Jason - that's a much better picture that you linked above. It is definitely primrose (Ludwigia spp).
Here's a photo of a healthier specimen.
Watershield differs considerably in appearance from primrose (below).
Glyphosate isn't the best product for primrose, but will do a fair job. But, be cautioned that only aquatic-labeled formulations of glyphosate (usually containing 53.8% active-ingredient) should be applied to ponds & lakes, in combination with an aquatic surfactant.
The tallow-amine surfactants contained within many terrestrial-labeled glyphosate formulations (usually those containing 41% active-ingredient) are not intended nor desirable for direct application to aquatic sites.
