Factors influencing the growth of Mougeotia in experimentally acidified mesocosms

Acidification causes profound changes in species composition in aquatic sys tems. We conducted mesocosm experiments in three northern Wisconsin lakes ( Trout Lake, Little Rock - Reference, Little Rock - Treatment) to test how d ifferent phytoplankton communities respond to acidification. Major differen ces exist among these lakes in water chemistry and phytoplankton community composition. In each lake, three pH treatments (control, press (sustained p H 4.7), and pulse (alternating pH 4.7 and ambient pH)) were maintained for 6 weeks. We observed a striking increase in species in the genus Mougeotia in all systems. Mougeotia is a filamentous green alga often found in acidif ied lakes. The magnitude of the Mougeotia increase differed among lakes and treatments, and we used an autoregressive model to identify potential fact ors responsible for these differences. Our results suggest that biotic fact ors such as competition with other algae played a relatively minor role in regulating Mougeotia dynamics. Instead, pH and abiotic factors associated w ith changes in pH (e.g., dissolved inorganic carbon) were important predict ors of Mougeotia dynamics.