Phytoplankton Primary Production and Light Attenuation Responses to the Experimental Acidification of a Small Canadian Shield Lake

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Abstract
Phytoplankton primary production and light attenuation were monitored over a 10-yr period in Lake 223, a small, softwater, shield, lake. After 2 yr with no treatment, the lake was treated for 8 yr with sulfuric acid to decrease the epilimnetic pH from about 6.7 to 5.0. Primary production, integrated over time and depth, varied considerably during the 2 pre-treatment years. However, it increased steadily during the first 6 yr of treatment, with a total increase of more than 250%. This production increase was coincident with a decrease in epilimnetic light attenuation. After the 6th year of treatment, the production of the lake decreased although the pH level was held relatively constant during this period. Nearby control lakes tended to show a similar, though less distinct, pattern during this 10-yr period. Thus, it is difficult to separate the effects of acidification from long-term natural variation. However, there was no apparent reduction in community phytoplankton production as a consequence of the acidification and the hypothesis that acidification causes oligotrophication was not supported.