NUTRIENT CYCLING AND THE GROWTH OF BENTHIC ALGAE IN EXPERIMENTALLY ACIDIFIED LITTLE ROCK LAKE, WISCONSIN

Changes in nutrient-cycling and the growth of benthic algae resulting from decreased pH in low alkalinity lake systems were analyzed by laboratory, mesocosm, and whole-lake studies on Little Rock Lake (Vilas County), Wisconsin. Nutrients, transparency, and algal growth in the experimentally acidified basin were compared with conditions in an untreated reference basin. During the first summer following acidification (1985), accumulation rates of attached algae were significantly higher in the acidified basin than in the reference basin during June-August, but not in September. Color and winter silica values were significantly lower in the acidified basin relative to the reference basin following treatment. In addition, the lack of a fall decline in SiO;The effect of pH on N or P levels in acidified mesocosms differed between littoral and pelagic sites and between years. Dissolved SiO;In studies of inorganic phosphorus sorption by sediments, predicted equilibrium P;Three types of evidence indicate that the invasion of benthic mats of Mougeotia occurred as the direct result of pH rather than as the result of indirect effects of acidification. (a) Growth rates of Mougeotia isolates were highest at low pH (5.0-5.5). (b) Increased transparency of the acidified basin relative to the reference basin was not sufficient to explain differences in benthic algal accumulation. Photosynthesis by Mougeotia-dominated communities was light-saturated throughout the littoral zone of both basins in 1984 and 1985. (c) No significant differences in grazing effects were detected between basins in exclosure experiments.