Effects of nitrate and the pathogenic water mold Saprolegnia on survival of amphibian larvae.

We tested for a synergism between nitrate and Saprolegnia, a pathogenic water mold, using larvae of 3 amphibian species: Ambystoma gracile (northwestern salamander), Hyla regilla (Pacific treefrog) and Rana aurora (red-legged frog). Each species was tested separately, using a 3 x 2 fully factorial experiment with 3 nitrate treatments (none, low and high) and 2 Saprolegnia treatments (Saprolegnia and control). Survival of H. regilla was not affected significantly by either experimental factor. In contrast, survival of R. aurora was affected by a less-than-additive interaction between Saprolegnia and nitrate. Survival of R. aurora was significantly lower in the Saprolegnia compared to the control treatment when nitrate was not added, but there was no significant difference in survival between Saprolegnia and control treatments in the low and high nitrate treatments, consistent with increased nitrate preventing Saprolegnia from causing mortality of R. aurora. Survival of A. gracile followed a similar pattern, but the difference between Saprolegnia and control treatments when nitrate was not added was not significant, nor was the nitrate x Saprolegnia interaction. Our study suggests that Saprolegnia can cause mortality in amphibian larvae, that there are interspecific differences in susceptibility and that the effects of Saprolegnia on amphibians are context-dependent.