They say liquid nitrogen, but that doesn't make sense. Then they mention ammonia....

An anhydrous ammonia would be thousand times worse for the environment than nitrogen. Yet, it's not really used for its nitrogen impact.

Most likely, this was a release of a liquid urea-ammonium nitrate (UAN solution). Commonly, this can be ammonium nitrate, urea-ammonium sulfate. One could dumb it down as mass synthetic pee, sort of.

This stuff kills plants, but is generally considered safe for humans and animals....in normal amounts / concentrations. I don't think the dead fish would agree this was normal amounts.

The largest long-term risk, that anyone will admit exists, is the toxic algae blooms that result from these kinds of spills.