Effects of synergistic disturbance upon phytoplankton communities and organic carbon accumulation in Sparkling Lake, northern Wisconsin

Background/Question/Methods

Understanding the sensitivity of north temperate lakes to past forest disturbance can inform efforts to sustainably manage lakes in the context of future forest change and increased drought frequency.  Our research on Sparkling Lake in the Northern Highlands Lake District (NHLD) focused on the following questions:  (a) Did phytoplankton communities and sediment organic carbon records in Sparkling Lake respond to historical fire episodes, catchment logging, or drought?  (b)  Were responses different when fire and logging occurred together?  (c) Were responses magnified in the presence of a drought?   To inform these questions, sediment cores were collected, dated with ²¹⁰Pb, and analyzed for total organic carbon, fossilized carotenoid pigments, charcoal, and pollen.  Charcoal and pollen records provided indicators of forest disturbance and fire episodes near the lake; the North American Drought Atlas provided historical drought records for the region. 

Results/Conclusions

Results show that three fire episodes prior to 1890 CE were not accompanied by changes in total organic carbon accumulation (CAR) rates; however, a dramatic increase in CAR was observed following several fire episodes that accompanied and followed clearcut logging (1890-1929 CE).  Selective logging that occurred in the absence of fire in the 1980s was not accompanied by a similar change in organic carbon sedimentation.  Multivariate statistical analyses showed that turn of the 20^th century forest disturbance significantly structured Sparkling Lake's pigment community (p = 0.001, non-parametric MANOVA).  Phytoplankton community structure and sediment organic carbon in Sparkling Lake appeared to be resilient to effects of fire activity (in the absence of logging) and selective logging (in the absence of fire).  Two fire episodes over the logging period also coincided with drought periods ca. 1890 and 1910 CE.  We conclude that the significant changes observed in these records at the turn of the 20^th century were driven by the synergistic effects of logging and fire in addition to drought.