Biodiversity embraces multiple facets of the variability of nature, although most research has dealt separately with population-, species- and assemblage-level measures. This review concentrates on aquatic insect biodiversity and the assemblage-level measures, such as species richness, assemblage compositional variation, taxonomic distinctness and functional diversity. Most studies on aquatic insects have considered biodiversity patterns based on surveys of local assemblages along geographical and environmental gradients, while there is a virtual lack of studies that have considered regional grain sizes (i.e. the size of the observational unit). Latitudinal gradients at both regional and local grain are ambiguous in aquatic insects, as different studies have found either higher or lower local diversity in the tropics than in the temperate zone. Other geographical patterns in aquatic insect diversity may also be relatively weak, as suggested by subtle among-ecoregion differences in both local species richness and assemblage composition. An ecological explanation for the absence of strong geographical gradients is that local environmental features may not necessarily vary with geography, and these factors may override the influences of historical and climatic influences on local diversity. Evidence from large-scale studies suggests, however, that not only various local habitat and ecosystem variables, but also those measured at the watershed, regional and geographical scales are needed to account for variation in the species richness and assemblage composition of aquatic insects. Within regional species pools, species richness and assemblage composition in both lotic and lentic ecosystems vary most strongly along gradients in habitat size and acidity. Knowing how α-, β- and γdiversity of aquatic insects vary along geographical and environmental gradients has important implications for the conservation of biodiversity in freshwater ecosystems. Such knowledge is not yet well-developed, and two aspects should be considered in future research. First, further survey research on lotic and lentic ecosystems is necessary for improving our understanding of general biodiversity patterns. Second, given that freshwater ecosystems are facing a severe biodiversity crisis, the implementation of representative networks of freshwater protected areas would certainly benefit from increased understanding of patterns in aquatic insect biodiversity.

The increasing incidence of mass developments of Cyanobacteria in fresh- and brackish water is a matter of growing concern due to the production of toxins that threaten human and livestock health. The toxins that are produced by freshwater Cyanobacteria comprise hepatotoxins (cyclic peptides such as microcystins and nodularin, as well as alkaloids such as cylindrospermopsin) and neurotoxins (alkaloids such as anatoxin-a, anatoxin-a(S) and saxitoxins). The variation in toxicity between and within species of Cyanobacteria has been recognised for a long time. However, the toxic and non-toxic genotypes within a species cannot be discriminated under the microscope, which has been a major obstacle in identifying those factors that influence toxin production both in the laboratory and in the field. During the last decade, major advances were achieved due to the elucidation and functional characterisation of genes, such as the gene cluster encoding the synthesis of the hepatotoxic heptapeptide, microcystin. Genetic techniques, in particular, have been used to explore (i) the genetic basis, biosynthesis pathways, and physiological regulation of toxin (microcystin) production, (ii) gene loss processes resulting in a patchy distribution of the microcystin synthetase gene cluster among genera and species, as well as (iii) the distribution and abundance of the microcystin genes in the environment. In recent years, experience in detecting microcystin genes directly in the field has increased enormously and robust protocols for the extraction of DNA and the subsequent detection of genes by PCR (polymerase chain reaction)-based methods are now available. Due to the high sensitivity of PCR, it is possible to detect toxic genotypes long before a toxic cyanobacterial bloom may occur. Consequently, waterbodies that are at risk of toxic bloom formation can be identified early on in the growing season along with environmental factors that can potentially influence the abundance of toxin producing genotypes.

Current work on adaptation responses for conservation management in the face of predicted climate change has a distinctly terrestrial focus. Existing evidence for the potential impact of climate change on freshwater ecosystems indicates that it is the interaction between direct climate change and current anthropogenic pressures that is likely to define the way in which freshwater biodiversity is affected. A brief overview of the likely effect of climate change upon fresh waters is presented. In light of this review, possible actions to safeguard freshwater biodiversity in the face of climate change are discussed. Management challenges and proposed responses are presented at the level of individual sites, at the landscape scale and according to management policy drivers. Many of the principles underlying these proposed adaptation responses need a more extensive and robust evidence base and an attempt is made here to highlight the key research gaps.

Attention is directed to the many potential uses of specific electrical conductance (‘conductivity’) in the study of inland waters. Its measurement is capable of a precision useful in the detection of differences in a standing or flowing water-mass, but cannot be translated into measures of chemical concentration with equivalent absolute accuracy. Reasons — not infrequently neglected — include approximations in temperature correction, in allowance for a depression effect at higher ionic strength (salinity), and especially in the differences of specific conductance of chemically different ions. The last can be reduced by treating ionic concentration in chemical equivalents (e.g. meq L^-1) rather than the usual units of mass (e.g. mg L^-1) or molarity (e.g. mmol L^-1); also by making allowance for the exceptionally high equivalent conductance of H⁺and OH- ions of significance in markedly acid and alkaline waters.

Measurement in the field has been helped by the development of small portable instruments with inbuilt temperature compensation, flow-through electrode systems and electrical output. Examples of both field and laboratory measurements, for the charting and interpretation of various field situations, are illustrated chiefly from the author's experience. They include broad chemical surveys; interrelation with normal chemical analysis; longitudinal change, water travel and nutrient uptake in river and stream systems; ionic particulate conversion; horizontal and vertical differentiation in lakes; and ionic changes induced by photosynthesis.

The Shropshire and Cheshire meres of north-west England are characterised by high phosphorus concentrations. This review assesses the importance of phosphorus and nitrogen concentrations in determining the water and ecological quality of the meres. Palaeolimno—logical evidence indicates that the meres may be naturally eutrophic, but that phosphorus concentrations have increased in the past century. Results show that nitrogen concentrations have also increased and support the hypothesis that high concentrations of nitrogen contribute to reduced macrophyte species richness. In light of the evident significance of nitrogen, the potential role of nitrogen concentrations in driving eutrophication in the meres is discussed.