A Source of Terrestrial Organic Carbon to Investigate the Browning of Aquatic Ecosystems

Abstract

There is growing evidence that terrestrial ecosystems are exporting more dissolved organic carbon (DOC) to aquatic ecosystems than they did just a few decades ago. This “browning” phenomenon will alter the chemistry, physics, and biology of inland water bodies in complex and difficult-to-predict ways. Experiments provide an opportunity to elucidate how browning will affect the stability and functioning of aquatic ecosystems. However, it is challenging to obtain sources of DOC that can be used for manipulations at ecologically relevant scales. In this study, we evaluated a commercially available source of humic substances (“Super Hume”) as an analog for natural sources of terrestrial DOC. Based on chemical characterizations, comparative surveys, and whole-ecosystem manipulations, we found that the physical and chemical properties of Super Hume are similar to those of natural DOC in aquatic and terrestrial ecosystems. For example, Super Hume attenuated solar radiation in ways that will not only influence the physiology of aquatic taxa but also the metabolism of entire ecosystems. Based on its chemical properties (high lignin content, high quinone content, and low C:N and C:P ratios), Super Hume is a fairly recalcitrant, low-quality resource for aquatic consumers. Nevertheless, we demonstrate that Super Hume can subsidize aquatic food webs through 1) the uptake of dissolved organic constituents by microorganisms, and 2) the consumption of particulate fractions by larger organisms (i.e., Daphnia). After discussing some of the caveats of Super Hume, we conclude that commercial sources of humic substances can be used to help address pressing ecological questions concerning the increased export of terrestrial DOC to aquatic ecosystems.

A test of the subsidy–stability hypothesis: the effects of terrestrial carbon in aquatic ecosystems

“…See Table 3 for the statistical model, parameter estimates, and summary statistics. Third, we observed that bacterial respiration (BR) significantly increased along the terrestrial DOC supply gradient, but bacterial productivity (BP) was unaffected (Lennon et al 2013); this finding suggests that terrestrial DOC may be more important for subsidizing catabolic microbial processes than anabolic microbial processes. Values in this table correspond with results presented in Fig.…”

“…Dashed lines are Gaussian functions fit to observed data using maximum likelihood parameter estimation. Second, using the terrestrial carbon that was added to the ponds (i.e., Super Hume), we were able to isolate a diverse assemblage of aquatic bacteria that readily grew on humic substances as their sole source of carbon and energy (Lennon et al 2013). Notes: Data were fit using maximum likelihood with a Gaussian function that had the following form: y ¼ a 3 exp(À(supply À b) 2 /(2 3 c 2 )).…”

“…Notes: Data were fit using maximum likelihood with a Gaussian function that had the following form: y ¼ a 3 exp(À(supply À b) 2 /(2 3 c 2 )). Life-table experiments demonstrated that supplementing an algal-based diet with humic substances (i.e., Super Hume) increased the fitness of a Daphnia clone that was isolated from a DOC-enriched pond (Lennon et al 2013). 3.…”

Oxic water column methanogenesis as a major component of aquatic CH4 fluxes

“…For DOC, Superhume brand humic slurry was added to reach a target concentration of 15 mg of DOC per litre. For a full review of Superhume properties and applicability to freshwater experimentation, see Lennon et al 56 . The mesocosms were allowed to stabilize for 1 week before initial sampling and re-stocking zooplankton at ambient lake concentrations.…”

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“…We identified the lake chemical characteristics that influenced bacterial dormancy in hypersaline and freshwater lakes using multiple regression with lake as a categorical 224 predictor variable (Neter et al, 1996, Lennon andPfaff, 2005;Lennon et al, 2013). We tested whether or not a variable (i.e., dissolved oxygen, pH, salinity, TN and TP) related 226 12 to the percentage of dormant bacteria occurring in the five hypersaline and five freshwater lakes and the recovery of dormant taxa using forward selection procedure and 228…”