Evidence for temporally coherent increases in the abundance of small Discostella (Bacillariophyceae) species over the past 200 years among boreal lakes from the Experimental Lakes Area (Canada)

Increased relative abundances and accumulation rates of the small, centric diatom Discostella have been recorded in numerous paleolimnological investigations of north temperate lakes that span the last century. Yet, conflicting observations in the seasonal succession of small Discostella from monitoring studies, and opposite patterns in their abundance in sediment cores from some nearby lakes have prevented consensus on the mechanisms responsible for this trend. Differences in lake and catchment characteristics that filter biological responses to regional environmental changes have likely played a role in this variability. We present detailed sub-fossil diatom data in dated sediment cores from eight small lakes in the undisturbed Experimental Lakes Area of northwest Ontario, Canada. These lakes were chosen because they experienced large (> 25%) increases in relative abundance of small Discostella taxa since pre-industrial times, enabling a clearer assessment of their change over time and linkages with climate. Our data showed increased abundances of small Discostella in all lakes, with changes in the majority of lakes beginning in the mid-1800s. Application of a hierarchical generalized additive model structure provided statistical evidence that this pattern was shared among all lakes, although lake-specific departures from this trend were also apparent. Based on the coincidence of trends with historical temperature records and results from previous phytoplankton monitoring studies in the ELA, we suggest that the observed recent increases in the proportion of Discostella may be related to earlier ice-off and extended periods of spring mixing, and that the small amount of between-lake variability is attributable to differences in lake morphometry.