Patterns of Early Lake Ontogeny in Glacier Bay as Inferred from Diatom Assemblages

We studied a series of recently formed lakes along a deglaciation chronosequence in Glacier Bay National Park to examine changes in water chemistry, primary production, and biotic composition that accompany the early ontogeny of north-temperate lakes. Successional trends in these freshwater ecosystems have been explored with a two-tiered approach that includes (1) the comparison of limnological conditions among lakes of known age and in different stages of primary catchment succession, and (2) the inference of water-chemistry trends in individual sites based on fossil diatom stratigraphy. This paper emphasizes the reconstruction of limnological trends from fossil diatom assemblages. The modem distribution of diatoms in relation to water-chemistry gradients within 32 lakes of varied age is used to derive a transfer function for the reconstruction of chemical trends from fossil assemblages in sediment cores. The modem data suggest that pH and TN (total nitrogen) exert significant and independent controls on diatom distributions, and thus trends in these variables are reconstructed for Bartlett Lake, as an example of our approach. Core reconstruction corroborates pattems in pH suggested by the modem chronosequence and shows a gradual decline in lakewater pH after about 100 years. The Bartlett Lake core also follows the chronosequence pattern in TN concentration, with an initial increase followed by a decline after ca. 100 years. Reconstructions from other sites, however, suggest that trends in total nitrogen concentration are variable, and thus that localized pattems of plant colonization and soil development may result in regional variability in lakewater nitrogen concentration over time.