Having a ring of hairs surrounding the end of the petiole distinquishes N. amazonum from another common water lily here, Nymphaea prolifera.  Both commonly have purple specks on the upper surface of the leaves, and have entire, un-toothed, margins, unlike N. pulchella and N. ampla. 

Common in the shallower, west fringes of the wetland, I hadn’t encountered more than N. amazonum and N. pulchella prior to this year at Palo Verde.  There are at least four other lily species I’m missing that have been documented in the Tempisque basin according to Crow.