ICOS Science Conference, Session 4

1. Dissolved organic matter (DOM) dynamics
across North Sea river salinity gradients
Chiara Cooper,
Dorothee C. E. Bakker, Richard J. Cooper, Naomi Greenwood, Silke Kröger, Andrew
Mayes, Carol Robinson, Martin Johnson

2. Rationale
2
Modified from Mori et al., 2019; doi: 10.3389/fmars.2019.00481
Global Carbon Atlas. Data sources: Hastie et al., 2018; Lauerwald et al., 2015;
Laruelle et al., 2013; Raymond et al., 2013; Regnier et al., 2013.
• Uncertainties about the processes affecting DOM
• Composition of DOM gives information about its
origin

3. 3
Aim and objectives
The aim of this study is to investigate the composition and dynamics of
Dissolved Organic Matter in two lowland UK rivers as it is transported
downstream.
• 13 month period of sampling over a salinity gradient (November 2018 – December 2019)
• Analysis of the optical properties of DOM (coloured and fluorescent)

4. 4
Study area
River Yare
• catchment area = 757 km2;
• mean annual discharge = 1.47
m3 s-1;
• Baseflow Index (BFI) = 0.64
River Waveney
• catchment area = 943 km2;
• mean annual discharge = 1.77
m3 s-1;
• BFI = 0.46
Tidal rivers subject to several
stressors due to land-use and
alterations.

5. 5
Methods
Optical properties of DOM
• Parallel Factor analysis (PARAFAC)
• Fluorescence indices
• Coloured dissolved organic matter absorption coefficients
Chemical properties of DOM
• Dissolved organic carbon concentration
Additional analyses
• Chlorophyll-a
• Total suspended solids (TSS)
179 samples collected at monthly intervals between November 2018 and December 2019

6. 6
Results: DOM composition from PARAFAC
C1: Humic-like compounds,
terrestrially derived;
C2: Humic-like substances
related to anthropogenic
activities and indicative of
sewage impacted rivers;
C3: Protein-free aminoacids, in
particular tryptophan-like, related
to autochthonous DOM and
heterotrophic bacteria.
C1 C2 C3

7. 7
Results: spatial dynamics of DOM
• Decrease of CDOM and FDOM as the salinity increases (dilution?)
• Variability could be due to biotic and abiotic processes (flocculation, photodegradation, production?)

8. 8
• Evidence from literature shows flocculation at salinities 0-2 leads to losses of DOM of up to 15% (Sholkovitz,
1976; Asmala et al., 2014)
Results: DOC and CDOM vs TSS at low salinities for the River Yare

9. 9
Results: CDOM spectral slopes
• Increases in S275-295 indicate photodegradation
• Studies show this parameter to be unaffected by chemical
changes in the DOM environment (Fichot and Benner,
2012)
• Some studies report plankton DOM production could affect
S275-295 increase
• Strong relationship between α(300) and S275-295 indicative of
high CDOM in rivers and low in more saline waters

10. 10
Results: FDOM fluorescence indices
• BIX > 0.7 indicates dominance of autochthonous
and fresh DOM, BIX < 0.7 indicates less fresh
algal DOM1
• HIX > 10 indicates humified compounds
terrestrially sourced, HIX < 4 indicate fresh DOM
produced in-situ2
• Photodegradation reduces the molecular weight of
DOM and HIX3
1. Parlanti, 2000; Huguet et al., 2009; Coble, 2014.
2. Zsolnay et al., 1999; Huguet et al., 2009
3. Cory et al., 2010; Para et al., 2010; Chen et al., 2011

11. 11
Conclusions
 PARAFAC model identifies 2 humic-like components and a protein-like component
 DOM in these two lowland rivers decreases as it travels downstream to the sea
 During its transport DOM is affected by abiotic and biotic processes which alter its chemical
properties
 Possible flocculation of DOM when freshwater meets saline water
 DOM becomes less aromatic and its molecular weight decreases by photodegradation
 Microbial processing possibly increases the molecular weight of DOM
 There is a loss of DOM as the rivers flow out at sea
 Optical properties of DOM are useful for investigating DOM dynamics

12. 12
Future work
i. Investigate the seasonal variability of DOM
ii. Investigate how the tidal cycle affects DOM
iii. Calculate the fluxes of DOC, DON and DOP
iv. Evaluate the relative influence of meteorological variables, river discharge, land-use
and vegetation
v. Explore how physical and biogeochemical processes influence DOM dynamics