Seasonal and decadal patterns in Discostella (Bacillariophyceae) species from bi‐weekly records of two boreal lakes (Experimental Lakes Area, Ontario, Canada)

Abstract

A recent rise in the relative abundance of Discostella species (D. stelligera and D. pseudostelligera) has been well documented from sedimentary diatom assemblages across the Northern Hemisphere. This unprecedented change over the last ~150 years has been linked to rises in atmospheric temperatures, changes in ice cover, and/or increases in thermal stability, among other factors. The bi-weekly monitoring data from two boreal lakes at the Experimental Lakes Area (ELA) in northwestern Ontario were analyzed across seasons (spring, summer, and fall) and decades (1970s-2000s). We found that Discostella species are primarily spring/early summer bloomers (i.e., late April to June) in these lakes and changes in concentrations of Discostella over time were most pronounced in the spring or early summer months. Increases in Discostella abundance over time may be linked to earlier ice-off and a longer period of spring turnover, resulting from increased winter and spring temperatures. It is also possible that a trophic mismatch between the spring diatom bloom and zooplankton is occurring, thus reducing diatom loss rates, and resulting in greater overall abundance. Moreover, the spring dominance of Discostella in our study lakes occurred at a time of the year when nutrient concentrations were at their highest seasonally, suggesting that these taxa are neither limited directly by nutrients, nor responding to enhanced stratification during the summer months in these lakes.

How Does Changing Ice-Out Affect Arctic versus Boreal Lakes? A Comparison Using Two Years with Ice-Out that Differed by More Than Three Weeks

“…The same pattern was observed for L. bodanica, even though mixing depths showed only small differences across the two ice-out years. Boeff et al [18] also found that D. stelligera was more abundant in some Maine lakes in late ice-out years, in contrast to the early ice-out patterns found in some other areas [14,17]. The effects of the complex interactions between light and nutrients on Cyclotella taxa are well known and reviewed by Saros & Anderson [72], and are likely behind the weaker links between thermal structure and taxon responses in this boreal lake compared to those observed in Arctic lakes.…”

Using a decadal diatom sediment trap record to unravel seasonal processes important for the formation of the sedimentary diatom signal

“…Indeed, variable seasonal patterns have been observed in some other boreal lakes (Boeff et al 2016;Wiltse et al 2016), although we note that the ice-covered period has been ignored in most previous studies, making conclusions about seasonality incomplete. Because the physical drivers examined in this study affect lakes throughout the boreal region, many of the general processes observed in this study should be able to inform interpretations of sediment diatom records in a wide variety of lakes, although specific drivers are likely to be more or less important in individual lakes.…”

“…Early spring runoff may also increase nutrient availability under ice through the lateral dispersion of solutes into the stratified under ice surface layer, in contrast to a flush through the system within a more rapid spring flood (Cortés et al 2017;Jeppesen et al 2009). Other recent studies have also identified a relationship between warm early-spring conditions and the increased prevalence of C. glomerata (Wiltse et al 2016) although the mechanisms hypothesized to explain the observed trends were somewhat incomplete, in part because continuous measurements were not made throughout the winter period.…”

“…During these warm late-winter periods, stratified conditions in the surface water under the ice, increased light availability due to reduced snowpack and thinning ice, and excess nutrient concentrations likely favored the growth of phytoplankton (Salmi and Salonen 2016), including C. glomerata, which is known to favor conditions of shallow stratification and high light availability (Hutchinson 1967;Saros et al 2012). These conditions are usually observed during early shallow spring stratification after over-turn (Wiltse et al 2016), but also occur in under-ice surface water layers which are rarely sampled. Early spring runoff may also increase nutrient availability under ice through the lateral dispersion of solutes into the stratified under ice surface layer, in contrast to a flush through the system within a more rapid spring flood (Cortés et al 2017;Jeppesen et al 2009).…”

“…As most of the studies do not include the pre-spring conditions as one of the major contributing processes, they often attribute C. glomerata growth to the mixing period, rather than to the stable, light-saturated surface layer (under ice) conditions which preceded it. This potential oversight may explain the controversy regarding potential drivers of trends in C. glomerata (Saros and Anderson 2015;Wiltse et al 2016;Ruehland et al 2008). This controversy centers around the debate between the relative importance of earlier mixing and shorter shallow spring stratification, but neglects the fact that the bloom is an effect of a process which is not necessarily synchronous but occurred with a time lag before the bloom is measured.…”