Experimental evidence reveals impact of drought periods on dissolved organic matter quality and ecosystem metabolism in subalpine streams

Abstract

Subalpine streams are predicted to experience lower summer discharge following climate change and water extractions. In this study, we aimed to understand how drought periods impact dissolved organic matter (DOM) processing and ecosystem metabolism of subalpine streams. We mimicked a gradient of drought conditions in stream‐side flumes and evaluated implications of drought on DOM composition, gross primary production, and ecosystem respiration. Our experiment demonstrated a production and release of DOM from biofilms and leaf litter decomposition at low discharges, increasing dissolved organic carbon concentrations in stream water by up to 50%. Absorbance and fluorescence properties suggested that the released DOM was labile for microbial degradation. Dissolved organic carbon mass balances revealed a high contribution of internal processes to the carbon budget during low flow conditions. The flumes with low discharge were transient sinks of atmospheric CO2 during the first 2 weeks of drought. After this autotrophic phase, the metabolic balance of these flumes turned heterotrophic, suggesting a nutrient limitation for primary production, while respiration remained high. Overall our experimental findings suggest that droughts in subalpine streams will enhance internal carbon cycling by transiently increasing primary production and more permanently respiration as the drought persists. We propose that the duration of a drought period combined with inorganic nutrient availability are key variables that determine if more carbon is respired in situ or exported downstream.

Drought alters the biogeochemistry of boreal stream networks

“…Specifically, drought caused a significant reduction in aerobic respiration along the experimental stream reach, which decreased from −403 ± 172 to −130 ± 81 mmol O 2 m −2 d −1 . Previous studies testing the effects of drought, either experimentally or under natural conditions, have shown that low flows can either enhance 31,32 or reduce 33 rates of stream aerobic respiration. In our case, aerobic respiration decreased nonlinearly with WRT (r 2 = 0.41; p < 0.001; n = 111), ostensibly because the biochemical O 2 demand driven by aerobic respiration greatly exceeded the resupply of O 2 to hyporheic sediments as drought ensued.…”

Dynamic transfer of soil bacteria and dissolved organic carbon into small streams during hydrological events

“…As an alternative explanation, enrichment of DOC along the flow length could also be generated by enhanced primary production that enhances DOC concentrations, especially during spring and summer low flow (Harjung et al 2019). However, many previous studies have shown that first order streams presented very low autochthonous primary production that accounted for less than 5% of the annual organic carbon inputs in these ecosystems (Fisher and Likens 1973;Mulholland 1997;Richardson 2019).…”

Pollution profile of waterborne bacterial and fungal community in urban Rivers of Pearl River estuary: Microbial safety assessment

“…The components C1 (<240/ 475 nm) and C3 (<240/385 nm) were attributable to terrestrial humic-like substances (Murphy et al 2011;Kowalczuk et al 2013) and microbial humic-like substances (Yamashita et al 2011;Retelletti Brogi et al 2019), respectively. The components C4 (<240/360(450) nm) and C6 (275/325 nm) were indicators for the presence of tyrosinelike and tryptophan-like proteins and were presumably derived from plankton-derived organics or wastewater-impacted waters according to the references (Harjung et al 2019;Wang et al 2019). Overall, it was found that humic-like components C1, C2, C3, and C5 represented a total of 81 ± 3% (mean ± SD), while the two protein-like components (C4 and C6) accounted for 19 ± 3% of the six components, suggesting the dissolved organic matter is dominated by humic-like substances.…”