RELATING DIATOM COMMUNITY SHIFTS TO ENVIRONMENTAL CHANGE IN ICELANDIC FRESHWATER LAKES

Polar regions are responding rapidly to climate change (Post et al. 2009), and freshwater lakes may serve as sentinels that archive these changing environmental conditions in sediment records (Williamson et al. 2009). Over the last 50 years, Iceland, the region examined in this study, has experienced linear increases in annual maximum and minimum temperatures and decreased mean wind speed (Icelandic Meteorological Office 2012). In addition to linear trends, Iceland is also strongly affected by the North Atlantic Oscillation (NAO) and experiences decadal oscillations in temperature, wind speed, and precipitation (Hurrell 1995).

Here, we study changes in diatom community composition in five lakes located in southern and western Iceland over the last century to ask: how do diatom communities in different regions of Iceland change over time, and if this change is synchronous, is it associated with decadal cycles of the NAO? We selected lakes that range in physical, chemical, and biological characteristics to test for synchronous responses to climate change across varying systems. If there are synchronous diatom community changes across lakes, we expect there to be a single broad-scale driver, such as climate change. If we do not see synchronous responses across lakes, we expect lake-specific characteristics, such as morphometry (Boeff et al. 2016), may modify the sensitivity of a particular system to climate change.

Diatom species indicative of changes in lake mixing (Aulacoseira spp. (e.g. Stone et al. 2016)) and nutrient availability (Fragilaria spp. (e.g. Saros et al. 2005)) were the most abundant components of the diatom community across lakes. Decreases in Aulacoseira spp. in some study lakes suggests increased frequency or duration of stratification in lakes that are historically polymictic. Increases in Fragilaria spp. in some study lakes suggested increased nutrient availability. Long-term shifts in Aulacoseira spp. and Fragilaria spp. were not observed synchronously across all lakes. Instead, more dynamic shifts occurred in deeper lakes. These findings suggest lakes across Iceland are responding differently to recent climate change, with deeper lakes experiencing reduced lake mixing and increased nutrient availability that was not observed in shallow systems.