Mechanisms controlling long-term changes in periphytic diatom community structure

Rapid deglaciation in Glacier Bay National Park and Preserve, AK, has left areas suitable for primary terrestrial succession. Changes in lakewater chemistry and surface-sediment diatom assemblages correlated with primary succession suggest mechanisms for shifts in diatom communities over successional time. Lakewater nitrogen (N) and modifications in the light regime, due to increases in dissolved organic carbon (DOC), are correlated with primary succession. In this study we manipulated nutrients (phosphorus (P) and N) and the light regime experienced by periphytic diatom communities in a chronosequence of lakes.; We hypothesized that there would be a shift in the importance of N and P in structuring the diatom communities in early stages of primary succession. N and/or N + P were important in structuring these communities in all six lakes studied, whereas P was important in one of these lakes.; We also hypothesized that the quality and quantity of light experienced by the communities would affect their composition. The response of a community to alterations in the light regime depended on the prior light history of the community. Significant effects due to the quality (UV and visible) and the quantity of light were detected in communities transplanted from high- to low-DOC lakes. Interactive effects of UV and nutrients were detected in lakes less than 100 years old.; Analysis of communities on artificial substrates among lakes in the chronosequence suggest that decreasing pH, or correlated environmental variables (e.g alkalinity), are primary factors influencing the community composition among lakes. However, within lakes species dominance is determined by N:P ratios, the light regime, or interactions of these variables.