Assessing eutrophication and reference conditions for Scottish freshwater lochs using subfossil diatoms

1. The European Council Water Framework Directive requires reference conditions to be determined for all water body types including lakes. We examined the role of palaeolimnology, specifically the diatom record, as a tool for assessing eutrophication and for defining lake reference conditions and ecological status.2. Sediment cores (representing c. AD 1850 to present day) were taken from 26 Scottish freshwater loch basins. Radiometric dating techniques (Pb-210 and Cs-137) established a chronology for each core. Two levels of diatom analysis were employed: a relatively high resolution (15-20 samples) at 21 lochs considered of high interest, and a lower resolution (four to five samples) at the remaining sites.3. Detrended correspondence analysis and dissimilarity measures were applied to the core top (present day) and bottom (reference state, c. AD 1850) samples to assess floristic change at each site. Significant floristic change, indicative of nutrient enrichment, occurred in 18 lochs along a broad trophic gradient.4. Two-way indicator species analysis (TWINSPAN) was applied to the bottom (c. AD 1850) samples to classify the 'reference' diatom assemblages and thereby characterize the reference floras of the different lake types. TWINSPAN identified four site end-groups, each with a characteristic diatom assemblage, although there was some overlap in the taxa present in the four groups. Water depth and productivity were key factors that explained the groupings.5. Diatom transfer functions that reconstructed total phosphorus (TP) concentrations were used to evaluate eutrophication. Nineteen lochs had increases in diatom-inferred (DI) TP of > 5 mug l(-1) (five of these > 20 mug l(-1)), six lochs had no change or negligible increases in DI-TP (<2 mug l(-1)), and there was evidence of a decline in DI-TP in one loch over the period represented by the sediment cores. The inferred increases were significant at 12 lochs.6. Synthesis and applications. Our data indicate that it may be difficult to find minimally impacted waters to act as reference sites, particularly for shallow, lowland lake types, in the current population. The derivation of site-specific reference conditions from the sediment record is a particularly valuable approach in such cases. Ordination, clustering and dissimilarity measures applied to palaeodata, combined with transfer functions, offer powerful techniques for characterizing lake types, defining ecological and chemical reference conditions, and assessing deviation from the reference state.