Effects of boron and nitrate on hatching success of amphibian eggs

As part of a land-application wastewater disposal system in central Pennsylvania, vernal ponds are commonly exposed to wastewater effluent containing high levels of boron and nitrate. We examined the individual effects of these compounds on the eggs of amphibians breeding in these ponds. Wood frog (Rana sylvatica), Jefferson salamander (Ambystoma jeffersonianum), spotted salamander (A. maculatum), and American toad (Bufo americanus) eggs were exposed to boron (0, 50, and 100 mg L-1) and nitrate (0, 10, 25, and 40 mg L-1) under laboratory conditions. There was no difference in the hatching success or proportion of deformed larvae among the four nitrate levels in any of the species. High boron concentrations reduced the hatching success of B. americanus and produced more deformed offspring in R. sylvatica, A. jeffersonianum, and A. maculatum. Deformed R. sylvatica and A. jeffersonianum hatchlings displayed a "curling defect" in which the larvae had a crescent body shape and expressed difficulty in swimming. A. maculatum larvae from high boron treatments displayed a different abnormality, possibly because of longer times to hatching, characterized by a swelling of the thoracic region and altered gill morphology. Although the concentrations of boron and nitrate tested were within the range measured in wastewater effluent, maximum levels of boron and nitrate in breeding ponds were only 1.5 mg L-1 and 26.7 mg L-1 respectively, likely due to rainwater dilution and biological and chemical processes reducing the levels of these compounds in vernal ponds. Nonetheless, these studies are the first to examine the effects of boron and nitrate on amphibian egg hatching success, and to demonstrate that elevated boron concentrations can detrimentally affect the development of amphibian embryos.