Sizes, shifts, & structures: A statistical exploration of changes in diatom morphologies and communities

Diatoms, single-celled eukaryotic algae, are abundant in aquatic ecosystems and are readily preserved in sediments because of their silica frustules. This combination of traits results in a robust fossil record of diatom species and community evolution through time. In this dissertation, I evaluated how both diatom species and communities from high-elevation lakes evolve in response to changing environments. In the second chapter, I showed how a dominant endemic diatom from Lake Titicaca, Cyclostephanos andinus, evolved over a ~ 400,000-year period. Major morphological change in C. andinus took place during two periods, at ~220 and ~145 ka BP. Both of these shifts in body size were rapid non-linear punctuations in the evolution of C. andinus. The first shift was in response to geomorphological change of the lake basin, and the second shift was in response to lake-level lowering brought about by interglacial warming and drying in Marine Isotope Stage 5e. The third and fourth chapters of my dissertation look at the rapid reorganization of diatom community structure from sediment recovered from Foy Lake, Montana. In chapter three, I evaluated the data using multiple statistical approaches to identify and describe the dynamics of the system. I found that univariate analyses were successful for some, but not all, species analyzed, but multivariate approaches were consistently successful at predicting the observed regime shift in the Foy Lake diatom community composition. In the fourth chapter, I showed that size discontinuities were also an effective predictor of a regime shift in the Foy Lake diatoms. Thus, multiple components of the diatom community in paleoecological records can be used to identify and test for regime shifts in biological structure. All three research projects highlight how the changing environment produced non-linear shifts in diatom sizes and community structures