Separate and Combined Effects of Nitrate and Pathogenic Water Mold Saprolegnia on Survival of Amphibian Larvae

John M. Romansic, Kristin A. Diez, Elise M. Higashi,

and Andrew R. Blaustien


Department of Zoology

Oregon State University

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ABSTRACT

ROMANSIC, JOHN*, KRISTIN DIEZ, ELISE HIGASHI, and ANDREW BLAUSTEIN. Department of Zoology, Oregon State University, Corvallis, OR, romansij@science.oregonstate.edu

Separate and Combined Effects of Nitrate and the Pathogenic Water Mold Saprolegnia on Survival of Amphibian Larvae

Synergisms between stressors and infectious organisms can have important effects on host organisms. Nitrate pollution is an environmental stressor of global significance. We tested for a synergism between nitrate and Saprolegnia, a pathogenic water mold, using larvae of three amphibian species: Ambystoma gracile (northwestern salamander), Hyla regilla (Pacific treefrog) and Rana cascadae (Cascades frog). Each species was tested separately, using a fully factorial experiment with three nitrate addition treatments and two Saprolegnia treatments. Nitrate treatments had nominal nitrate concentrations of 0, 5, and 20 mg/L. Saprolegnia treatments were three hemp seeds laden with Saprolegnia (Saprolegnia treatment) and three sterile hemp seeds (control treatment). Each experiment lasted for one week. No significant effects of nitrate addition or Saprolegnia were found on survival of H. regilla. In contrast, survival of R. aurora was affected by an interaction between Saprolegnia and nitrate. With no nitrate, survival of R. aurora was significantly lower in the Saprolegnia treatment compared to the control treatment. However, there were no significant effects of the Saprolegnia treatment on survival when nitrate was added. A. gracile followed a pattern similar to R. aurora, but the difference between the Saprolegnia treatment and the control treatment when nitrate was not added was not significant, nor was there a significant nitrate by Saprolegnia interaction. Our results suggest that nitrate addition and Saprolegnia had less-than-additive effects on R. aurora survival. Less-than-additive interactions between stressors and infectious organisms have been described in other systems. Future studies should attempt to determine what controls the nature of interactions between stressors and infectious organisms.