Going with the flow: Planktonic processing of dissolved organic carbon in streams.

A large part of the organic carbon in streams is transported by pulses of terrestrial dissolved organic carbon (tDOC) during hydrological events, which is more pronounced in agricultural catchments due to their hydrological flashiness. The majority of the literature considers stationary benthic biofilms and hyporheic biofilms to dominate uptake and processing of tDOC. Here, we argue for expanding this viewpoint to planktonic bacteria, which are transported downstream together with tDOC pulses, and thus perceive them as a less variable resource relative to stationary benthic bacteria. We show that pulse DOC can contribute significantly to the annual DOC export of streams and that planktonic bacteria take up considerable labile tDOC from such pulses in a short time frame, with the DOC uptake being as high as that of benthic biofilm bacteria. Furthermore, we show that planktonic bacteria efficiently take up labile tDOC which strongly increases planktonic bacterial production and abundance. We found that the response of planktonic bacteria to tDOC pulses was stronger in smaller streams than in larger streams, which may be related to bacterial metacommunity dynamics. Furthermore, the response of planktonic bacterial abundance was influenced by soluble reactive phosphorus concentration, pointing to phosphorus limitation. Our data suggest that planktonic bacteria can efficiently utilize tDOC pulses and likely determine tDOC fate during downstream transport, influencing aquatic food webs and related biochemical cycles.

Storage effects on quantity and composition of dissolved organic carbon and nitrogen of lake water, leaf leachate and peat soil water.

This study aimed to evaluate the effects of freezing and cold storage at 4 °C on bulk dissolved organic carbon (DOC) and nitrogen (DON) concentration and SEC fractions determined with size exclusion chromatography (SEC), as well as on spectral properties of dissolved organic matter (DOM) analyzed with fluorescence spectroscopy. In order to account for differences in DOM composition and source we analyzed storage effects for three different sample types, including a lake water sample representing freshwater DOM, a leaf litter leachate of Phragmites australis representing a terrestrial, 'fresh' DOM source and peatland porewater samples. According to our findings one week of cold storage can bias DOC and DON determination. Overall, the determination of DOC and DON concentration with SEC analysis for all three sample types were little susceptible to alterations due to freezing. The findings derived for the sampling locations investigated here may not apply for other sampling locations and/or sample types. However, DOC size fractions and DON concentration of formerly frozen samples should be interpreted with caution when sample concentrations are high. Alteration of some optical properties (HIX and SUVA254) due to freezing were evident, and therefore we recommend immediate analysis of samples for spectral analysis.

Carbon dynamics and their link to dissolved organic matter quality across contrasting stream ecosystems.

Streams represent active components of the carbon cycle as emitters of carbon dioxide (CO2) and methane to the atmosphere at a global scale. However, the mechanisms and governing factors of these emissions are still largely unknown, especially concerning the effect of land use. We compared dissolved and gaseous carbon dynamics in streams bordered by contrasting types of land use, specifically agriculture and forest. Carbon dioxide and methane partial pressures (pCO2 and pCH4, respectively) in the water body and carbon emissions via both gases were studied for 24h during four field expeditions. pCH4 did not differ between the two system types. pCO2 was constantly oversaturated in all streams and significantly higher in agricultural streams (annual mean 4282 ppm) compared to forest streams (annual mean 2189 ppm) during all seasons. However, emissions of CO2 were not significantly different between the stream types due to significantly higher gas transfer velocity in forest compared to agricultural streams. pCO2 was significantly positively correlated to the concentrations of dissolved organic carbon, dissolved nitrogen and soluble reactive phosphorus in the water. Furthermore, pCO2 was correlated to optical parameters of dissolved organic matter (DOM) quality, e.g., it increased with indicators of molecular size and an allochthonous fluorescent component identified by Parallel Factor Analysis (PARAFAC). This study demonstrates that different forms of land use may trigger a cascade of effects on the carbon production and emission of streams linked to changes in DOM quality.

Global effects of agriculture on fluvial dissolved organic matter.

Agricultural land covers approximately 40% of Earth's land surface and affects hydromorphological, biogeochemical and ecological characteristics of fluvial networks. In the northern temperate region, agriculture also strongly affects the amount and molecular composition of dissolved organic matter (DOM), which constitutes the main vector of carbon transport from soils to fluvial networks and to the sea, and is involved in a large variety of biogeochemical processes. Here, we provide first evidence about the wider occurrence of agricultural impacts on the concentration and composition of fluvial DOM across climate zones of the northern and southern hemispheres. Both extensive and intensive farming altered fluvial DOM towards a more microbial and less plant-derived composition. Moreover, intensive farming significantly increased dissolved organic nitrogen (DON) concentrations. The DOM composition change and DON concentration increase differed among climate zones and could be related to the intensity of current and historical nitrogen fertilizer use. As a result of agriculture intensification, increased DON concentrations and a more microbial-like DOM composition likely will enhance the reactivity of catchment DOM emissions, thereby fuelling the biogeochemical processing in fluvial networks, and resulting in higher ecosystem productivity and CO2 outgassing.

Comparison of organic matter composition in agricultural versus forest affected headwaters with special emphasis on organic nitrogen.

Agricultural management practices promote organic matter (OM) turnover and thus alter both the processing of dissolved organic matter (DOM) in soils and presumably also the export of DOM to headwater streams, which intimately connect the terrestrial with the aquatic environment. Size-exclusion chromatography, in combination with absorbance and emission matrix fluorometry, was applied to assess how agricultural land use alters the amount and composition of DOM, as well as dissolved organic nitrogen (DON) forms in headwater streams, including temporal variations, in a temperate region of NE Germany. By comparing six agriculturally and six forest-impacted headwater streams, we demonstrated that agriculture promotes increased DOC and DON concentrations, entailing an even more pronounced effect on DON. The major part of DOC and DON in agricultural and forest reference streams is exported in the form of humic-like material with high molecular weight, which indicates terrestrial, i.e., allochthonous sources. As an obvious difference in agricultural streams, the contribution of DOC and particularly DON occurring in the form of nonhumic high-molecular-weight, presumably proteinous material is clearly elevated. Altogether, DOM in agricultural headwaters is mainly complex-soil-derived and aromatic material with a low C:N ratio, which is more microbial processed than its counterpart from forest reference catchments. Our results emphasize the importance of agricultural land use on DOM loss from soils and identify agricultural soils as important DOC and particularly DON sources to headwater streams.