Do diatoms in the Swiss Alps reflect the length of ice-cover?

Diatom analyses in the water column, sediment traps, surficial sediments as well as in a short sediment core from Hagelseewli (2339 m asl, Swiss Alps) give information about the present-day seasonal cycle of diatom blooms, ta phonomic processes in the lake basin and the lake's history. Analyses of su rficial sediments show that water depth and thus light and nutrient availab ility is the most important factor influencing the production and distribut ion of diatom assemblages in Hagelseewli, and that periphytic diatom valves deposited in the deeper part of the basin originate from the shallow, litt oral parts and are transported to the central part by processes such as lat eral currents or sediment focussing. The lake is characterised by a very sh ort period (2-3 months) of open water. Water-column and sediment-trap data revealed that planktonic diatoms bloom during and after the ice break-up, w hereas mainly periphytic Fragilaria species entered the traps during the ic e-covered period. These results suggest that plankton development is strong ly inhibited by the ice-cover, with longer periods of ice-cover favouring F ragilaria species in Hagelseewli. The diatom analysis of a short sediment c ore that includes the last five centuries revealed several changes in the p roportion of planktonic diatoms to Fragilaria species. The colder phases of the Little Ice-Age correspond to phases of lower concentration of plankton ic diatoms. The highest, statistically significant amount of variance in th e downcore diatom data is explained by winter precipitation, which directly influences the length of the ice-cover but inversely influences the light regime.