Nutrient dynamics in Adirondack lakes recovering from acidification by acid deposition

Background/Question/Methods

Despite being a historic hotspot for effects of acidification, the Adirondack Mountains of New York State have seen marked decreases in acidic deposition over the past few decades. This change has coincided with increased acid neutralizing capacity, increased leaching of dissolved organic carbon from watersheds, and increased productivity of aquatic ecosystems previously impacted by acidic deposition.  Decades earlier, researchers hypothesized that acidification is an oligotrophication process; with decreased acid deposition and lowered nitrogen inputs, this process may be reversed.  It is possible that nitrogen:phosphorus ratios in lakes could subsequently be lowered, potentially resulting in decreased phosphorus limitation of phytoplankton and altered food web dynamics.  We used Mann-Kendall Tao associations between variables (total phosphorus, chlorophyll a, dissolved organic carbon, acid neutralizing capacity, and nitrate concentrations) in Adirondack lakes to determine if decreases in atmospheric deposition alter phosphorus dynamics in freshwater ecosystems impacted by acid deposition.  We sought to assess whether the lakes show decreases in phosphorus limitation as nitrogen:phosphorus ratios change and whether these changes differ based upon lake-watershed classification (seepage, chain drainage, non-chain drainage, thin glacial till, medium glacial till).

Results/Conclusions

We found that associations were highly dependent upon lake class (p<0.05), with associations for variables for seepage lakes differing greatly from drainage lakes. The strongest association for drainage lakes was between nitrate and chlorophyll a, while the strongest association for seepage lakes was between chlorophyll a and dissolved organic carbon.  Differentiations between non-chain drainage and chain drainage, as well as till depth, were also important factors in determining associations.  Though associations could be found between variables using annual values, high seasonal dependence was evident as well; for example, negative association was found between chlorophyll a and nitrate in winter, while total phosphorus and chlorophyll a were positively associated in summer.  For all lakes and classifications examined, acid neutralizing capacity did not appear to be associated with either total phosphorus or chlorophyll a.  Information on total phosphorus and chlorophyll a from the Adirondacks will inform how changes in atmospheric nitrogen deposition influence trophic status of lake systems throughout the region.