Diatom Community Dynamics In Cold-Region Stream Ecosystems: Examining Changes in Composition and Diversity in Response to Shifts in Environmental Factors

Diatoms, photosynthetic algae with silica-based cell walls, play a critical role in aquatic primary production and act as sensitive indicators of ecosystem conditions. Stream ecosystems in cold regions, including the Arctic and northeastern U.S., are experiencing shifts of different environmental factors driven by global climate change that influence diatom communities. The research in this dissertation used existing, unanalyzed samples collected over many years from two well-studied, cold-region stream ecosystems — the Upper Kuparuk River in the Arctic and the Hubbard Brook Experimental Forest in the northeastern U.S. — to examine how variations in key environmental drivers, such as nutrient availability and temperature, affect diatom community composition and diversity. The analyses of the samples from Hubbard Brook provided the first ever classification of diatom species in streams draining the watersheds at this well-known research forest. The research revealed diatom diversity varied across streams due to differences in pH, dissolved organic carbon, and light availability, underscoring the influence of local environmental conditions. These findings highlight the importance of some diatom species as indicators of environmental change and provide a baseline to track future ecological changes. Analysis of samples from the long-term phosphorus fertilization experiment in the Upper Kuparuk River were used to assess diatom community responses to enrichment of the ecosystem with the limiting nutrient. Phosphorus addition led to shifts in diatom communities, favoring species adapted to higher phosphorus levels, while also promoting bryophyte growth that altered diatom habitats. Despite the cessation of phosphorus inputs, the fertilized reach showed lasting differences in community structure, suggesting that changes in nutrient levels can have enduring effects on diatom composition. Other samples from this long-term experiment enabled a unique 25-year analysis of diatom communities in the unmanipulated reach of the Upper Kuparuk River, in a region that is undergoing rapid climate change. The study identified alkalinity, nitrate + nitrite, total dissolved phosphorus, dissolved organic carbon, and wind speed as key drivers of community change, with specific taxa acting as bioindicators of warming conditions. This long-term perspective enhances understanding of diatom resilience and adaptability to climate-driven environmental changes in Arctic headwater streams. Overall, the findings indicate diatom communities are sensitive to localized nutrient enrichment and broader environmental shifts, with their responses differing based on the nature and scale of these changes. This work emphasizes the value of long-term monitoring for understanding ecological dynamics in cold-region streams and the role of diatoms as indicators of ecosystem health in the context of climate change.