CENTRIC DIATOM SIZE SHIFTS IN RESPONSE TO EARLY HOLOCENE CLIMATIC CHANGE

Over the last ~30 years, lake monitoring has shown a size decrease in some centric diatom species in Northern Hemisphere lakes. This size decrease is manifested in two ways: first, the centric diatom community assemblage shifts to smaller-sized species; second, centric species show a decrease in median individual valve size. Climate change is hypothesized as a major factor in the concurrent trend observed in many lakes worldwide (Rühland et al., 2008). In this study we examined whether the genus Cyclotella became smaller in response to early-Holocene warming in the Rocky Mountain region of North America, and thus whether size shifts are a ubiquitous response to dramatic climate warming. To explore this, we investigated the valve size and planktonic community structure in Blacktail Pond, an alpine lake located in Yellowstone National Park, Wyoming, during the intense summer warming of the early Holocene. The Cyclotella assemblage in Blacktail Pond during the early Holocene did not show an overall decrease in valve size or a shift towards smaller-size species, as observed in lakes during the last few decades. In fact, many species increased in median valve size, and the centric diatom community assemblage shifted to larger species. A likely explanation is that increased size reflects the transition from a newly formed lake created by glacial retreat to a lake with altered nutrient availability as the landscape stabilized and the climate warmed. In addition, differences in the rate of climate change may be a factor; this study encompasses several thousand years as opposed to warming spanning only a few decades. Another possibility is that recent changes are influenced by both climate and concurrent changes in nitrogen and/or carbon inputs from anthropogenic activities that differ from those during the early Holocene. Increased atmospheric nitrogen loading to lakes is of particular interest in this respect (Rühland et al., 2008; Saros et al. 2012). In any case, the differences between the recent record and the early Holocene point to a complex set of influences on Cyclotella size.