Spawning salmon as a nutrient pulse to Pacific Rim riparian ecosystems.
Gende, Scott*,1, Hood, Eran2, 1 National Park Service, Juneau2 University of Alaska Southeast, Juneau
ABSTRACT- Spawning Pacific salmon represent a nutrient and energy pulse to Pacific Rim stream ecosystems because they gain nearly all of their adult body mass while foraging in the ocean but return to spawn (anadromy) and die (semelparity) in their natal freshwater systems. This pulse is highly predictable in space and time allowing organisms to structure life-history attributes and behavior around this resource availability. The effects of spawning salmon on stream food webs has been well established, including its role in elevating productivity at virtually all trophic levels. A less-developed but emerging issue is the ecological consequences of salmon to riparian ecosystems. Because salmon spawning occurs in streams, the nutrients need to be made accessible to riparian food webs, with the transfer mechanisms dictating the strength of the pulse. Although mechanisms such as hyporheic flow provide some available nutrients, the primary mechanism identified to date has been the transfer of salmon carcasses by the foraging activities of bears. The bear populations exhibit a numerical (aggregative) and functional response to salmon availability, with predation rates exceeding 50% at some streams. Once captured, bears generally carry the carcasses from the stream to the riparian forest where they are only partially consumed, leaving the remaining biomass (often >70% of original carcass mass). A diversity of other organisms utilize the carcasses as food and the nutrients leach into the riparian soils. Soil cores near salmon revealed ammonium and nitrate levels several orders of magnitude higher near the carcasses compared to adjacent control plots with no carcasses. Although the spatial extent of the salmon nutrient shadow was approximately 20 cm (lateral), the carcasses continued to leach nutrients into the soil for up to 3 months following deposition. Combining soil nutrient results with carcass surveys revealed that, at least over extended time periods, salmon-derived N and P can approach levels of commercial fertilizer for some riparian areas. The salmon-derived nutrients are utilized by many riparian shrubs and trees, suggesting this pulse may have long-term consequences for riparian vegetation and their associated communities.
Key words: salmon, riparian, nutrients, ammonium
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