Ants and aphids

Spotted these ants "tending" aphids on an iris leaf in our pond. I watched them for a bit through my macro lens but couldn't work out exactly what they were doing. The leaf is about 6 inches from the edge of the pond and the ants were running along a dead leaf which was touching the edge, then up a stem and onto another leaf which was touching that. Then from that leaf they were climbing onto this one and then touching the aphids.