Breaking Down PCB's in lakes
According to this abstract$\mathrm{^{[1]}}$, both aerobic and anaerobic processes play a role in the natural degradation of PCB's.  


  Studies have identified two distinct biological processes capable of
  biotransforming polychlorinated biphenyls (PCBs): aerobic oxidative
  processes and anaerobic reductive processes. It is now known that
  these two complementary activities are occurring naturally in the
  environment. Anaerobic PCB dechlorination, responsible for the
  conversion of highly chlorinated PCBs to lightly chlorinated
  ortho-enriched congeners, has been documented extensively in the
  Hudson River and has been observed at many other sites throughout the
  world. The products from this anaerobic process are readily degradable
  by a wide range of aerobic bacteria, and it has now been shown that
  this process is occurring in surficial sediments in the Hudson River.
  The widespread anaerobic dechlorination of PCBs that has been observed
  in many river and marine sediments results in reduction of both the
  potential risk from and potential exposure to PCBs. The reductions in
  potential risk include reduced dioxinlike toxicity and reduced
  carcinogenicity. The reduced PCB exposure realized upon dechlorination
  is manifested by reduced bioaccumulation in the food chain and by the
  increased anaerobic degradability of these products.


1) Aerobic and anaerobic PCB biodegradation in the environment, Environ Health Perspect. 1995 Jun; 103(Suppl 5): 97–99