Low-level dissolved organic carbon subsidies drive a trophic upsurge in a boreal stream

1. Energy pathways in stream food webs are often driven by allochthonous basal resources. However, allochthonous dissolved organic carbon (DOC) is generally viewed as a minor if not insignificant basal resource because much of the DOC pool comprises high molecular weight, recalcitrant compounds and is inefficiently incorporated into biomass. Nevertheless, there is increasing evidence that the relatively small, labile fraction of DOC may indeed fuel microbial activity to a level that stimulates productivity across multiple trophic levels, resulting in a trophic upsurge. Here, we tested the trophic upsurge hypothesis by subsidising the labile DOC pool of an Alaskan boreal stream that had relatively high nutrient availability but low levels of naturally occurring DOC. 2. We continuously added ecologically relevant (0.250 mg C/L, c. 10% increase above ambient bulk DOC) concentrations of labile DOC (acetate-C) for 62 days to a treatment reach that was statistically indistinguishable in its channel form and chemistry from an upstream reference reach. We measured responses of pe-riphyton production and biomass, whole reach metabolism and nutrient uptake, benthic invertebrate abundances, and juvenile salmonid (Dolly Varden, Salvelinus malma) abundance and growth. 3. Measurements of basal ecosystem responses collectively indicated increased en-ergy mobilization at the base of the food web in response to labile DOC addition. Periphyton bacterial production in the treatment reach was generally >1.5× refer-ence reach values, and periphyton ash-free dry mass, chlorophyll-a, and chloro-phyll-a:ash-free dry mass were all greater in the treatment reach by the end of the study. Throughout dosing, ecosystem respiration was 1.3× greater in the treat-ment reach and dissolved inorganic nitrogen uptake was greater in the treatment reach on eight out of nine measurements. 4. Benthic invertebrate counts, dominated by Baetis spp. and Chironomidae, were c. 4× greater after 28 dosing days and c. 8× greater after 56 days in the upstream portion of the treatment reach. Abundance generally declined with increasing dis-tance from the dosing station. Dolly Varden fry and parr age classes were nearly 2× more abundant in the upstream portion of the treatment reach than in any section of the reference reach and also declined with increasing distance from the dosing station. Further, Dolly Varden tagged with passive integrated transponders prior to the experiment had significantly higher instantaneous growth rates in the treatment reach than those recaptured in the reference reach. 5. The strong consumer responses to small quantities of labile DOC mirrored sig-nificant treatment reach increases in basal ecosystem function and therefore demonstrated a response consistent with a trophic upsurge. Terrestrial DOC has historically been viewed as contributing little to metazoan consumers, instead modulating the influence of nutrients and being respired out of a disconnected microbial loop. Because we dosed the treatment reach with a relevant concentra-tion of labile DOC, based on measurements in nearby peatland-draining streams, we suggest that terrestrial DOC deserves more attention as a basal resource for whole food webs, akin to nutrients fuelling green (autochthonous) pathways.Alaska Sustainable Salmon Fund, Grant/Award Number: 4470

The importance of low-level nitrogen and carbon to periphyton in Alaskan headwater streams.

Alaskan headwater streams are ecologically and economically important habitats for juvenile salmonid development. These low-nutrient environments are relatively untouched by anthropogenic effects and contain little marine-derived nutrients due to their distance inland and narrow channels. Reduction in these nutrient sources places high dependence of stream productivity on allochthonous resources supplied by landscape features within the catchment. The current research investigates the importance of low-level nitrogen and carbon (simulating nutrient levels derived from wetlands and alder trees) by observing the effects of these nutrients on microbial biomass, enzyme activity and metabolism. Results indicate that these low-level nutrient sources may be undervalued resources driving stream productivity and that such features should be considered in land management plans within vulnerable stream catchments. The research presented here may be essential in protecting and maintaining habitats that promote healthy salmonid populations in the Kenai lowlands of Alaska and elsewhere