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Biodiversity is a key measure of environmental quality in lake ecosystems. Lake biodiversity can be assessed using modern survey data, but typically these data only provide a 'snapshot' measure and in most cases it is not possible to reconstruct temporal trends in biodiversity, so that human impacts can be detected. Palaeoecological techniques offer an alternative means of identifying changes in biodiversity over the period of historical records and far beyond, but there are problems associated with this approach. This is because only a select set of organisms leave a trace in the sediment record such that it is not usually possible to make reliable assessments of diversity changes within an entire taxonomic order (e.g. the algae). Moreover these organisms are typically from the lower levels of the trophic hierarchy (i.e. plants and insects).The problems of identifying changes in biodiversity from the palaeolimnological record are addressed with reference to Groby Pool, a shallow, eutrophic, medieval lake in the English Midlands, which has been subjected to eutrophication over the last 150 years. 210 Pb and 137 Cs-dated sediment cores have been used to estimate short-term alterations in the composition and diversity of three groups of indicators, representing different levels in the trophic cascade, namely diatoms, aquatic pollen and chironomids. By exploring relationships, both between these indicators and with archival macrophyte records, an assessment is made of eutrophication-related changes in overall habitat diversity at the ecosystem level. These data suggest that the lake has undergone considerable nutrient enrichment, resulting in the loss of a diverse, mesotrophic macrophyte flora from at least the turn of the century onwards and its replacement by a few highly competitive species tolerant of high nutrient concentrations. Reductions in macrophyte diversity seem to be reflected palaeoecologically by a decline in the diversity of fossil chironomid assemblages, related to the breakdown of particular host-plant relationships amongst the phytophagic species. However, diatom assemblages generally exhibit the opposite trend, which may be related to increases in macrophyte cover and increasing opportunities for the colonization of diverse epiphyte communities. The different fossil indicators have different limitations and merits, and for this reason a 'multi-proxy' approach is essential if meaningful inferences are to be made of changes in lake biodiversity using palaeoecological data.

A 10-year study of groundwater-dominated headwater streams in the chalklands of southern England has highlighted the major influence that stream flow has on the composition of in-channel macrophyte communities. Macrophytes supply much of the physical structure within these headwater channels, as well as providing habitats for a rich assemblage of higher life forms, some of them specialized and rare. The overall influence of stream flow however, is modulated by many environmental factors operating at diverse spatial scales. Data describing the relevant environmental variables were accumulated for six chalk streams in the Thames basin and related directly to the vegetation record by means of Canonical Correspondence Analysis, correlation tests and other analyses, to reveal the circumstances in which communities would be likely to thrive and those in which they would be negatively impacted by variations in stream flow. It was found that local topography and channel dimensions had a direct influence on the composition of communities at local level and that land use and rates of water abstraction had an underlying, but no less important, influence at catchment level. The species most associated with positive species diversity were those of sluggish flow and damp margins indicating the importance of velocity and moisture gradients across the channel in producing a full manifestation of the flora. Strong negative correlations were produced by non-aquatic grasses and herbs, which represent prolonged periods of channel desiccation. The environmental parameters most associated with positive diversity were high local water stages, wide channels and a high degree of semi-natural land use within the catchment. Steep channel gradients were strongly associated with negative diversity. The results have confirmed that optimal channel conditions, as exemplified by the classic 'winterbourne' with its low-flow channel, extensive margins, gently sloping banks and high seasonal inundation, will produce good conditions for species diversity and make communities more resistant to the effects of drought. These conditions can be enhanced through sensitive catchment management that encourages rainfall percolation, limits abstractions and seeks to maintain a low intensity of landcover management.

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