SPATIAL HETEROGENEITY OF DIATOM STRATIGRAPHY IN VARVED AND NON-VARVED SEDIMENTS OF A SMALL, BOREAL-FOREST LAKE

Spatial and temporal heterogeneity of recent diatom stratigraphy were assessed for a small (23 ha) dimictic lake in Northern Sweden (Kassjon). Varves are confined to water depths > 10 m. Six freeze cores were taken along a transect covering a range of water depths (3-12 m) and both varved and non-varved sediments. Core profiles were compared for dry mass accumulation, loss-on-ignition (LOI), and diatom relative frequency stratigraphy and accumulation rate. Excluding the 3.2 m water depth, non-varved core, all parameters showed good repeatability between cores, apart from diatom accumulation rates which were more variable. The 3.2 m core was atypical and had lower LOI values, low planktonic diatom percentages and high values of benthic taxa that were not abundant in the deep-water sediments. Detrended Correspondence Analysis (DCA) and Canonical Correspondence Analysis (CCA) ordinations were compared; both methods clearly differentiated the shallow water core, and showed the general similar ecological trends of the deeper-water cores. CCA axes constrained by ''environmental'' (i. e. core location) data resulted in slightly lower eigenvalues than that obtained by Correspondence Analysis (CA), but the axes were significantly non-random. A Partial-CCA of the four varved cores alone (with effects of sediment depth, i.e. age, partialled out), indicated that there was no significant difference between their diatom assemblages.