CHANGES IN THE IMPORTANCE OF LOTIC AND LITTORAL DIATOMS IN A HIGH ARCTIC LAKE OVER THE LAST 191 YEARS

Both habitat preferences and community associations for some of the mo st prominent pennate taxa in our profile were established using sample s from the modern lake and its watershed. Habitat preferences were use d to establish a simple indicator for the relative contribution of lot ic communities in this lake (Lotic Index) and indicator taxa for both littoral (Achnanthes spp. and Cymbella spp.) and lotic (Hannaea arcus and Meridion circulare) communities. When profiled, the Lotic Index sh owed a clear positive relationship to sedimentation rate as recorded i n the varves, while profiles of littoral indicator taxa show the oppos ite trends. It seems likely that the patterns we observed in the Lotic Index are related to changes in runoff. Apparently, there was a perio d of declining runoff beginning ca. two centuries ago and ending in th e late 1800's. This was followed by increasing runoff lasting until th e middle of the 20th century. A brief minimum occurred in ca. 1970 fol lowed by a recovery by ca. 1980. A strong positive relationship was al so found between the dates of major turbidites, exceptionally thick va rves and the concentration of valves in the sediment. It is possible t hat many of the thicker varves in the profiles contain littoral materi al transported to the site of deposition by turbid interflows and unde rflows. For this reason, the concentration of valves in the sediment i n our cores appears to be a proxy for sediment deposition from turbidi ty currents.