PELAGIC RESPONSES TO CHANGES IN DISSOLVED ORGANIC-CARBON FOLLOWING DIVISION OF A SEEPAGE LAKE

Within 2 yr of dividing a multibasin lake into discrete lakes for expe rimentation, dissolved organic carbon (DOC) concentrations and water c olor (absorption coefficient g at 440 nm, g(440)) increased in the eas t basin and decreased slightly in the west basin. These changes were n ot explainable by watershed vegetation or groundwater chemical composi tion. However, g(440) increased from 0.7 to 4.2 m(-1) for water moving through the sediment-water interface in the east basin. In the west b asin, g(440) of groundwater (0.6 m(-1)), in-seeping water (0.7 m(-1)), and lake water (0.7 m(-1)) were all similar. Patterns of DOC distribu tion matched time trends in the surface waters. In the east basin, DOC concentration doubled and g(440) increased 3-fold from 1990 to 1993. Trends in the west basin were more complex, but in general, there was a decrease in DOC and g(440) over the same period. Changes in the ligh t regime affected phytoplankton vertical distribution, but total areal chlorophyll and epilimnetic chlorophyll concentrations were not alter ed. The depth of oxygenation was changed by altered mixing characteris tics and phytoplankton distribution. Changes in light-attenuating DOC affected pelagic responses to nutrient inputs.