A HOLOCENE RECORD OF DIATOM SPECIES PLASTICITY AND EVOLUTION FROM CUMBRES BOG (COLORADO, USA)

We used landmark shape analysis of fossil diatom assemblages belonging to the Eunotia formica species group to explore how diatom valve morphology has changed through a 9,500-year sediment record from Cumbres Bog. Cumbres Bog is unusually deep (modern Z_max = 5 m, Holocene range ~12 m) for a sub-alpine bog; it is located on a high-elevation plateau (elevation=3050 m a.s.l.) in a remote region of the San Juan Mountains of southeastern Colorado. Prior research on the sediment record from this site has shown that the bog has a history of water level fluctuations throughout the Holocene, causing it to repeatedly alternate between lake and peat-bog states, gradually evolving into the bog that currently occupies the modern basin. Eunotia taxa belonging to the E. formica group dominated the diatom assemblages during shallower-water bog intervals, and have continuously existed in diatom assemblages throughout the sediment record from at least the last 9,500 years. Scanning electron microscope analyses of these Eunotia show similar ultrastructural characteristics exist between all specimens, but substantial variation in valve outline occurs throughout the record. Because of the high morphological variability commonly observed in this diatom group and difficulty in distinguishing simple reliable characteristics to define speciation and species boundaries, we used traditional morphometrics and landmark analyses to evaluate differences in valve size and outline from a set of representative samples collected approximately every 1,000 years throughout the Holocene. Morphometric analyses show that long-term patterns in valve breadth and outline varied substantially throughout the Holocene, but stabilized by approximately the mid-Holocene around features that define at least one distinct new species that evolved from the original early Holocene populations.