Differentiating the degradation dynamics of algal and terrestrial carbon within complex natural dissolved organic carbon in temperate lakes

Abstract

[1] It has often been hypothesized that the dissolved organic carbon (DOC) pool of algal origin in lakes is more bioavailable than its terrestrial counterpart, but this hypothesis has seldom been directly tested. Here we test this hypothesis by tracking the production and isotopic signature of bacterial respiratory CO 2 in 2 week lake water incubations and use the resulting data to reconstruct and model the bacterial consumption dynamics of algal and terrestrial DOC. The proportion of algal DOC respired decreased systematically over time in all experiments, suggesting a rapid consumption and depletion of this substrate. Our results further show that the algal DOC pool was used in proportions and at rates twice and 10 times as high as the terrestrial DOC pool, respectively. On the other hand, the absolute amount of labile terrestrial DOC was on average four times higher than labile algal DOC, accounting for almost the entire long-term residual C metabolism, but also contributing to short-term bacterial C consumption. The absolute amount of labile algal DOC increased with chlorophyll a concentrations, whereas total phosphorus appeared to enhance the amount of terrestrial DOC that bacteria could consume, suggesting that the degradation of these pools is not solely governed by their respective chemical properties, but also by interactions with nutrients. Our study shows that there is a highly reactive pool of terrestrial DOC that is processed in parallel to algal DOC, and because of interactions with nutrients, terrestrial DOC likely supports high levels of bacterial metabolism and CO 2 production even in more productive lakes.Citation: Guillemette, F., S. L. McCallister, and P. A. del Giorgio (2013), Differentiating the degradation dynamics of algal and terrestrial carbon within complex natural dissolved organic carbon in temperate lakes, J. Geophys. Res.

Selective consumption and metabolic allocation of terrestrial and algal carbon determine allochthony in lake bacteria

“…The results of our mass balance approach suggest that bacteria disproportionally removed algal DOC from a largely terrestrially dominated DOC pool, even in lakes of low primary productivity (Supplementary Figure S2). This preferential consumption of algal DOC by lake bacterial communities has been previously postulated, but past inferences were based on the isotopic signal of bacterial biomass (Kritzberg et al, 2004), of the respiratory CO 2 (Guillemette et al, 2013) or on modeling (Berggren et al, 2010b), rather than on the combined amounts of respired and incorporated DOC as we present here. Our study thus provides novel and more robust support to the contention that algal DOC is preferentially consumed by bacteria (Figure 3).…”

“…A widespread view in aquatic ecology and carbon (C) biogeochemistry is that bacterial communities exposed to a mix of algal and terrestrial compounds should develop a strategy of resource utilization whereby algal C is preferentially consumed and incorporated into biomass over terrestrial C owing to its greater accessibility and nutritional quality (Hobbie, 1988;Bianchi, 2011;Guillemette et al, 2013). However, the preferential consumption of algal C and its subsequent allocation to growth rather than to respiration remain to be empirically tested, as past studies have never simultaneously quantified the sources of DOC that support these different metabolic pathways.…”

Higher late summer methane emission from central than northern European lakes

“…Lateral transport of CH 4 from the littoral zone might be an important process in small central European lakes, as indicated by the CH 4 concentrations and stable carbon isotopic composition (δ 13 C) profiles in the water column reported by Rinta et al (2015), and may provide a source for high diffusive fluxes in the pelagic zone as well (Hofmann, 2013). The quality of the organic matter (OM) in the central European lakes may be more suitable for methanogenesis than in the boreal lakes, since autochthonous OM tends to be a better substrate for methanogenesis than OM of terrestrial origin (Duc et al, 2010;West et al, 2012;Guillemette et al, 2013). In-lake primary production is expected to be higher in the central European lakes due to the higher nutrient inputs from nonforested catchments dominated by arable land, pastures, and alpine grasslands often used for seasonal summer pasturing than from the more forested catchments in the boreal regions (Tab.…”

Improving the precision of lake ecosystem metabolism estimates by identifying predictors of model uncertainty

“…Low rho uncertainty on days with low PAR levels may be a product of organic carbon source availability. There is some evidence that ecosystem respiration changes over the course of the day, potentially due to changing sources of organic carbon since autochthonous organic matter is generally more biolabile than allochthonous sources (Sadro et al 2011;Guillemette et al 2013). On days with low productivity, there may be little autochthonous organic carbon production and the carbon source fueling respiration may vary less, resulting in a more consistent respiration rate.…”