I guess my concern would be if you had a film covering the entire pond. I would think that could inhibit the pond's ability to exchange gases with the atmosphere.
Edit: Spraying an oil film on BOWs is/was a common practice of controlling mosquitos by preventing O2 from entering the water and suffocating the larvae.
Last edited by Bill D.; 03/19/15 09:16 PM. Reason: Clarification