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 We synthesised published and unpublished data from studies that had experimentally rehydrated sediments collected from dry riverbeds, to establish the importance of the seedbank in promoting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

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

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

Lipid Bilayers on Deformable Elastic Substrates

In this thesis an experimental model for the interface between the cell membrane and the supporting cytoskeleton has been developed and analysed. The experimental platform is a novel approach to the design of supported membrane based devices and technologies. The system consists of a single component lipid bilayer coupled to an elastic substrate, the area of which can be reversibly increased and decreased. We uncover three independent mechanisms that the membrane may use to respond to changes in substrate area. If the elastic support is partially hydrophilic, the area of the planar portion of the membrane is strongly coupled to the substrate area. The membrane responds to increasing substrate area by absorbing lipid protrusions, and when the substrate area is decreased the excess membrane area is projected back out in the form of lipid tubes. This mechanical remodelling of the membrane occurs above the plane of the support and mimics the passive means of membrane area regulation recently uncovered in live cells. In contrast, when the surface support is completely hydrophilic, two further mechanisms of substrate stress relaxation are uncovered. When the pH of the solu- tion is greater than 7 the membrane is able to slide over the expanding and contract- ing substrate. This membrane sliding motion occurs in the plane of the support and is dynamic. The effectiveness at which membrane tension is relaxed is dependent on the rate at which the substrate area is changed. When the pH is reduced below pH 7, the membrane area becomes strongly coupled to that of the support and the membrane dramatically ruptures, opening large circular pores, in response to substrate deformation. The pores exhibit a dynamic area change, revealing a complex flow of membrane across the support to equilibrate stress. This novel supported membrane behaviour reveals the rich physics possessed by supported lipid systems, that may assist in the design of new supported lipid based technologies

Benthic and hyporheic invertebrate community responses to seasonal flow recession in a groundwater-dominated stream

Natural hydrological variability in lotic ecosystems can include prolonged periods of flow recession. A reduction in discharge is accompanied by abiotic changes in benthic and hyporheic habitats, often including reductions in s habitat availability. Whilst the benthic invertebrate community response to low flows is well documented, little research has considered how the composition of the community within the hyporheic zone is affected. We examined benthic and hyporheic invertebrate community composition during flow recession in a temperate karst stream, at sites with contrasting historic flow permanence regimes. Changes in the benthic invertebrate community composition primarily reflected changes in habitat availability associated with discharge variability; in particular, the population density of the dominant amphipod, Gammarus pulex, increased as the area of submerged benthic sediments declined. Concurrent significant increase in the hyporheic abundance of G. pulex, and moderate increase in the proportion of the total G. pulex population inhabiting the hyporheic zone were recorded. It is postulated that G. pulex migrated into the hyporheic zone to reduce exposure to intensifying biological interactions in the benthic sediments. Increase in the hyporheic abundance of G. pulex was particularly pronounced at sites with historic intermittent flow, which could be attributed to downwelling stream water dominating vertical hydrologic exchange. The increase in G. pulex abundance reduced community diversity in the benthic sediments, but had no apparent detrimental effects on the hyporheic invertebrate assemblages

Hyporheic invertebrates as bioindicators of ecological health in temporary rivers: a meta-analysis

Worldwide, many rivers cease flow and dry either naturally or owing to human activities such as water extraction. However, even when surface water is absent, diverse assemblages of aquatic invertebrates inhabit the saturated sediments below the river bed (hyporheic zone). In the absence of surface water or flow, biota of this zone may be sampled as an alternative to surface water-based ecological assessments. The potential of hyporheic invertebrates as ecological indicators of river health, however, is largely unexplored. We analysed hyporheic taxa lists from the international literature on temporary rivers to assess compositional similarity among broad-scale regions and sampling conditions, including the presence or absence of surface waters and flow, and the regional effect of hydrological phase (dry channel, non-flowing waters, surface flow) on richness. We hypothesised that if consistent patterns were found, then effects of human disturbances in temporary rivers may be assessable using hyporheic bioindicators. Assemblages differed geographically and by climate, but hydrological phase did not have a strong effect at the global scale. However, hyporheic assemblage composition within regions varied along a gradient of higher richness during wetter phases

Sticking and sliding of lipid bilayers on deformable substrates

We examine here the properties of lipid bilayers coupled to deformable substrates. We show that by changing the extent of the substrate hydrophilicity, we can control the membrane–substrate coupling and the response of the bilayer to strain deformation. Our results demonstrate that lipid bilayers coupled to flexible substrates can easily accommodate large strains, form stable protrusions and open reversibly pores. These properties, which differ significantly from those of free standing membranes, can extend the applications of the current lipid technologies. Moreover, such systems better capture the mechanical architecture of the cell interface and can provide insights into the capacity of cells to reshape and respond to mechanical perturbations