little tiny black bugs--look almost like oil--on irrigation water

I saw today little tiny black bugs floating on the surface of irrigation water as it came out the gate of my gated pipe. They look like oil or black paint at first. When you get closer you can see they are very small black bugs. I am irrigating alfalfa. These bugs are extremely small--like a pin prick. Am seeking advice about what they might be, if they will harm my alfalfa, and if so what to do about them. Thanks.

little tiny black bugs--look almost like oil--on irrigation water