Assessing the Ecological Effects of Increased Dissolved Organic Carbon in Maine Lakes Over Recent Decades

As a result of changing climate and atmospheric deposition patterns, dissolved organic carbon (DOC) concentrations have increased in many lakes situated in forests of northeastern North America and northern Europe since the 1990s. While this increase has been well documented, the associated ecological implications remain unclear. In particular, DOC strongly influences the vertical temperature structure of lakes, with increasing DOC commonly leading to a shallower epilimnion. I investigated the effect of increased DOC concentrations on lake thermal structure using fossil diatom records from six remote Maine lakes. Three pairs of small (\u3c50 ha), morphologically similar lakes were compared, with each pair containing one lake with a documented significant increase in DOC and the other experiencing no change in DOC since the early 1990s. Lake thermal structure was inferred by changes in the relative abundance of Discostella stelligera and Aulacoseira species, two diatom taxa that are associated with changes in thermal stratification. Two of the three lakes with documented increases in DOC showed the highest turnover in diatom community structure and directional change in diatom- inferred mixing depth over the past 20 years. In contrast, the three lakes without increasing DOC since the early 1990s showed no change in diatom-inferred mixing depth during this time. This study demonstrates that recent increases in DOC have the potential to alter the biological and physical structure of lakes and the high regional variability in how these lakes are responding to changing atmospheric deposition and/or climate