Low‐level addition of dissolved organic carbon increases basal ecosystem function in a boreal headwater stream

Abstract

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.

Pulse of dissolved organic matter alters reciprocal carbon subsidies between autotrophs and bacteria in stream food webs

“…A small increase in GPP following sucrose addition has been observed before under similar experimental conditions (Robbins et al. ). This transient response may be explained by the increase in supply of CO 2 by bacteria and an initial sufficiency of nutrients (N, P).…”

“…The potential benefit of bacterial CO 2 for primary producers has been hypothesized to explain a small increase (16–20%) in gross primary production following dissolved organic carbon addition and associated increase in bacterial activities (Robbins et al. ). This is a reasonable assumption (e.g., Redfield , Butenschoen et al.…”

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

“…We added ecologically relevant concentrations of a labile form of DOC to an Alaskan boreal headwater stream that supports juvenile Dolly Varden, an anadromous fish in the family Salmonidae, to estimate how stream consumers depend on and respond to allochthonous DOC. We demonstrated in a companion paper that labile DOC addition subsidised stream microbes, increasing respiration, gross primary production, biofilm production, and whole‐stream N demand, also presented here in the context of this study's goals (Robbins et al, ). We expected this increase in basal ecosystem function to drive a trophic upsurge (sensu Tanentzap et al, ) of energy to invertebrates and fish.…”

“…Dissolved inorganic nitrogen (DIN, almost entirely NO 3 ‐N) at the tops of each reach was typically within 10 µg/L of each other. Similarly, PO 4 ‐P was never more than 3 µg/L different between the tops of the reaches, and background DOC was highly comparable between reaches (usually «0.2 mg/L different; see Robbins et al, for detailed nutrient and DOC data). Sampling locations within each reach were designated by metres from the top of the reference (R) and treatment (T) reaches, and weekly water chemistry measures for DOC, DIN, and PO 4 ‐P were taken at the 0, 37.5, and 75 m points in each reach, with an additional 10 m sampling point in the treatment reach (i.e.…”

“…We did not attempt to maintain a constant dosing concentration of acetate‐C because DOC naturally fluctuates with discharge, typically being diluted by surface‐water runoff in these headwater streams (R. S. King, unpublished data). We estimated the flux of dosed C into the treatment reach using the change in total DOC concentration from the top of the reach (based on known discharge, dosing rate, and background DOC concentrations [T0]) to the bottom of the reach (T75; data in Robbins et al, ). Dissolved organic carbon was measured using a Shimadzu TOC‐VCSH.…”