The need to assess the current status of fresh waters relative to some baseline state in the past (Moss, Johnes & Phillips 1997; Battarbee 1999) is encompassed in recent water legislation such as the US Clean Water Act (CWA; Barbour et al. 2000) and the European Council Water Framework Directive (WFD; European Union 2000). Both require that biological, hydromorphological and chemical elements of water quality should be based on the degree to which present-day conditions deviate from those expected in the absence of significant anthropogenic influence, termed reference conditions. The WFD states that, in the absence of long-term data, reference conditions based on modelling may be derived using hindcasting methods, and palaeolimnology (the study of the lake sediment record) is given as one such technique (Pollard & Huxham 1998; European Union 2000).
Diatoms (Bacillariophyceae), unicellular, siliceous algae, are one of the most widely used biological groups in palaeolimnological studies (Stoermer & Smol 1999; Battarbee et al. 2001). Of the biological elements relevant to the WFD, diatoms represent components of both the phytoplankton and phytobenthos. However, shifts in the diatom community often correspond closely to changes in other biological groups (Kingston et al. 1992). Diatoms are sensitive to water quality and are therefore good indicators of past lake conditions, such as lake pH (Battarbee et al. 1999) and total phosphorus (TP) concentrations (Hall & Smol 1999). In recent years, transfer functions have been developed to model the relationship between diatom assemblage composition and water chemistry in a training set of lakes. Once calibrated, such functions are then applied to fossil diatom assemblages in sediment cores to infer past water chemistry. Weighted averaging (WA) regression and calibration (ter Braak & van Dam 1989) and its extension WA partial least squares (WA-PLS) (ter Braak & Juggins 1993) are the most widely used techniques for reconstructing past environmental variables in this way (Birks 1998). The diatom record is therefore a potentially useful tool for assessing water quality and defining lake reference conditions, both chemical and ecological (Kauppila, Moisio & Salonen 2002).
Fresh waters are a major feature of the Scottish landscape (approximately 30 000, with a surface area of at least 0·4 ha) and are an important resource for industry, recreation and conservation (Maitland, Boon & McLusky 1994). In the last few hundred years or so human activity has changed the ecology of Scottish lochs and it is likely that almost all surface waters in Scotland have been impacted to some extent (Bennion et al. 2002). A state-changed scheme for the classification of Scottish standing waters has recently been developed whereby lakes are classified according to degree of change in trophic status, acid neutralizing capacity and contamination with toxic substances (Fozzard et al. 1999). In a survey of 174 Scottish standing waters in 1995, and a subsequent survey in 2000, phosphorus (P) enrichment was assessed as the major cause of downgrading using the scheme (Fozzard et al. 1999; Doughty, Boon & Maitland 2002), with diffuse pollution being a major contributor to increases in TP concentrations (Ferrier & Edwards 2002). Historical TP concentrations were estimated by the land use/loss coefficient method for around ad 1850 (Ferrier et al. 1997). This is a simple approach that calculates the total load of P as a sum of the individual loads exported from each separate nutrient source in the catchment.
Palaeolimnological studies offer an alternative method for establishing reference conditions and assessing the degree of nutrient enrichment. In contrast to the loss coefficient method (Ferrier et al. 1997; Moss, Johnes & Phillips 1997), they give an indication of the ecological response to anthropogenic impacts and thus can provide an ecological target for management purposes, a concept fundamental to the WFD and CWA. While palaeolimnology has been key in establishing the extent of acidification in Scotland from acid deposition (Flower & Battarbee 1983; Battarbee et al. 1985), there have been very few such studies to assess eutrophication in Scotland (Battarbee & Allott 1994; Bennion et al. 2002). The focus of eutrophication research to date has been on productive lochs where the symptoms of enrichment, such as algal blooms, loss of submerged plants and deoxygenation, are most obvious, e.g. Loch Leven (Haworth 1972; Bailey-Watts 1994). It is now recognized, however, that factors such as fish farming, forestry fertilization, agriculture and sewage effluent disposal can potentially enrich the large, oligotrophic lochs, which are perceived as pristine waters. There is a need to assess further the extent of cultural eutrophication in Scottish fresh waters. This study examined the fossil diatom record in dated sediment cores from 26 Scottish loch basins. The role of palaeolimnology as a tool for assessing eutrophication, and for defining reference conditions and ecological status, was then evaluated.