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PNW Project Overview 1997
Nutrient Enrichment of Riparian Areas by Spawning Salmon
Principal
Investigators:
Dr.
Robert
E. Bilby, Weyerhaeuser Company, Tacoma, Washington
Dr.
Peter
A. Bisson, U.S. Forest Service, PNW Research
Station, Olympia, Wa
Awarded: $19,160
Project
Description:
Pacific
salmon die after spawning. It has recently been demonstrated that the eggs
and carcasses deposited in freshwater habitats by spawning salmon makes a
significant contribution to the nitrogen (N) capital in these systems.
Thus, the spawning migration of these fishes represents a mechanism
of nutrient transport from the highly fertile, North Pacific marine
ecosystem to relatively nutrient-poor, freshwater ecosystems. The
contribution of nutrients from spawning salmon can be considerable.
However, relatively little work has been done examining the influence of
marine-derived nutrients on riparian vegetation. Bilby, et al (1996) found
that 18% of the N in the foliage of riparian plants along a stream where
coho salmon spawned was of marine origin. These nutrients may be made
available to terrestrial plants by removal of the fish from the stream by
scavengers, deposition on the floodplain during high flows, or migration
of dissolved nutrients into riparian soils in hyporheic flow.
The availability of additional nutrients in riparian areas may influence the plant community composition and structure and primary production. Elevated N levels have been associated with reduced understory herbaceous plant species diversity and increased cover (Huston 1994). Increased soil N levels are often reflected in higher levels of N in the foliage of plants on the site (Aber and Mellilo 1991).
This project will examine the effect of marine-derived nutrients from
spawning salmon on the vegetation of riparian areas. The three objectives
are to:
- Determine
the extent to which marine-derived N is distributed upslope from a
stream system where large numbers of salmon spawn.
- Compare relative availability of N between areas where salmon spawn and comparable locations upstream of blockages to anadromous fishes.
- Compare
the plant species diversity and understory cover at comparable riparian
sites above and below blockages to anadromous fishes.
Project Status:
A
manuscript for this project has been submitted for review to the Canadian
Journal of Fisheries and Aquatic Sciences.
Submitted
Manuscript Abstract:
Riparian
plant communities were compared along streams in two watersheds (Griffin
Creek and Kennedy Creek) in western Washington above and below barriers to
spawning salmon.
Salmonberry (Rubus spectabilis) foliage at the channel edge
and at 20, 50, and 100 meters upslope from the stream was analyzed for
nitrogen stable isotope ratio (an indicator of salmon-derived nitrogen)
and total nitrogen (N) and phosphorous (P) content.
In addition, researchers compared cover, plant density and species
richness of shrub and understory vegetation between sites with and without
salmon.
The nitrogen stable isotope ratio declined with distance from the
channel at sites with and without salmon at both locations.
The nitrogen stable isotope values in salmonberry leaves at sites
with salmon were higher than at corresponding distances from the channel
at sites without salmon at Kennedy Creek, but not Griffin Creek.
Salmonberry foliage possessed significantly higher levels of total
N and P adjacent to salmon spawning reaches in both watersheds.
Shrub cover and understory cover, species diversity and density
were greater at sites with salmon at Griffin Creek.
Shrub species diversity and understory plant diversity and density
were higher at sites with salmon at Kennedy Creek.
Results indicate that Pacific salmon carcasses contribute to
nutrient availability in riparian areas and influence the cover, density
and diversity of riparian shrub and understory vegetation.