Low-level addition of dissolved organic carbon increases basal ecosystem function in a boreal headwater stream.Ecosphere 8(4):e01739. 10. 1002/ecs2.1739 Abstract. Comprehension of basic stream ecosystem function relies on an understanding of aquaticterrestrial linkages. One major component of such linkages is the incorporation of landscape-derived energy and nutrients into the aquatic food web via microbes. In many boreal streams, wetlands and alder are known to be primary sources of dissolved organic carbon (DOC) and dissolved inorganic nitrogen (DIN), respectively. To simulate the influence of the highly labile portion of wetland-derived DOC subsidies on microbial production and ecosystem processes in a stream with high landscape-derived nutrient inputs, we enriched a boreal headwater stream situated in a high-alder, low-wetland cover catchment (i.e., high DIN, low DOC) with low levels (~0.25 mg/L) of labile DOC (as acetate-C) for 9 weeks. We compared nutrient uptake, bacterial biomass production, and photosynthesis of periphyton and ecosystem metabolism in physicochemically similar upstream (reference) and downstream (treatment) reaches. DIN uptake was greater in the treatment than in reference reach on six out of nine dates during the dosing period. Bacterial biomass production positively responded to C enrichment. Ecosystem respiration increased up tõ 50% after dosing began. Gross primary production responded positively to DOC enrichment early in the study when riparian vegetation did not limit light availability, but negatively later on in the growing season. We conclude that even low levels of labile DOC may act as a strong subsidy to headwater stream ecosystems, particularly those with high levels of DIN inputs from alder. Headwater streams influenced by high contributions of both alder and wetlands may represent biogeochemical hotspots, and these landscape features should be viewed as vital and complementary in their roles for ecosystem function.

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Headwater streams are the most numerous in terms of both number and length in the conterminous United States and play important roles as spawning and rearing grounds for numerous species of anadromous fish. Stream temperature is a controlling variable for many physical, chemical, and biological processes and plays a critical role in the overall health and integrity of a stream. We investigated the controls on stream temperature in salmon‐bearing headwater streams in two common hydrogeologic settings on the Kenai Peninsula, Alaska: (1) drainage‐ways, which are low‐gradient streams that flow through broad valleys; and (2) discharge‐slopes, which are high gradient streams that flow through narrow valleys. We hypothesize local geomorphology strongly influences surface‐water and groundwater interactions, which control streamflow at the network scale and stream temperatures at the reach scale. The results of this study showed significant differences in stream temperatures between the two hydrogeologic settings. Observed stream temperatures were higher in drainage‐way sites than in discharge‐slope sites, and showed strong correlations as a continuous function with the calculated topographic metric flow‐weighted slope. Additionally, modeling results indicated the potential for groundwater discharge to moderate stream temperature is not equal between the two hydrogeologic settings, with groundwater having a greater moderating effect on stream temperature at the drainage‐way sites.

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Abstract. Spawning salmon deliver annual pulses of marine-derived nutrients (MDN) to riverine ecosystems around the Pacific Rim, leading to increased growth and condition in aquatic and riparian biota. The influence of pulsed resources may last for extended periods of time when recipient food webs have effective storage mechanisms, yet few studies have tracked the seasonal persistence of MDN. With this as our goal, we sampled stream water chemistry and selected stream and riparian biota spring through fall at 18 stations (in six watersheds) that vary widely in spawner abundance and at nine stations (in three watersheds) where salmon runs were blocked by waterfalls. We then developed regression models that related dissolved nutrient concentrations and biochemical measures of MDN assimilation to localized spawner density across these 27 stations. Stream water ammonium-N and orthophosphate-P concentrations increased with spawner density during the summer salmon runs, but responses did not persist into the following fall. The effect of spawner density on d 15 N in generalist macroinvertebrates and three independent MDN metrics (d 15 N, d 34 S, and x3:x6 fatty acids) in juvenile Dolly Varden (Salvelinus malma) was positive and similar during each season, indicating that MDN levels in biota increased with spawner abundance and were maintained for at least nine months after inputs. Delta 15 N in a riparian plant, horsetail (Equisetum fluviatile), and scraper macroinvertebrates did not vary with spawner density in any season, suggesting a lack of MDN assimilation by these lower trophic levels. Our results demonstrate the ready assimilation of MDN by generalist consumers and the persistence of this pulsed subsidy in these organisms through the winter and into the next growing season.