The development of diatom-nutrient palaeolimnological inference models for UK lowland reservoirs
Deterioration of UK lowland reservoir water quality through cultural eutrophication is of concern for both water supply and conservation. This study represents the first attempt to develop diatom-based palaeolimnological inference models for the assessment of eutrophication in UK lowland reservoirs. Models are developed from a 46-reservoir calibration set comprising surface sediment diatom assemblages and contemporary environmental data measured seasonally between May 1999 and October 2000. Following removal of outliers the dataset spans a total phosphorus (TP) gradient of 12-242 µgl ˉ¹ (mean = 63), a chlorophyll-α (Chla) gradient of 1.8-25.5 µgl ˉ¹ (mean = 63) and a conductivity (EC) gradient of 119-781 µScm ˉ¹ (mean = 424). Reservoir mean annual epilimnetic TP, Chla and EC are reconstructed using weighted averaging (WA) and weighted average partial least squares (WAPLS) techniques. Jack-knifed error statistics for the best performing models (WA inverse deshrinking), are comparable to those reported from similar studies. The plankton provides the dominant habitat for diatom growth in reservoirs, thus plankton-only models are also created and perform almost as well as models developed using all taxa. Examination of seasonal diatom plankton populations indicates that many taxa display defined seasonal growth preferences. The transition of frustules from live to sedimentary assemblages reveals that reservoir productivity and hence sedimentation rates affect subsequent representation of seasonal diatom populations in surface sediment samples. Fossil diatom assemblages in sediment cores from two contrasting reservoirs are analysed and the UK lowland reservoirs TP, EC and Chla inference models applied to reconstruct reservoir nutrient histories, yielding a fuller account of ecological change than either model alone. Twentieth-century reconstructions from Blackbrook reservoir illustrate an early history of limited impact mesotrophy, followed by a shift to cultural eutrophication during recent decades. Daventry reservoir shows a history of nutrient enrichment to hypertrophy, followed by subsequent re-oligotrophication as a result of pollution-reduction measures. Available historical data support the broad trends inferred by the diatom-based inference models. UK lowland reservoirs are considered suitable environments for the development of diatom-nutrient inference models and their application where intact sediment profiles exist.