Although we are still developing our understanding, there is increasing recognition that groundwater is essential to many ecological communities. Groundwater is a connector, not just in the aquifer itself, but within, across, and between surface waters and many terrestrial ecosystems. Where the water table intersects or comes close to the land surface, contributions of water and nutrients to plant roots and aquatic ecosystems can be critical to their persistence. Consider that precipitation is the dominant source of water in nearly all wetland systems, yet the influence of the lesser groundwater flow component can be sufficient from an ecological perspective to yield an entire new type of wetland, the fen. Influxes of groundwater to lakes, rivers, and wetlands can change whole-system physico–chemical properties such as temperature and salinity, while also providing more subtle influences on microenvironments and their ecological processes. Infiltration of water from surface aquatic ecosystems and rainfall can have an equally significant effect on aquifer ecology, especially on microbes and subsurface invertebrates. Whether water is flowing into or out of an aquifer, or is moving from one part to another, it is the extent and intensity of connectivity that often determines its importance to ecosystems. Moreover, the same location in space can have all three types of flows at different periods of time. Surface ecological processes (such as evapotranspiration) can significantly impact hydrological responses and related hydrochemical function. Thus, the relation of groundwater hydrology to patterns and processes in ecology is a 'two-way street' where understanding the feedback of one to the other serves as a powerful lens through which to evaluate and explain the functioning of natural ecosystems.

Freshwater ecosystems are under increasing pressure as human populations grow and the need for clean water intensifies. The demand for ecologists and environmental managers who are trained in basic freshwater ecology has never been greater. Students and practitioners new to the field of freshwater ecology and management need a text that provides them with an accessible introduction to the key questions while still providing sufficient background on basic scientific methods. This book represents the only freshwater ecology textbook that is specifically aimed at an introductory level. It will also be a useful primer for students who have not previously taken a specialized freshwater course but who require an accessible overview of the subject.

Many river restoration projects seek to address issues associated with impaired hydrological and ecological connectivity in longitudinal (e.g. effects of dams, weirs) or lateral (e.g. alienated floodplain) dimensions. Efforts to restore the vertical dimension of impaired stream–groundwater exchange are rare, hampered by limited understanding of the factors controlling this linkage in natural alluvial rivers. We propose a simplified two-axis model of the 'primary drivers' (sediment structure and vertical hydraulic gradient) of stream–groundwater exchange that acknowledges their interaction and provides a practical template to help researchers and river managers pose hypothesis-driven solutions to restoration of damaged or lost vertical connectivity. Many human activities impact on one or both of these drivers, and we review some of the tools available for treating the causes (rather than symptoms) in impacted stream reaches. For example, creating riffle-pool sequences along stream reaches will enhance vertical hydraulic gradient, whereas flushing flows can remove clogging layers and sustain sediment permeability. Our model is a first step to specifying mechanisms for recovery of lost vertical connectivity. Assessing results of river restoration using this approach at reach to catchment scales will provide scientific insights into the interplay of hydrology, fluvial geomorphology and river ecosystem function at appropriately broad scales.

Exotic plants have invaded the riparian zones of many streams worldwide, but their consequences for stream fauna are seldom fully appreciated, especially when effects are sublethal. In northern New South Wales, Australia, the exotic tree camphor laurel ('Cinnamomum camphora') has aggressively invaded riparian zones of many subtropical streams, often forming monocultures and outcompeting native vegetation. In forested streams, leaf litter provides a major source of food for some stream fauna and is broken down by shredding invertebrates. We hypothesized that shredder densities would be reduced in streams where the riparian zone was dominated by camphor laurel because of the reputed chemical toxicity of the leaf litter. We also investigated the sublethal effects of camphor laurel litter by comparing growth rates of larvae of the common shredding caddisfly 'Anisocentropus' (Calamoceratidae: Trichoptera) reared on a diet of camphor laurel vs native leaf litter. Shredder densities were significantly lower in streams where camphor laurel constituted .38% of the benthic leaf litter than in streams with a lower percentage of camphor laurel litter (0.43 ± 0.13 vs 1.43 ± 0.28 individuals/g dry mass of litter; mean ± SE). Shredder densities were greater on artificial packs of native leaf litter than on packs of camphor laurel litter in camphor laurel–infested streams (3.37 ± 1.01 vs 1.40 ± 0.46 ind./g dry mass of litter). However, shredder densities did not differ between leaf-litter types in streams lined with native vegetation. 'Anisocentropus' growth was retarded when larvae were fed only camphor laurel compared to those reared on native leaf litter. This native caddisfly used camphor laurel leaves for case construction, but impaired larval growth in infested streams implies a sublethal impact that has repercussions for energy transfer to higher trophic levels (e.g., predatory fish). Our findings suggest effects similar to those often reported for shredder assemblages in temperate streams subject to invasion by exotic riparian plants, indicating that the tropical– temperate differences in this aspect of leaf breakdown might not be as marked as previously suspected. Carefully managed river restoration to remove camphor laurel to allow recovery by native vegetation is recommended in these streams. Population dynamics of 'Anisocentropus' could provide a useful indicator of the ecological success of such restoration.

The pervious lateral bars (parafluvial zone) and beds (hyporheic zone), where stream water and groundwater exchange, are dynamic sites of hydrological and biological retention. The significance of these biogeochemical 'hotspots' to stream and groundwater metabolism is largely controlled by filtration capacity, defined as the extent to which subsurface flowpaths and matrix hydraulic conductivity modify water characteristics. Where hydraulic conductivity is high, gradients in biogeochemistry and microbial activity along subsurface flowpaths were hypothesized to be less marked than where hydraulic conductivity is low. This hypothesis was tested in two riffles and gravel bars in an Australian subtropical stream. At one site, gradients in chemical and microbial variables along flowpaths were associated with reduced hydraulic conductivity, longer water residence time and reduced filtration capacity compared with the second site where filtration capacity was greater and longitudinal biogeochemical trends were dampened. These results imply that factors affecting the sediment matrix in this subtropical stream can alter filtration capacity, interstitial microbial activity and biogeochemical gradients along subsurface flowpaths. This hydroecological approach also indicates potential for a simple field technique to estimate filtration capacity and predict the prevailing hyporheic gradients in microbial activity and biogeochemical processing efficiency, with significant implications for stream ecosystem function.

Much of the hydroecological and ecohydrological literature refers to 'overt' interactions in surface ecosystems such as the response of fish to hydraulics in a river (Nestler et al., Chapter 12 in this volume) or the hydroecological variability evident in vegetation distribution in wetlands (Burt et al., Chapter 14 in this volume). It is often easy to see how ecological features such as fish behaviour or water plant distribution are associated with hydrological variables. Less obvious are the 'covert' interactions where groundwater hydraulics play a role in the ecology of surface ecosystems, either through the direct supply of water for so-called groundwater dependent ecosystems (GDEs, Boulton and Hancock, 2005) or by acting as a vector for critical nutrients (Jones et al., 1995) or thermal energy (Power et al., 1999) required by surface biota and processes. However, as many lakes and rivers are largely fed by groundwater (Winter et al., 1998: Wood et al., 2001) the ecohydrological significance of this covert interaction is greater than often appreciated, and this has important ramifications for management of these acquatic ecosystems (Woessner, 2000).

1. Growing recognition of the ecological significance and biodiversity of groundwater fauna in Australia has led to statutory requirements for monitoring these communities prior to resource development. However, the efficiency of methods for assessing community composition and taxa richness remains untested, hampering the collection of reliable data for compliance. 2. We assessed the efficiency of two commonly-used methods to sample groundwater fauna (net hauls and pumping) in bores in two contrasting regions (Dubbo and upper Hunter Valley) of eastern Australia. Repeated sets of seasonal samples from the upper Hunter Valley bores were used to test whether once-off sampling was sufficient for compliance purposes. 3. All taxa collected by the net method were also captured in pump samples. In addition, pumping yielded more taxa, higher total abundances, and higher numbers of the two most common taxa (syncarids and copepods) than the net. Ten net hauls in the upper Hunter Valley bores collected, on average, 64% of the taxa and 44% of total abundance. When combined with the first 100 L from pumping, cumulative totals rose to 92.5% and 74.5% respectively. Similar trends occurred in bores near Dubbo, indicating that net hauls alone under-sampled groundwater faunal communities. Multivariate analyses revealed substantial differences in community composition as each incremental subsample was added. 4. Temporal changes in community composition in the Hunter Valley aquifer were marked yet inconsistent across the bores. Initial once-off sampling collected 30–87% of the total taxa from each bore and new taxa were still being collected after four sampling periods in over half the bores. This shows that more than one sampling occasion is needed for comprehensive assessments of groundwater biodiversity. 5. Successful compliance monitoring of groundwater fauna requires cost-effective sampling that will not overlook potentially vulnerable taxa or miss ecologically relevant changes in community composition. Currently, a lack of suitable bores seems to be the main constraint on accurate assessment of groundwater invertebrate community composition in these two areas of eastern Australia.

Despite the widely accepted importance of the hyporheic zone as a habitat for stream macroinvertebrates during floods, few data exist regarding community composition and distribution during periods of low flow or drought in perennial streams. Integrating research on hyporheic invertebrates with results from a long-term study of a UK river provided the opportunity to examine how benthic and hyporheic macroinvertebrate communities respond to inter-annual variability in river flow and periods of groundwater drought. Changes in the riverine macroinvertebrate community associated with low flow included a reduction in species richness and the number of individuals per sample, particularly aquatic insects. The hyporheic community was characterized by a relatively homogeneous composition during a period of severe low flow, punctuated by short-term changes associated with variation in water temperature rather than changes in discharge. We present a conceptual model of the processes influencing benthic and hyporheic invertebrates under low-flow conditions. Previous studies have seldom integrated these two assemblages and their interactions. The model presented highlights the potential importance of surface water and hyporheic zone linkages for riverine invertebrate communities under a range of flow conditions.

Most conservation efforts in aquatic ecosystems focus on surface waters. This is understandable given their public visibility, accessibility and, in many parts of the world, stark evidence of their vulnerability to human impact. On the other hand, programmes to protect and conserve groundwaters, and the ecosystems that rely wholly or partially on access to groundwater, are far less common. Contaminated groundwater is less obvious to the public than a blue-green algal bloom or the floating corpses from a fish kill. Increasingly, as surface waters become polluted, groundwater resource use is intensifying in many parts of the world, often mining resources faster than they are replenished (Danielopol et al., 2003). We are realizing that groundwater is not an inert, lifeless resource, but instead can harbour considerable biodiversity of invertebrates and microbes (Marmonier et al., 1993; Danielopol et al., 2000) whose activities provide crucial ecosystem services in nutrient transformation and biological filtration (Boulton, 2000; Hancock, 2002). There are growing numbers of examples of catastrophic crashes in groundwater ecosystems due to saline intrusion (Ergil, 2000) and over-extraction (Kromm and White, 1992) while in other areas, contaminants such as heavy metals are rendering groundwater toxic to millions of humans (Nickson et al., 1998; Sengupta et al., 2001). The key issue is one of public and political education about the conservation values, ecosystem services, and subsurface-surface linkages of groundwaters. While knowledge gaps still exist, I suggest that we know enough to be able to advise policy-makers and governments about the benefits of conservation of groundwaters and their dependent biota. Recent editorials in this journal have highlighted the ecological significance of hydrologic connectivity (Pringle, 2003) and restoration science at large scales (Ormerod, 2004). My contribution builds on these to provoke research and debate about conservation of groundwaters and their linkages at suitable scales, potentially requiring restoration of groundwater quality and water regime to benefit significant groundwater-dependent ecosystems (GDEs).

Human demands for water reduce total discharge and extend the natural durations of low flows in streams and rivers worldwide. Although these 'artificial droughts' are predicted to increase, their ecological effects are poorly understood, hampering management of the timing and volume of extractions to minimise damage to stream ecosystems. This study hypothesised that artificial drought would reduce wetted area, increase electrical conductivity and nutrient concentrations, and promote algal growth in a heavily extracted river when compared to a river relatively unaffected by water extraction. These responses were predicted to affect aquatic macroinvertebrate community composition, producing declines in taxon richness and density and increases in representation by drought-tolerant groups such as odonates and semi-aquatic coleopterans and hemipterans. At five sites, samples were collected weekly to monthly from riffles before, during and after greatest water extractions until flows increased again. As discharge declined, wetted area and velocity fell while conductivity increased. Changes in physical habitat were most marked at sites on the impacted river but temporal variations in wetted area strongly correlated with site-specific riffle morphology, mediating local effects on periphyton biomass and benthic invertebrates. Periphyton biomass increased as flow declined except at the site with the most severe artificial drought. This latter unexpected response apparently reflected nutrient limitation, likely due to restricted supply as runoff and nutrient transfer fell. The richness and density of aquatic macroinvertebrates did not decline with discharge, but community composition changed as taxa tolerant of low flow became more abundant, with some sensitive taxa not found in the impacted river. These changes were more strongly associated with the number of low-flow events over the long term (365 d) than shorter terms (< 90 d), suggesting cumulative effects of artificial droughts on aquatic invertebrate composition in the impacted river.