Biodiversity changes in a shallow lake ecosystem: a multi-proxy palaeolimnological analysis

Biodiversity is a key measure of environmental quality in lake ecosystems. Lake biodiversity can be assessed using modern survey data, but typically t hese data only provide a 'snap-shot' measure and in most cases it is-not po ssible to reconstruct temporal trends in biodiversity, so that human impact s can be detected. Palaeoecological techniques offer an alternative means o f 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 re cord such that it is not usually possible to make reliable assessments ol: diversity changes within an entire taxonomic order (e.g. the algae). Moreov er these organisms are typically from the lower levels of the trophic hiera rchy (i.e. plants and insects). The problems of identifying changes in biodiversity from the palaeolimnolog ical record are addressed with reference to Groby Pool, a shallow, eutrophi c, medieval lake in the English Midlands, which has been subjected to eutro phication over the last 150 years. Pb-210 and Cs-137-dated sediment cores h ave been used to estimate short-term alterations in the composition and div ersity of three groups of indicators, representing different levels in the trophic cascade, namely diatoms, aquatic pollen and chironomids. By explori ng relationships, both between these indicators and with archival macrophyt e records, an assessment is made of eutrophication-related changes in overa ll habitat diversity at the ecosystem level. These data suggest that the la ke has undergone considerable nutrient enrichment, resulting in the loss of a diverse, mesotrophic macrophyte flora from at least the turn of the cent ury onwards and its replacement by a few highly competitive species toleran t 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 r elationships amongst the phytophagic species. However, diatom assemblages g enerally exhibit the opposite trend, which may be related to increases in m acrophyte cover and increasing opportunities for the colonization of divers e epiphyte communities. The different fossil indicators have different limi tations and merits, and for this reason a 'multi-proxy' approach is essenti al if meaningful inferences are to be made of changes in lake biodiversity using palaeoecological data.