Land cover and nutrient enrichment regulates low‐molecular weight dissolved organic matter turnover in freshwater ecosystems

Abstract

Dissolved organic matter (DOM) is a complex mixture of carbon-containing compounds. The low-molecular weight (LMW) fraction constitutes thousands of different compounds and represents a substantial proportion of DOM in aquatic ecosystems. The turnover rates of this LMW DOM can be extremely high. Due to the challenges of measuring this pool at a molecular scale, comparatively little is known of the fate of LMW DOM compounds in lotic systems. This study addresses this knowledge gap, investigating the microbial processing of LMW DOM across 45 sites representing a range of physicochemical gradients and dominant land covers in the United Kingdom. Radioisotope tracers representing LMW dissolved organic carbon (DOC) (glucose), dissolved organic nitrogen (DON) (amino acid mixture), dissolved organic phosphorus (DOP) (glucose-6-phosphate), and soluble reactive phosphorus (SRP, measured as orthophosphate) were used to measure the microbial uptake of different DOM compounds in river waters. The amount of DOM biodegradation varied between different components (DON ≥ DOC > DOP), with the rate of turnover of all three increasing along a gradient of N and P enrichment across the range of sites. Conversely, the uptake of SRP decreased along this same gradient. This was ascribed to preferential utilization of DOP over SRP. Dominant land cover had a significant effect on DOM use as a resource, due to its control of nutrient enrichment within the catchments. We conclude that nutrient enrichment of river waters will lead to further DOM removal from the water column, increased microbial growth, and a decrease in stream oxygen saturation, exacerbating the effects of eutrophication in rivers.

Characterisation of riverine dissolved organic matter using a complementary suite of chromatographic and mass spectrometric methods

“…There is evidence that deoxysugars, fucose and rhamnose can be microbially derived (Guggenberger et al 1994b;Biersmith & Benner 1998), and these compounds are relatively more abundant at both Priors Farm and Chew Stoke where the microbial communities are hypothesised to be particularly active because of the contribution of farm and human wastes. This is supported by the microbial response to 14 C-labelled tracers introduced to water and sediments in the earlier work of Brailsford and colleagues in both the Conwy catchment, including Nant y Brwyn (Brailsford et al 2019(Brailsford et al , 2021 and Hampshire Avon (including Priors Farm) catchment (Brailsford et al 2021).…”

“…The total hydrolysable amino acid pool was an order of magnitude higher in the Conwy compared with Priors Farm, measuring 0.46 mg L −1 (58 µg N L −1 , 0.17 mg C L −1 , ~ 5% DOC) and 0.04 mg L −1 (5.1 µg N L −1 , 5 µg C L −1 , ~ 0.3% DOC), respectively, suggesting that that the DOM at Priors Farm is more degraded and perhaps proteins are more rapidly utilised by an active and/or larger microbial community. This is supported by further work conducted by Brailsford et al (2021) where water was collected from both Nant y Brwyn and Priors Farm and 14 C-labelled amino acids were added in an incubation experiment. The results showed after 24 h double the amount of labelled amino acids had been degraded in the Priors Farm water compared with Nant y Brwyn, suggesting that turnover rates of these compounds were greater in the agricultural catchment.…”

“…Dissolved organic matter (DOM) plays a fundamental role in nutrient cycling dynamics in riverine systems, accounting for the majority of the reactive carbon (C) present in aquatic ecosystems (Evans et al 2017) and a bioavailable source of nitrogen (N) and phosphorus (P) for both stream autotrophs and heterotrophs (Brailsford et al 2019(Brailsford et al , 2021Mackay et al 2020). As a result, DOM plays an important role in stream elemental nutrient cycling and the ecological functioning of aquatic systems (Cole et al 2007;Prairie 2008;Battin et al 2009;von Schiller et al 2017).…”

Determining patterns in the composition of dissolved organic matter in fresh waters according to land use and management

“…They conclude that concentrations are strongly linked to soil character and to land use and management, with elevated concentrations observed in agricultural soils, particularly those used for livestock production. In our own work we also see DOP concentrations in streams linked to landscape character and elevated in agricultural streams versus those draining through natural or semi-natural catchments (Yates et al 2019a;Lloyd et al 2019;Brailsford et al 2021), and both microbial and plant uptake of both DOP and DON and as a nutrient resource (Brailsford et al 2019a, b;Mackay et al 2020). Such elevated nutrient concentrations are commonly observed in UK agricultural catchments (Yates et al 2019a).…”

Utilization of Low Molecular Weight Carbon Sources by Fungi and Saprolegniales: Implications for Their Ecology and Taxonomy

“…In the other path, however, low molecular weight (LMW) compounds are leached from OM and are easily utilized by a wide range of microorganisms. Generally, LMW OM accounts for <20% of the entire carbon pool and contributes to biogeochemical processes in two different ways [ 8 ]. Firstly, they accelerate the humification process by providing an instant energy source to microbial communities to degrade the more recalcitrant polymers.…”