Nutrients and Oregon Spotted Frog

Evaluation of Nutrient Enrichment and
Frog Response at Conboy Lake National Wildlife Refuge


At the base of Mount Adams in Washington State, Conboy Lake National Wildlife Refuge (NWR) encompasses a major portion of a large seasonal marsh nestled within the mixed pine and fir forest of the Glenwood Valley. Conboy Lake NWR seasonally supports numerous migratory waterfowl (ducks, geese, and swans); the only significant breeding population of greater sandhill cranes (Grus canadensis) in Washington State; and a diverse assemblage of wetland-associated birds, including black terns (Chlidonias niger), yellow-headed blackbirds (Xanthocephalus xanthocephalus), sora (Porzana carolina), Virginia rails (Rallus limicola) and American bitterns (Botaurus lentiginosus). The refuge also supports 12 amphibian and reptile species, including the Oregon spotted frog (Rana pretiosa), a candidate species under the federal Endangered Species Act.

Photo - Oregon Spotted Frog (USFWS).The Oregon spotted frog population at Conboy Lake NWR is the largest remaining population of this species across its geographic range, having disappeared almost entirely from lowland marsh habitats. Because the Oregon spotted frog is almost always found in water, it is vulnerable to exposure to chemicals entering their wetland habitats. Recent laboratory investigations demonstrate that agriculture-linked nitrogen-based compounds may contribute to amphibian population declines.

Agriculture is a major land use in the Glenwood Valley, mostly as haying and livestock grazing, but limited crop farming also exists. Such land uses have the potential to increase nutrient loading in nearby wetlands. Nutrients like nitrogen and phosphorus are essential for plants, but at higher concentrations these nutrients can encourage excessive plant growth, promote low dissolved oxygen levels, or directly impact organisms in the aquatic habitat. Consequently, concern existed that nutrient enrichment of wetlands had the potential to impact the Oregon spotted frog or its habitat at Conboy Lake NWR.Photo - Collecting water for nutrient analysis (USFWS).

We sampled nutrients (total and dissolved forms of nitrogen and phosphorus), examined waterquality parameters (temperature, dissolved oxygen, pH, and conductivity) and surveyed Oregon spotted frogs at its various life stages (eggs, larvae and recently metamorphosed juveniles) to identify the potential for agriculture-related nutrient enrichment at Conboy Lake NWR.

Aside from selective conditions (exacerbated summer eutrophication during a drought year [2000]), primary nutrients levels (nitrogen and phosphorus) were generally low. Values for both nutrients and other water quality parameters were generally lower than those recognized as compromising the life stages of western North American ranid frogs. Available data suggest no significant problem with eutrophication. Nonetheless, Conboy Lake NWR comprises part of a wetland system that exhibits a naturally eutrophic cycle that peaks in late summer. With climate change scenarios predicting increasingly droughty conditions and a high potential existing for human development in the Glenwood Valley, periodic monitoring for inadvertent nutrient enrichment should be implemented to avoid jeopardizing this largest remaining population of Oregon spotted frogs.

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Appendix I:
Sampling Methods

Appendix II:
Spot Data for Water Quality Parameters Measured at Conboy Lake NWR, 2000

Appendix III:
Continuous Data fpr Water Quality Parameterss Measured at Conboy Lake NWR, 2000

Appendix IV:
Sampling Effort for Post-Larval Ranid Frogs at Conboy Lake NWR, 2000

Appendix V:
Comparison of the Concentrations for Forms of Nitrogen and Phosphorus Dectected

Appendix VI:
Comparison of the Concentrations for Forms of Nitrogen and Phosphorus

Appendix VII:
List of Abbreviations and Acronyms