Abstract
Climate change over the last two centuries has been associated with significant shifts in diatom community structure in lakes from the high arctic to temperate latitudes. To test the hypotheses that recent climate warming selects for species of smaller size within communities and a decrease in the average size of species within populations, we quantified the size of individual diatom valves from 10 depths in a sediment core covering the last ∼200 years from a pristine subarctic lake. Over the last ∼200 years, changes in the relative abundance of species of different average size and changes in the average valve size of populations of species contribute equally to the changes in community size structure, but are often opposite in sign, compensating for one another and moderating temporal changes in community size structure. In the surface sediments that correspond to the recent decades when air temperatures have warmed, the mean size of valves in the diatom community has significantly decreased due to an increase in the proportion of smaller-sized planktonic diatom species.
Highlights
The high latitudes of North America have experienced some of the largest changes in climate over the last century
Surface area, and volume of the diatom valves The estimate of average size of the diatom community (A) is derived from measurements of the two-dimensional area enclosed by the outline drawn around the images of intact valves
A recent meta-analysis of bacteria, phytoplankton, zooplanton and fish found there were community, population and individual level decreases in organism size with recent increases in temperature, leading the authors to conclude that reduced body size is a universal ecological response to warming in aquatic systems (Daufresne, Lengfellner & Sommer, 2009)
Summary
The high latitudes of North America have experienced some of the largest changes in climate over the last century. The subarctic, covering regions from 50–70◦N, is amongst the least understood and studied regions in North America (Rouse et al, 1997; Ruhland & Smol, 2005). Dendroclimatic reconstruction indicates summer temperatures have risen ∼2 ◦C since the middle of the 19th century in the subarctic central Yukon and northern and middle Mackenzie Basin (Rouse et al, 1997; Szeicz & MacDonald, 1995). Climate projections indicate that summer temperatures in the subarctic may increase an additional 2–6 ◦C with a doubling of atmospheric CO2 (summary in Rouse et al, 1997). Recent warming has been linked with decreases in the duration of river and lake ice cover (Magnuson et al, 2000), alteration in the thermal structure and a lengthening of the growing season in arctic and subarctic lakes
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