| Years of atmospheric deposition of sulfur from industrial activity resulted in the acidification of thousands of lakes around the world. One of the most affected areas was Sudbury, Ontario, Canada. Beginning in the 1970's, emission control programs led to impressive chemical recovery of most Sudbury area lakes. However, biological recovery has lagged, particularly in zooplankton communities. While there appear to be several factors that govern the trajectory of zooplankton recovery, one major influence is distorted predation regimes. As lakes acidified, often acid-tolerant planktivorous fish or macroinvertebrates were left as the top predator, strongly altering community structure and limiting successful colonization of zooplankton species. These changes form the two separate but similar theories explored in this thesis. I begin by examining the Biological Resistance Hypothesis (BRH) which theorizes that unusual predator regimes are preventing zooplankton recovery from acidification. I then examine the idea that macroinvertebrate predators are important to the energy flow in lakes through an infra-guild predation (IGP) linkage. To study the effects of these predation regimes on zooplankton communities, I undertook a mesocosm experiment with a factorial design (Fish x Chaoborus sp). For the BRH study; the treatments were three levels of fish biomass (absent, low, and high) x two levels of Chaoborus (absent, present). For the IGP study, the treatments were two levels of fish (present, absent) x three levels of Chaoborus (absent, small-bodied, large-bodied). Results from the BRH study suggest that lake food webs dominated by either planktivorous fish or Chaoborus, can indeed limit zooplankton recovery. The IGP study showed effects of IGP dynamics, in that macroinvertebrate predators can have major impacts on the zooplankton of lake food webs. Therefore, a normal fish community comprised of a species mix of piscivores, planktivores, and benthivores may be essential for the restoration of a normal zooplankton community. Such a fish community would prevent the extremes of planktivory by planktivorous fish and by macroinvertebrate predators. |
