Relationships between bathymetry, water quality and diatoms in some Hebridean lochs

SUMMARY 1. Computer methods for mapping and calculating morphometric characteristics are described and applied to seventeen previously unsurveyed South Uist lochs.
2. Deposited sea salt is shown to have a major effect on water quality and the effect varies according to distance from the west coast. Calcium concentrations appear to be determined by dissolution of locally abundant shell debris. Sulphate and particularly nitrate concentrations are influenced significantly by catchment sources.
3. Surface sediment diatom assemblages from all seventeen sites are analysed using multi-variate methods (Hill, 1979). Assemblages are shown to be clearly distributed along DECORANA axis 1 and are clustered into five groups by TWINSPAN.
4. Correlation analysis of water quality, morphometric characteristics, and DECORANA axes scores indicates that conductivity has the strongest influence on surface sediment diatom assemblage composition whereas morphometric characteristics have no demonstrable effect.