment. We predicted that benthic assemblage composition between sites would converge as epigean taxa were selectively eliminated, whereas between-site hyporheic diversity would change little. We sampled benthic and hyporheic invertebrates concurrently from 4 sites along a groundwater-fed stream during the final stages of a severe supraseasonal drought punctuated by a record heat wave. Abiotic conditions in benthic habitats deteriorated as flow declined, but changes were less pronounced in the hyporheic zone. Benthic diversity declined during drought, whereas hyporheic diversity changed little. However, benthic within-site diversity increased as the drought progressed because of localized variation in the abundance of common taxa. Temporal trends in hyporheic diversity were less consistent. Benthic assemblages at individual sites became more similar, especially during the heat wave, reflecting low diversity and abundance. Hyporheic assemblages changed markedly because of temporary increases in abundances of epigean and hypogean amphipods. These contrasting responses of benthic and hyporheic assemblages to drought should be recognized when developing management strategies for drought-impacted streams

Characterization of the density and body size of a Gammarus pulex (Crustacea: Amphipoda) population in subsurface sediments reflects the sampling technique used

The hyporheic zone and its invertebrate fauna play vital roles in the functioning of lotic ecosystems. However, although sampling invertebrates from subsurface sediments is recognized as challenging, few studies have quantified the effectiveness of common sampling techniques. We conducted laboratory experiments to compare two common, semi-quantitative pump-sampling techniques – Bou-Rouch and vacuum-pump sampling. We determined the proportion of a Gammarus pulex (Crustacea: Amphipoda) population sampled by each method in five sediment treatments comprising coarse (16-32 mm), medium (8-16 mm), and fine (4-8 mm) gravel and combinations thereof. We compared the body size of individuals sampled and not sampled by each technique to the population mean. Density estimates obtained using both methods were low: 33 ± 5 and 5 ± 1% of the population present for Bou-Rouch and vacuum-pump samples, respectively. Density estimates were significantly higher for Bou-Rouch than for vacuum-pump samples in four sediment treatments, but were comparable in coarse gravel. The body size of organisms captured by the Bou-Rouch technique was comparable to the population mean, whereas those in vacuum-pump samples were smaller. With hyporheic invertebrates suggested as future biomonitors of ecosystem health, we recommend Bou-Rouch sampling as the more effective pump sampling method for community characterization

Drying duration and stream characteristics influence macroinvertebrate survivorship within the sediments of a temporary channel and exposed gravel bars of a connected perennial stream

Intermittent rivers, which experience periods of flow cessation and streambed drying, occur globally. Given that the frequency and duration of stream drying events is likely to increase as a result of anthropogenic pressures and global climate change, riverbed sediments may become increasingly important as refuge habitat for benthic macroinvertebrates. Our study examined the effect of surface water loss and increasing drying duration on the survivorship of the most abundant benthic invertebrate, Gammarus pulex (L.) (Amphipoda: Gammaridae), inhabiting the wet subsurface sediments of exposed gravel bars within a perennial stream and a connected temporarily flowing side channel. G. pulex survivorship declined more over time during drying conditions compared to control conditions (flowing water present). Survivorship was greater in the temporary channel and may reflect the greater water retention capacity of fine sediments in the subsurface and abiotic stability compared to the free-draining exposed gravel bars on the main channel. Our results illustrate that saturated subsurface sediments may facilitate G. pulex persistence during surface drying events and highlight the need for effective refuge management and conservation for instream fauna during drying events

Flow intermittence in river networks: understanding the ecohydrological diversity of aquatic–terrestrial ecosystems

Temporary streams comprise dynamic mosaics of lotic, lentic and terrestrial habitats and dominate global river networks, occurring across regions with contrasting climate types. Recent advances in our ecohydrological understanding of temporary streams have focused on systems in arid, semi-arid and mediterranean climates. In this special issue, we present new temporary stream research from underrepresented regions, primarily cool, wet temperate climates but also continental central Europe and the mediterranean-climate region of South Africa. We bring together observational case studies, laboratory experiments, and field surveys spanning surface water and groundwater habitats. Papers within the special issue explore ecological responses to flow intermittence; examine biodiversity patterns of rare and endemic species at broad spatial scales; characterize diverse responses to drying events within and among populations; demonstrate the value of long-term observational data in understanding the hydrological drivers that underpin biotic responses; and present opportunities to improve temporary stream monitoring and management. Collectively, these contributions complement dryland research to advance global understanding of temporary stream ecohydrology. However, the terrestrial communities that inhabit dry channels remain a notable research gap, which we address in a review of global literature. As global change causes an increase in their extent across climate regions, we urge researchers and stakeholders to collaborate to implement recommendations that address the challenges associated with the effective management of temporary streams as aquatic–terrestrial ecosystems