Date of Award

Spring 1-1-2011

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

First Advisor

Diane M. McKnight

Second Advisor

Diana R. Nemergut

Third Advisor

Sarah A. Spaulding

Fourth Advisor

Patrick Kociolek

Fifth Advisor

Michael Grant

Abstract

Diatoms are vital primary producers in aquatic ecosystems and useful indicators of environmental change. In climatically sensitive polar areas, diatoms have been used as beacons of climate change, allowing us to monitor physical, chemical, and biological changes. This research aims to improve our understanding of diatom ecology in the pristine and dynamic McMurdo Dry Valleys of Antarctica, where diatoms reside in stream microbial mats. These results aid in understanding how changes in hydrologic regime will affect stream microbial communities, biodiversity, and ecosystem function in a changing environment.

Relationships between hydrologic regime and diatom community composition were investigated using long-term data. Diatom communities were structured by stream physical features, and streams with more similar hydrologic characteristics had more similar communities. Variation in diatom community composition was best explained by hydrologic regime. Small diatoms increased in relative abundances with increasing streamflow, suggesting a role of diatom size in structuring communities. Overall, diatom communities were resistant to flood and drought-like conditions, suggesting an adaptation to frequent disturbances.

The importance of hydrologic regime on drift activity was investigated during three 24-hour experiments. Diel variations in drift could be attributed to diurnal flow peaks. Biomass and diatom cell densities followed a clockwise pattern with stream discharge and support the dominant role of hydraulic processes. The quality of source material differed between seasons and throughout the day. Drifting diatom communities were dominated by Fistulifera pelliculosa, which is rarely found in stream mats and suggests a different source. Modeling results suggest that the less firmly anchored marginal mats contribute more to the drift than channel mats in low-flow seasons, while the channel mats become more important during high-flow seasons.

The relationships between diatom and bacterial assemblages in microbial mats were assessed based on phylogenetic and functional relatedness in five Dry Valley streams. Significant relationships between diatom and bacterial communities were found, and co-occurrence analysis identified numerous correlations between individual diatom and bacterial taxa. A consistency in metabolic lifestyles of correlated taxa suggests that the relationships are ecologically relevant. Diatom and bacterial diversity showed opposite patterns, which indicate differences in environmental drivers of diversity for bacteria and eukaryotes.

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