Indirect effects of lake acidification on Chaoborus population dynamics: The role of food limitation and predation

A twofold increase in Chaoborus punctipennis abundance following experimental acidification of one basin of Little Rock Lake, Wisconsin, provided an opportunity to examine the hypotheses that C. punctipennis dynamics were regulated by (i) food resources for instars I and II, (ii) fish predation on instars III and IV, and (iii) invertebrate predation on instars I and II. Neither food limitation nor fish predation appeared to be important constraints on C. punctipennis abundance. Although rotifer biomass increased significantly during the acidification, C. punctipennis did not respond in a consistent manner to variation in food availability. Bioenergetics estimates of C. punctipennis consumption by rock bass (Ambloplites rupestris), the most acid-sensitive fish species, were small compared with C. punctipennis production. In contrast, invertebrate predation appeared to be an important constraint on C. punctipennis abundance. Chaoborus punctipennis abundance increased dramatically when abundance of the predator Mesocyclops edax declined to near zero. Comparison of M. edax predation rates and first instar production suggested that M. edax could exert strong predation pressure on C. punctipennis Thus, invertebrate predation may create a juvenile bottleneck for populations of small-bodied Chaoborus species in moderately productive acidified lakes.