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

Francesca L Brailsford,Alun T Owen,Christopher A Yates,Penny J Johnes,Miles R Marshall,Peter N Golyshin,Helen C Glanville,Davey L Jones

doi:10.1002/lno.11852

Abstract

AbstractDissolved 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.

Highlights

Dissolved organic matter (DOM) can be divided into two size categories, namely those compounds that are of high molecular weight (HMW; > 1000 Da) and those that are of low molecular weight (LMW; < 1000 Da) (Cui and Choo 2013)
Comparison of the depletion of DOM components (DOC, dissolved organic nitrogen (DON), dissolved organic phosphorus (DOP)) from solution demonstrated that dissolved organic carbon (DOC) removal from solution proportionally increases with increases in DON and DOP depletion, with DON removal being slightly more rapid and DOP removal being slightly slower than DOC removal, respectively
The depletion of all three DOM fractions increases as nutrient enrichment increases, and as the proportion of HMW DOM decreases along the river enrichment gradient

Summary

Introduction

Land cover and nutrients regulate DOM turnover derived breakdown products from woody debris, is considered to be relatively recalcitrant and not readily degraded by the microbial biomass (Zhang et al 2017) It is often present at high concentrations, in peat-rich headwater catchments. In natural and seminatural systems, individual LMW DOM compounds (e.g., amino acids) are present at very low concentrations in freshwaters (1–10 nmol LÀ1; Marie et al 2015; Hornak et al 2016). These concentrations are close to the influx-efflux equilibrium point for microbial transport systems (i.e., point of zero net flux). The results of the study were used to evaluate how DOM and SRP are processed across catchment-scale gradients of disturbance and nutrient enrichment

Objectives

Methods

Results

Discussion

Conclusion