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

Abstract

Citation: Demars, B. O. L., N. Friberg, and B. Thornton. 2020. Pulse of dissolved organic matter alters reciprocal carbon subsidies between autotrophs and bacteria in stream food webs. Ecological Monographs 90(1):Abstract. Soils are currently leaching out dissolved organic matter (DOM) at an increasing pace due to climate and land use change or recovery from acidification. The implications for stream biogeochemistry and food webs remain largely unknown, notably the metabolic balance (biotic CO 2 emissions) and carbon cycling between autotrophs and bacteria. We increased by 12% the flux of DOM in a stream for three weeks to mimic a pulse of natural DOM supply from soils rich in organic matter. We were able to track its fate into the food web through the use of a before and after control impact experimental design and the addition of DOM with a distinctive d 13 C signature. We used whole-stream metabolism to quantify carbon fluxes. Both photosynthesis and heterotrophic respiration increased rapidly following C addition, but this was short lived, likely due to nutrient limitations. Carbon exchange between autotrophs and bacteria in the control stream accounted for about 49% of bacterial production and 37% of net primary production, under stable flow conditions. Net primary production relied partly (19% in the control) on natural allochthonous dissolved organic carbon via the CO 2 produced by bacterial respiration, intermingling the green and brown webs. The preferential uptake of labile carbon by bacteria and excess bacterial CO 2 relative to nutrients (N, P) for autotrophs shifted the reciprocal carbon exchange between bacteria and autotrophs to a predominantly one-way carbon flow from bacteria to autotrophs, increasing the C:N:P molar ratios of autotrophs, the latter likely to become less palatable to consumers. The bacterial response to sucrose addition shifted the metabolic balance toward heterotrophy increasing biotic CO 2 emissions (+125%), shortened the average distance travelled by a molecule of organic matter (À40%), and thus provided less organic matter of lower quality for downstream ecosystems. Even a small increase in labile dissolved organic matter supply due to climate and land use change could significantly alter in-stream carbon cycling, with large effects on stream food web and biogeochemistry in small streams draining catchments with soils rich in organic carbon.

Aggregated filter‐feeders govern the flux and stoichiometry of locally available energy and nutrients in rivers

“…Individuals that did not excrete measurable concentrations of DOC were not included in our model, which may be an overestimation of DOC excreted by mussel aggregations. Nevertheless, even small additions of labile DOM (~12% of total DOC) can profoundly alter reciprocal carbon subsidies between microbes and autotrophs in streams such that autotrophic production is increased by heightened microbial respiration (Demars et al., 2020). Together with previous studies, it appears that high biomass aggregations of animals supply a large source of microbially available energy in streams, although more explicit test of the importance of animal derived DOM to microbes is needed.…”

High-Frequency Monitoring Reveals Multiple Frequencies of Nitrogen and Carbon Mass Balance Dynamics in a Headwater Stream

“…DOC uptake in this study was associated with PAR (positive), discharge (negative), and magnitude of previous event (positive). The relationship with PAR is interesting as this could represent DOC uptake due to light availability stimulating metabolism (Demars et al, 2020) or conversely could be related to increased solar radiation reaching the stream and greater potential for photodegradation of DOC (Moody and Worrall, 2017). Further work is required to explicitly test this however the elevated delta DO during early spring (Figures 3A,C) suggests the former mechanism is more likely.…”

Brown meets green: light and nutrients alter detritivore assimilation of microbial nutrients from leaf litter

“…Further work must parcel out biomolecules that explain varying microbial P quality. We acknowledge our assumptions that Pycnopsyche did not feed selectively on microbial biomass, and that radiolabel 14 C did not transfer across microbial pools during labeling (e.g., label transfer from autotrophs to heterotrophs via exudation, or internal cycling of labeled CO 2 ; Demars et al 2020). While these processes merit investigation, the controlled and relatively short labeling conditions, especially of algae and bacteria, should have limited these dynamics during our study.…”

“…This is due to the assumption that autotrophs play minimal roles in detrital‐based food webs, especially in ecosystems of low light availability and low algal biomass, such as headwater streams. Yet, increasing research suggests that algae can play key roles in brown food webs by stimulating heterotrophic activity (Danger et al 2013, Kuehn et al 2014, Demars et al 2020). Moreover, food web data suggest that detritivores partly rely on green energy pathways, suggesting that autotrophs play greater roles in detrital food webs than is classically assumed (Wolkovich et al 2014).…”