According to Blaustein, health effects such as those caused by nitrates and nitrites may also work in concert with other environmental insults, such as acid rain or UV-B exposure, to compound problems. For instance, Blaustein said the furor that has arisen over frog deformities such as extra legs has been linked to a treated parasite known as a fluke. These flukes have been around forever and we never observed the level of problem we're now seeing with deformed frogs. It's quite possible this fertilizer issue relates to that, along with killing tadpoles directly. The flukes that can cause amphibian deformities live part of their life cycle in a snail, Blaustein said.
Snails eat algae.
And higher levels of nitrogen-based fertilizers can cause increased alga growth, increasing the snail populations. And this was in a wildlife management area, which was not intensively farmed but was only surrounded by agricultural lands. The researchers stated in their report that chemicals used for various purposes, including crop agriculture, may permeate lakes, ponds, and streams, making them unsuitable for many amphibians. But the fact remains that nitrogen fertilizers by themselves, used at levels considered safe in drinking water, are enough to kill some amphibians. So clearly that's part of the answer and a fairly serious concern in its own right.