Dissolved organic carbon biolability decreases along with its modernization in fluvial networks in an ancient landscape

Abstract

The metabolism of dissolved organic carbon (DOC) along fluvial networks determines what fraction of organic matter is exported to the ocean. Although it is thought fresh rather than older DOC is preferred by bacteria, old DOC can also be highly bioavailable to stream bacterial communities. In strongly seasonal and oligotrophic regions, we argue that groundwater inputs of old DOC may increase the bioavailability of stream organic matter. We sampled 22 streams along a gradient of size (wetted widths from 1 to 60 m) and one groundwater spring in the Kimberley region of northwest Australia to determine how the age and bioavailability of streamwater DOC varied with stream size. Our hypothesis was that stream DOC would become more enriched in 14C (younger) and less bioavailable as streams increased in size and depleted 14C‐DOC was metabolized by stream microbial communities. We also used fluorescence characterization of DOC, ultraviolet absorbance at 254nm (SUVA254), δ13C‐DOC and lignin phenol yields to assess how these indicators of DOC character influenced the bioavailability and age of stream DOC. Stream evaporation/inflow ratios (E/I, used as a proxy for catchment water residence time), determined from changes in stream δ18O along the gradient of stream size, were positively related to DOC concentration and carbon‐normalized lignin yields, while δ13C‐DOC became more depleted with increasing E/I. Stream Δ14C‐DOC varied from −452.1‰ (groundwater) to 48.9‰ and showed progressive enrichment as streams increased in size and accumulated DOC mainly from terrestrial plant material. Older DOC corresponded to higher bioavailability (R2 = 0.67, P < 0.01), suggesting that old bioavailable DOC, which has escaped from subterranean food webs utilizing 14C‐depleted carbon, is common to one of the oldest landscapes on earth. Therefore, rapid biotic uptake of old bioavailable DOC originating in groundwater springs and the accumulation of modern, terrestrially derived DOC work in opposite directions affecting DOC dynamics along fluvial networks. We suggest the metabolism of old DOC along fluvial networks provides a biogeochemical link between non‐contemporary carbon fixation and modern river productivity.

Optical properties and bioavailability of dissolved organic matter along a flow-path continuum from soil pore waters to the Kolyma River mainstem, East Siberia

“…Further downstream in larger tributary and Kolyma mainstem waters, it has been shown that lower total amounts of bioavailable DOC is supported almost entirely from predominantly modern radiocarbon aged surface soils and vegetation sources . Aquatic microorganisms may therefore be readily acclimating to significant shifts in DOM composition caused by selective losses of unique DOM fractions (e.g., Kaplan and Bott, 1983;Spencer et al, 2015) alongside high internal demand for labile DOM by stream communities in lower-order streams, which would otherwise generally be expected to result in decreased DOM lability with increasing water residence time (Stepanauskas et al, 1999a, b;Wikner et al, 1999;Langenheder et al, 2003;Sondergaard et al, 2003;Fellman et al, 2010Fellman et al, , 2014.…”

Characterization of dissolved organic material in the interstitial brine of Lake Vida, Antarctica

“…Mao et al, 2007). Furthermore, methods that characterize the fluorescent molecules in DOM have been widely applied and have the advantage of providing results comparable across diverse aquatic ecosystems with minimal sample processing (Coble, 1996;McKnight et al, 2001;Stedmon et al, 2003;Gabor et al, 2014;Fellman et al, 2014;Jaffe et al, 2014). Recent application of electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI FT-ICR-MS) has provided accurate molecular formulas for numerous organic compounds comprising DOM and major DOM fractions (Mopper et al, 2007), including those that contain hetero-atoms, e.g.…”

Dynamics of Microbial Indicators in Waste Water

“…The development of biota (flora and fauna) in surface waters is governed by a variety of environmental conditions that determine species selection as well as the physiological performance of individual organisms (Morgan and Maitland, 1997). Primary production of organic matter, in the form of phytoplankton and macrophytes, is more intense in lakes and reservoirs and is usually more limited in rivers (Fellman et al, 2014;Shen et al, 2012). Degradation of organic substances and associated bacterial production may be a long-term process that may be important in groundwater and deep-sea water not exposed directly to sunlight (Chapman, 1996;Findlay and Sinsabaugh, 2003).…”