Influence of environmental and spatial variables on the distribution of surface sediment diatoms in an upland loch, Scotland

The spatial distribution of surface sediment diatoms was analyzed using ArcGIS in the Round Loch of Glenhead, an acid upland lake in south-west Scotland. The assemblages were composed almost entirely of benthic species. Tabellaria quadriseptata was fairly evenly distributed across the loch but some species (Navicula madumensis, Brachysira brebissonii, Aulacoseira perglabra and Eunotia vanheurckii var 1) showed rather patchy distributions. Ordination analysis was performed to assess the influence of environmental and spatial variables on the diatom composition of the samples. Loss of ignition was significantly negatively correlated with redundancy analysis species axis 1 (r = –0.77), indicating the influence of substrate on the diatom assemblages. The positive relationship between theoretical bottom shear stress resulting from wind stress and redundancy analysis (r = 0.31) suggests wind stress also influences the spatial distribution of diatoms within the loch. Spatial variables [(principal coordinates of neighbour matrices (PCNM 1 and PCNM3) positively correlated with redundancy analysis axis 2], indicated that spatial variables, ignored in former studies, are a further influence on diatom distribution. Unique environmental and spatial variables explained 27.3% and 8.6% of diatom variability respectively. Environmental and spatial interactive variables combined explained 4.8% of variation. Although the pure contribution of spatial variables was only 8.6%, the study highlighted the importance of differences in the spatial distribution of different benthic diatom species in this upland lake