Assessing the Historic Contribution of Marine-Derived Nutrients to Idaho Streams
Steven A. Thomas, Todd V. Royer, G. Wayne Minshall, and Eric Snyder
Abstract.—Recent studies have shown that anadromous fish deliver ecologically significant quantities of marine-derived nitrogen (N), phosphorus (P), and organic carbon (C) to lakes, rivers, and streams of the Pacific Northwest. These marine-derived nutrients (MDN) can influence the ecological functioning of receiving streams through nutrient release and food availability. In Idaho, populations of anadromous salmon have declined dramatically with many formerly salmon-bearing streams now receiving no MDN supplementation. In order to assess how the loss of MDN may influence Idaho streams and rivers, we examined the current nutrient status of streams and rivers in Idaho with particular emphasis on the limiting role of N and P. We also generated a range of estimates of the historic and current affects of MDN on selected basins of the Salmon River, Idaho. Our analysis indicates that 25–50% of Idaho’s streams are potentially nutrient limited. Further analysis suggests that N and P limitation occurred in an approximately equal number of streams. Historic contributions of MDN to the Salmon River had varying potential to influence N and P availability, ranging from undetectable to resulting in a doubling of N availability. The level of influence depended upon location within the basin and the choices made regarding some simplifying assumptions. Finally, we discuss the effectiveness of artificial fertilization as a means of compensating for lost MDN and suggest that a spiraling approach be used to design and monitor fertilization treatments.