COMPARING 40-YEAR SEDIMENT RECORDS OF AQUATIC ECOSYSTEM EVOLUTION IN TWO LARGE LAKES IN THE BLAST ZONE OF MOUNT ST. HELENS

The 1980 eruption of Mount St. Helens dramatically altered the surrounding landscape, removing vegetation and impacting hydrology. The pyroclastic debris flow partially filled Spirit Lake and dammed Coldwater Creek, creating Coldwater Lake. Spirit Lake was sterilized by the eruption, and fallen trees from the blast washed into the lake forming a floating log mat covering 20% of the surface. While significant research has gone into understanding the history of ecosystem recovery in Spirit Lake, nearby Coldwater Lake has not been studied as extensively. These two lakes provide a unique opportunity to study how volcanic eruptions alter freshwater environments. Recent research at Spirit Lake has focused on spatial heterogeneity in the post-eruption lake ecosystem related to patterns of log mat coverage, and our current work at Coldwater Lake provides an important point of comparison in a lake without the influence of woody debris. In order to study the evolution of the Coldwater Lake ecosystem, we collected four sediment cores (~30 centimeters each), extruded and sectioned the cores, and analyzed diatom abundance and diversity, and carbon and nitrogen biogeochemistry through time. These sediment records reflect changes in lake conditions, such as water chemistry, and in aquatic ecology. Our data show a substantial increase in diatom abundance and diversity through time, with highest levels in the top 15 centimeters of the cores. The sediment itself is organic-poor and composed mainly of ash and tephra. The percent carbon and nitrogen also increase in younger core sediments. In contrast to Spirit Lake, where there is high spatial variability in sediment biogeochemistry due to the floating log mat, at Coldwater Lake initial results show low spatial variability among sediment cores from different locations.