Depth-distribution of surface sediment diatoms in Lake Tovel, Italy

Lake surface sediment is a widespread microhabitat, for which specific algae quantification procedures have been developed. Sediment habitat can promote selection of diatom taxa with specific characteristics such as motility. Diatoms inhabiting the surface sediment of lakes can also perform vertical migrations. However, the ecology of depth distribution of surface sediment diatoms in lakes is a topic which has received limited attention. An aspect explored to some extent is the analysis of the depth distribution of taphocoenoses to reconstruct lake-level fluctuations often linked to climate change in the Holocene. Lake Tovel in the ltalian Alps was well-known to scientists and tourists because of the past summer reddening. This fostered -and still promotes- limnological research, including investigations of the epilithic diatoms of the eulittoral zone. The present study initiated with the aim to develop a training set to reconstruct water-level fluctuations, an important process in the formation of Lake Tovel. The training set was not yet applied due to unexpected marked silica dissolution phenomena affecting most cores. This setback, however, offered the opportunity to carry out a detailed study of the vertical physical and chemical gradient and of the depth distribution of surface sediment diatoms. In our surface-sediment diatom depth distribution study, a majority of truly benthic taxa was found at most depths. Only the depths below 20 m were characterized by an important contribution of plankton fallout. Several taxa were found to have a rather specific depth distribution. The depths at which the individual taxa were comparatively most abundant are seldom in agreement with the weighted average (optimum) values of water depth. Thus, morphological and physical and chemical features of the depth-transects sampled might greatly influence the depth-distribution patterns of species. It is possible to distinguish different zones along a depth-gradient on the basis of the composition and structure of surface sediment diatom assemblages, and to identify most determining factors by means of multivariate techniques. In the case of Lake Tovel these appear to be water level and light (P.A.R.) extinction. The diversity of surface sediment diatom assemblages is highest in the upper part of the gradient where the level of "disturbance" (mainly due to water-level fluctuations) is at maximum. This could be due to the fact that the epipelic habitat tends to select motile species which are particularly apt to colonize these parts of the lake subject to seasonal desiccation.