NATURAL AND MAN-CAUSED FACTORS AFFECTING THE ABUNDANCE AND CYCLING OFDISSOLVED ORGANIC-SUBSTANCES IN PRECAMBRIAN SHIELD LAKES

Effects of natural factors (drought and forest fire), and experimental perturbations (fertilization and acidification) on dissolved organic carbon (DOC) concentrations and ratios to other nutrients in lakes of the Experimental Lakes Area are examined using data obtained over a pe riod of 20 years. DOC concentration, and the ratio of dissolved iron t o DOC in lakes of the area were strongly correlated with the relative size of the catchment to the lake. DOC in many lakes of the area decli ned over 20 years, due to increased water residence times caused by in creasing average temperature and decreasing precipitation. Inexplicabl y, Lake 382 was an exception to this general observation. Acidificatio n of Lake 302S to below pH 5.0 also caused DOC to decrease. The lesser acidification of Lake 223 (min. pH 5.0) did not significantly affect DOC. Experimental acidification of a small peatland also caused a temp orary decline in DOC concentrations in bog pools. Changes in DOC appea r to affect the availability of mercury for methylation. Addition of a luminum to a small acidic lake caused a two-fold decline in DOC. Ferti lization of Lake 227 caused a considerable increase in DOC, and in rat ios of DOC to other carbon fractions. New stable ratios did not occur for a decade after fertilization began. Lake 226N, fertilized at a low er rate, showed similar but less pronounced increases in DOC, but the experiment was terminated after only eight years. Phosphorus fertiliza tion caused a dramatic increase in the lability of the DOC pool in Lak e 226N, where the autochthonous carbon pool was labelled with DIC-14. A large increase in autochthonous production of DOC and increased micr obial utilization of allochthonous DOC appear to have occurred. DOC co ncentrations in streams were higher after drought, but concentrations were unrelated to flow volume during wet periods. Due to lower streamf lows in drought years, annual yields of DOC from streams were unaffect ed by drought. Mesocosm experiments showed that DOC's primary effect o n iron is to inhibit sedimentation, possibly by suppressing flocculati on reactions that are known to control the cycles of many metals. The changes of DOC in lakes brought about by changes in water renewal, aci dification, or other perturbations can have major effects on the cycle s of metals, lake transparency, and phytoplankton production and stand ing crop.