1. LOW-LEVEL ADDITION OF DISSOLVED ORGANIC CARBON INCREASES NITROGEN UPTAKE AND BACTERIAL BIOMASS PRODUCTION IN AN ALASKAN HEADWATER STREAM Caleb J Robbins 1, Ryan S King 1, Alyse D Yeager 1, Coowe M Walker 2, Jeff A Back 1, Dennis F Whigham 3 Introduction Dissolved organic matter (DOM) is often overlooked as an important or limiting resource; however, it has been shown to be an important source of energy to stream food webs (Bernhardt and McDowell 2008). Furthermore, additions of labile organic carbon can increase inorganic nitrogen uptake and community respiration (Johnson et al 2012). Catchment characteristics of headwater streams strongly determine the availability of nutrients and organic matter, and thus can have a profound effect on ecosystem functioning. On the Kenai Peninsula, Alaska, catchment alder largely determines stream inorganic nitrogen (DIN) concentrations (Shaftel et al 2012), while wetland cover is a strong predictor of dissolved organic carbon (DOC) concentrations (Walker et al 2012). We hypothesized that algae and bacteria in streams with high alder-fixed DIN could be limited by wetland-derived DOC. We dosed a 75-m reach with acetate-C to simulate the influence of wetland derived, labile DOC on a salmon-rearing headwater stream with relatively high in alder-fixed DIN. Methods Study Site (Fig. 1)  The catchment of our study stream had high amounts of N-fixing alder and low wetland cover (low DOC).  We paired two similar 75m segments (Fig. 2), separated by an 80 m intermediate reach, as reference and treatment reaches.  Sampling locations were set at 0m, 37m and 75m in each reach, with an additional 10m location in the treatment reach (Fig. 1).  We dosed the treatment reach with ~0.250 mg/L acetate-C for 9 weeks, starting on 25 June 2013. Water Chemistry  Water samples were collected once per week from each sampling location from 4 June to 19 August. BBP and PS  Bacterial Biomass Production (BBP) and Photosynthesis (PS) of periphyton were measured using a dual radiolabel technique.  7 rocks and site water were collected from upstream locations in each reach and taken back to the lab.  Rocks were placed into septum-topped jars with site water, 3 were covered with foil for ‘Dark’ incubation, 3 uncovered for ‘Light’ incubation, and 1 injected with formalin as a killed control.  We injected jars with 14 C-bicarbonate, incubated for 1.5 h, then injected 3 H-leucine and incubated for another 0.5h before stopping incubation with formalin. All incubations occurred at 10 - 12°C under 305-350 µE light intensity.  We converted counts of radioactivity in samples to measurements of production. 1 Baylor University, Department of Biology, 2 Kachemak Bay Research Reserve; 3 Smithsonian Environmental Research Center Conclusions Our results suggest that labile DOC from wetlands are important drivers of DIN uptake and bacterial production in this region, particularly in areas with high alder cover. Because our study showed that even low levels of labile C can stimulate substantial increases in basal secondary production, wetlands should be a focal point for stream ecosystem management and salmonid conservation. Figure 1. Location of stream on the Kenai Peninsula in south-central Alaska with dosing schematic showing water sampling locations. The 0 m location was immediately upstream of the dosing station. The reference and treatment reaches were separated by 80 m. Figure 7. Mean daily temperature throughout the dosing study. Dosing commenced on June 25, 2013 (Day 1). Results Figure 3. DIN uptake velocity (mean, 95% CI error bars propagated from regressions). Negative days are prior to dosing. Onset of riparian shading shown by arrow (also see Figs. 6 & 7)  DIN uptake increased as a result of dosing. Overall uptake spiked on day 8 (June 30 th ) coincident with high temperatures (Fig 3) and low canopy cover (Fig 6). Uptake began to decrease when riparian vegetation shaded the stream.  PO 4 -P uptake was negligible throughout the study in both reaches. Figure 5. Canopy cover was still relatively open the day dosing started. Figure 6. By the third week of dosing, dense vegetation was shading large portions of the study reaches. Results – Water Chemistry Figure 4. PO4-P uptake velocity (mean; 95% CI error bars propagated from regressions). Negative days are prior to dosing. Figure 2. Means and standard deviations of various physical and chemical stream characteristics taken before dosing. Water chemistry measurements are the average of the samples from the three weeks prior to dosing. Results – BBP and PS  BBP was 2x higher in the treatment than the reference reach by week 2, and the magnitude did not change throughout the study.  BBP between light and dark incubations did not differ.  PS showed no response to dosing, even though it trended toward an effect on Week 4 (P=0.1193). Photosynthesis was negligible in week 8 after the streams were thoroughly shaded. References Bernhardt ES, and WH Mcdowell. 2008. Twenty years apart: Comparisons of DOM uptake during leaf leachate releases to Hubbard Brook Valley streams in 1979 versus 2000. Journal of Geophysical Research-Biogeosciences 113:G03032. Johnson LT, TV Royer, JM Edgerton, and LG Leff. 2012. Manipulation of the dissolved organic carbon pool in an agricultural stream: responses in microbial community structure, denitrification, and assimilatory uptake. Ecosystems 15: 1027-1038. Shaftel RS, RS King and JA Back. 2012. Alder cover drives nitrogen availability in Kenai lowland headwater streams, Alaska. Biogeochemistry 107:135-148. Walker CM, RS King, DF Whigham, and SJ Baird. 2012. Landscape and wetland influences on headwater stream chemistry in the Kenai Lowlands, Alaska. Wetlands 32:301-310. Pre-Dose ReferenceTreatment MeanStDevMeanStDev Physical Wetted Width (m)1.490.261.300.30 Average Depth (m)0.120.070.120.07 Reach Slope (%)5.28-4.88- Sinuosity1.07-1.06- Chemical DIC (mg/L)4.840.804.990.74 DOC (mg/L)2.530.532.590.51 DIN (µg/L)374.5882.25354.9296.73 PO4 (µg/L)58.967.8157.907.04 Acknowledgements Many thanks go to Jasmine Maurer for extraordinary and diligent help in the field and in the lab. Robert Doyle and CRASR at Baylor provided helpful insights, analytical support and cost-sharing. Project funding came from Alaska Sustainable Salmon Fund 44709 awarded to CM Walker, RS King, and DF Whigham. Figure 8. Discharge was measured at least once weekly. Dosing commenced on June 25, 2013 (Day 1). Shading Figure 9. BBP and PS during weeks 2,4 and 8 of dosing. Bars are means ±1 SE.