These plants look too robust for Callitriche stagnalis. Callitriche stagnalis is a weak plant, supported by water: it has rosettes floating on the water surface, and is too weak to form erect stems above the water surface. Its hard to be certain at this early stage of growth but I suggest that these are young Gratiola plants, most likely Gratiola pumilo. It would be good to go back later in the season and check the flowers.