Pacific salmon (Oncorhynchus spp.) play an important ecological role by providing a nutrient resource subsidy, primarily nitrogen and phosphorus, to their natal streams. Salmon-derived nutrients (SDN), delivered during salmon migration and subsequent decomposition after death, potentially fuel both aquatic and terrestrial primary and secondary production. However, forest management practices in North American temperate rainforests have resulted in significant physical changes in these ecosystems, potentially altering the ecological role of salmon. Our research investigates both the influence of salmon on aquatic ecosystems and the contrasting effect of forest management practices on the dynamics of SDN.
While SDN have been studied extensively, previous research has not addressed SDN dynamics in relation to forest management practices. Furthermore, few studies have quantified the change in downstream nutrient export associated with salmon spawning and carcass decomposition. Nutrient export is an especially useful metric in understanding the nutrient dynamics of streams where discharge fluctuates greatly. During 2007, we quantified dissolved and particulate nutrient concentrations (µg/L) and export (g/min) of dissolved inorganic nitrogen (both as ammonium and nitrate) and soluble reactive phosphorus (SRP) in seven streams from June to November draining watersheds of differing timber harvest legacy (from 5 to 70% of watershed area harvested) on Prince of Wales Island in Southeast Alaska.
Results/Conclusions
Stream nutrient concentrations increased during the salmon run across all streams (ANOVA p<0.005), regardless of harvest history, but did not differ before or after the salmon run. However, forest management did influence nutrient export, which increased only three-fold during the salmon run from minimally harvested watersheds (< 12% harvested), but increased ten-fold or more from the heavily harvested watersheds (> 20% harvested). Furthermore, SRP and nitrate export in heavily harvested sites remained elevated after the salmon run. Increased nutrient export post-salmon run is likely due to a combination of modified hydrology and terrestrial soil leaching, rather than a legacy effect of salmon carcasses. Our results indicate that examining only stream nutrient concentrations may mask the interacting effects of forest management practices and spawning salmon, and that nutrient export, which accounts for fluctuating discharge, is a more relevant metric to assess the effect of SDN on stream ecosystems.
Results from this study demonstrate that forest management practices can modify the effect of salmon on stream ecosystems. While salmon influence their natal watersheds, the effect of human-induced watershed alteration may potentially modify the ability of stream and terrestrial ecosystems to retain and process salmon-derived nutrients.