Introduction
Export of riverine organic matter (OM) to the coastal ocean contributes
significantly to heterotrophic metabolism in coastal marine ecosystems,
supplying approximately 0.5 Pg of organic carbon (OC) per year,
approximately half in dissolved and half in particulate form
(e.g., Cai, 2011). A significant portion of terrestrial OC is
metabolized and transformed by biological activity within terrestrial
aquatic ecosystems (Cole and Caraco, 2001; Battin et al., 2009;
Aufdenkampe et al., 2011), and the molecular composition of OM is thought to
exert controls on its bioavailability to in situ microbial communities (e.g.,
Sun et al., 1997; Fellman et al., 2009; Stepanauskas et al., 2005). The
concentration and composition of OM in freshwater systems also has
implications for mobilization and speciation of mercury (Hg), an important
element for its potential toxicity, particularly when converted to
monomethyl Hg and biomagnified within food webs (Fleck et al., 2014;
Zheng et al., 2012). Comprehensive time-series data sets for fluvial systems,
including nutrients, dissolved major elements, water isotopes, and suspended
sediments in addition to DOM (dissolved organic matter) properties are rare (e.g., Dornblaser and
Striegl, 2007, 2009; Spencer et al., 2012; Walling and Foster, 1975; Bagard
et al., 2011), yet are necessary to establish a robust foundation for
distinguishing biogeochemical from hydrologic and physical processes.
Rapid changes in riverine DOC concentrations are often associated with
variations in discharge on timescales of hours to weeks. Such hydrologic
effects on DOC concentration have been observed in large Arctic rivers
during the spring freshet (Holmes et al., 2012; Mann et al., 2012;
Wickland et al., 2012), as well as in tropical catchments experiencing
wet-season flushing of surface soils (Spencer et al., 2010; Laraque et
al., 2013), and in small streams and headwater catchments dominated by
episodic rain or snowmelt events (Sandford et al., 2013; Raymond and
Saiers, 2010; Lloret et al., 2013; Fellman et al., 2009; Battin, 1998). In
these settings, the composition of DOM is often observed to change
significantly, as changing hydrologic flow paths draw upon different DOM
pools (Striegl et al., 2005; Mann et al., 2012; Spencer et al., 2010).
The bulk concentration of DOC in the Fraser River has been investigated
through water quality monitoring by Environment Canada (Swain,
2007); however, more detailed information about the composition of Fraser
River DOM and its controlling processes are lacking. DOC constitutes the
larger pool of OC delivered to the coastal ocean by the Fraser River, at
concentrations ∼ 5 times higher than POC during the spring
freshet when suspended sediment concentrations peak, and ∼ 30
times higher during low discharge conditions.
The residence time of OC derived from fresh litterfall in soils depends on
its initial composition and environmental conditions, and varies between
less than 1 year to hundreds or thousands of years (Mills et
al., 2014). DOM that enters stream channels under base flow conditions
generally originates from deeper soil layers, where organic matter has been
exposed to microbial degradation and potential sorption surfaces for a long
time relative to fresh litter leachates in shallow soil layers (Hope et
al., 1994; Easthouse et al., 1992; Michalzik et al., 2003). Freshet DOM,
which derives largely from overland flow and more extensive soil inundation,
is composed of organic matter that has been exposed to soil microbial
communities for a shorter time than deep soil DOM (Wickland et al., 2007;
O'Donnell et al., 2010). This distinction is evident in the increase in DOC
radiocarbon content across the freshet transition in large Arctic rivers, as
shallow soil DOC containing bomb 14C is released during spring thaw
(Raymond et al., 2007). This relatively fresh DOM has
also been identified as more susceptible to microbial degradation (Holmes
et al., 2008; Mann et al., 2012), and is therefore a more potent source of
metabolic fuel to coastal ocean ecosystems and thus a potential flux of
CO2 to the atmosphere.
The Fraser River is a large, mountainous river basin in southwestern Canada.
Total basin area is 233 000 km2 and average annual discharge is
112 km3 a-1 (Meybeck and Ragu, 2012). The basin is largely
forested and experiences relatively little anthropogenic modification in the
form of dams, channelization, or land cover alteration for a basin of its
size and latitude. The time series record of DOC concentration in the Fraser
River from Environment Canada (Swain, 2007) has not been
analyzed and published in peer-reviewed literature, and we are not aware of
any published studies of DOM composition in the Fraser River. Several
aspects of the basin (e.g., physiographic features, hydrologic regime and
climatic setting) suggest it may exhibit somewhat different behavior from
established paradigms for large Arctic rivers and small streams. First, a
number of the major Arctic rivers (Yenisey, Ob', Lena, Kolyma, Mackenzie)
generally have catchments that drain northward and narrow flowing downstream
(a “northward funnel” shape), triggering a sudden peak in total basin
discharge in the downstream portions of these basins during spring melt. In
contrast, the southward-draining orientation of the Fraser basin and
northward migration of spring warming mutes the amplitude and prolongs the
duration of the freshet. Second, the freshet hydrograph in the Fraser River
is often punctuated by pauses in melting due to cold intervals and/or
precipitation pulses from spring storms, which, due to the presence of
mountain ranges, can exert longitudinal differences across the basin. The
resulting stepwise character of the freshet may lead to a more complex
transition between base flow and freshet DOC concentrations, and a less
extreme peak DOC concentration. Third, the smaller size of the Fraser basin
relative to large Arctic rivers also means that storm events typically
impact large portions of the basin, causing significant short-term increases
in total discharge. Fourth, the lack of extensive permafrost in the Fraser
basin excludes significant inputs from a potential pool of aged soil DOM
during late spring and summer months. Finally, in small streams,
storm-driven discharge events are short-lived and may deliver fresh DOM to
stream channels more efficiently than the long, relatively gradual rising
limb of the spring freshet in the Fraser River.
A consequence of variability in DOM concentration and composition in
freshwaters is the potential for dynamic behavior of dissolved Hg. Due to the
strong affinity of dissolved Hg for DOM, especially reduced organosulfur
moieties (Haitzer et al., 2002, 2003; Gerbig et al., 2011), concentrations of
DOC and total dissolved Hg are typically positively correlated in natural
waters (Schuster et al., 2011; Dittman et al., 2010; Burns et al., 2013;
Demers et al., 2010). We are not aware of any Hg observations in the main
stem Fraser River by federal or provincial government agencies, although Hg
monitoring has been recommended (MacDonald et al., 2011). Mercury
concentrations in water and fish tissue are not presently found at levels
deemed unfit for human consumption; however, it is a health concern for the
Fraser River fisheries and individuals who subsist on diets rich in fish
(Cohen, 2012; Kelly et al., 2008). Potential sources of Hg within the Fraser
basin include urban and industrial point sources (e.g., sewage effluent,
paper pulp mills), atmospheric deposition (particularly aerosols derived from
coal combustion in east Asia), and mobilization of legacy Hg contamination
from placer gold mining (including hydraulic mining in some areas), which was
widespread in the central portion of the basin in the 1850s–1910s. Given the
major role of rivers in global surface cycling of Hg (Amos et al., 2014), it
is important to constrain the flux of Hg from this regionally significant
river basin.
In this study we examine how (1) rapid changes in discharge impact DOC load
during the early stages of the spring freshet; (2) hydrology influences
changes in DOC flux and composition throughout the year; and (3) DOM and
suspended sediment dynamics influence the Hg load of the Fraser River. The
spring freshet is a critical period to quantify, as it is responsible for
the bulk of total annual fluxes of many constituents, including DOC and
suspended sediment (Swain, 2007). Given the ongoing and
anticipated changes in hydrological conditions in the Fraser basin under a
warming climate – particularly an increase in net annual and winter
precipitation, a shift towards more rain-dominated relative to
snow-dominated precipitation, and a corresponding decrease in snowpack, and
an earlier onset to the spring freshet (Morrison et al., 2002; Déry
et al., 2012; Shrestha et al., 2012; Riche et al., 2014) – understanding
freshet biogeochemical dynamics under present conditions is critical to
detecting future changes and anticipating their consequences.
Methods
Discharge and historical data
Continuous discharge and water temperature information (5 min frequency)
were obtained from the Environment Canada Water Office online real-time
hydrologic data source (http://www.wateroffice.ec.gc.ca). The record used
was the station at Hope (08MF005; 49.381∘ N, -121.451∘ E),
which is ∼ 100 km upstream of our sampling location at
Fort Langley (49.172∘ N,–122.577∘ E; Fig. 1), and the
farthest downstream station for which gauge height is not influenced by
tides. Discharge at Fort Langley is 10–20 % higher than at Hope (due
mainly to input from the Harrison River), and water temperature is
∼ 1.5 ∘C higher. The discharge and water temperature
records for the study year 2013 are shown in Fig. S1 in the Supplement.
The Fraser River basin, highlighting lakes (natural and man-made;
dark grey) and glaciated areas (light grey). Samples were collected at Fort
Langley and discharge data (from Environment Canada) are from Hope. River
contours and watershed boundaries provided by HydroSHEDS; lake and glacier
outlines are derived from the Digital Chart of the World and accessed
through Natural Earth Data (http://www.naturalearthdata.com).
Historical Fraser River DOC concentration data at Hope (1997–2014, station
BC08MF0001) were obtained from Environment Canada Pacific Yukon Freshwater
Quality Monitoring and Surveillance online data repository
(http://aquatic.pyr.ec.gc.ca/). A portion of this record is presented with
sampling and analytical information by Swain (2007). Average
DOC loads and discharge-weighted average DOC concentrations were calculated
from time series records using the LoadEst program (Runkel
et al., 2004) as described previously by Voss et al. (2014).
Sample collection and basic water chemistry
The data presented here include Fraser River samples collected during the
early portion of the 2013 spring freshet (Table 1), and a DOM-specific
data set for samples collected between June 2011 and September 2013
(Table 2). During the 2013 early freshet (26 March to 22 April), discrete samples
for basic water chemistry, SPM concentration, and DOM properties were
collected daily, while samples for Hg were collected every 4–5 days. The
longer record of DOM properties (2011–2013) constitutes samples collected
approximately twice monthly. No field duplicates or blanks were collected
for chemical analyses.
(a) Measured suspended particulate matter (SPM) concentration
determined from the mass of sediment recovered from filtered water
correlated strongly with turbidity measured with a nephelometer. (b) SPM
concentration difference was calculated as the difference between the SPM
concentration determined from the mass of sediment recovered from filtered
water and that estimated from the linear correlation in panel (a). (c)
Suspended sediment concentrations increased rapidly during the early freshet
period.
Geochemical data for the 2013 early freshet period. IGSN codes refer
to International GeoSample Numbers in the System for Earth Sample
Registration (SESAR) database; sample metadata can be accessed at
www.geosamples.org. TDHg = total dissolved mercury; THg = total
mercury (unfiltered water). For sediment %OC, δ13C, and
δ15N data, if calculated uncertainties (1 s.d. of the mean of
triplicate analyses) were less than instrumental uncertainty (0.1 for %OC,
0.3 ‰ for δ13C, and 0.4 ‰ for δ15N),
instrumental uncertainties are shown. All times are given in Pacific daylight
time (PDT), UCT-08:00.
Date
Parent IGSN
Qw at Hope
T
DO
Cond.
pH
Turb.
SPM conc.
δD
δ18O
DOC
POC
in situ FDOM
ex situ FDOM
(m3 s-1)
(∘C)
(mg L-1)
(µS cm-1)
(NTU)
(mg L-1)
(‰)
(‰)
(µmol L-1)
(µmol L-1)
(RFUB)
(RFUB)
26 Mar 13
GRO000920
980
6.8
12.7
132.0
7.53
3.49 ± 0.16
3.33 ± 0.15
-115.7
-15.35
277
9.2 ± 0.4
693
701
27 Mar 13
GRO000921
981
6.7
12.9
132.5
7.68
3.58 ± 0.17
3.48 ± 0.17
-120.7
-16.13
240
8.1 ± 0.5
628
583
28 Mar 13
GRO000922
984
6.7
12.7
134.2
7.63
3.45 ± 0.16
3.26 ± 0.16
-118.7
-15.87
206
703
722
29 Mar 13
GRO000923
1015
7.2
13.3
135.6
7.66
4.03 ± 0.26
4.24 ± 0.27
-119.8
-15.99
198
10.7 ± 0.7
647
596
30 Mar 13
GRO000178
1042
7.6
11.7
134.4
7.71
4.5 ± 0.6
5.0 ± 0.6
-120.7
-16.05
205
12.6 ± 1.7
605
591
31 Mar 13
GRO000924
1085
7.8
12.3
132.5
7.79
-123.1
-16.38
198
570
593
1 Apr 13
GRO000925
1182
8.4
11.6
130.8
7.80
-122.7
-16.20
205
567
539
2 Apr 13
GRO000926
1294
8.4
12.3
122.2
7.67
-120.7
-16.02
189
555
546
3 Apr 13
GRO000179
1363
8.3
12.3
123.9
7.73
5.7 ± 0.3
7.1 ± 0.4
-120.9
-15.93
199
17.2 ± 1.0
581
572
4 Apr 13
GRO000927
1485
8.2
12.4
122.3
7.71
6.7 ± 0.4
8.6 ± 0.5
-121.8
-16.08
205
19.7 ± 1.4
626
624
5 Apr 13
GRO000928
1774
8.1
12.0
121.3
7.68
8.1 ± 0.3
10.9 ± 0.4
-122.2
-16.04
217
26.4 ± 1.1
696
707
6 Apr 13
GRO000929
1861
8.0
12.2
117.2
7.69
12.6 ± 0.4
18.5 ± 0.6
-122.2
-16.02
234
28.1 ± 1.0
791
825
7 Apr 13
GRO000180
1879
7.2
12.5
109.1
7.67
22.4 ± 1.1
34.9 ± 1.7
-120.1
-15.95
266
41.6 ± 2.1
847
966
8 Apr 13
GRO000930
1936
6.8
12.5
109.2
7.66
39.5 ± 1.4
63.4 ± 2.2
-121.0
-15.78
286
62.8 ± 2.6
844
1142
9 Apr 13
GRO000931
2158
6.8
12.7
118.1
7.70
38.0 ± 1.3
60.8 ± 2.1
-121.6
-16.07
343
58.8 ± 2.2
951
1267
10 Apr 13
GRO000932
2354
7.0
13.0
124.1
7.77
53.0 ± 1.3
85.9 ± 2.1
-123.5
-16.32
415
73.4 ± 1.8
1030
1539
11 Apr 13
GRO000933
2337
6.7
13.2
120.6
7.74
56.6 ± 0.8
91.8 ± 1.4
-125.5
-16.58
413
82.3 ± 1.3
995
1553
12 Apr 13
GRO000181
2435
6.0
13.0
115.5
7.76
43.6 ± 1.1
70.2 ± 1.7
-125.3
-16.42
419
53.6 ± 1.4
1114
1537
13 Apr 13
GRO000934
2511
5.7
12.9
118.1
7.74
38.6 ± 1.1
61.9 ± 1.7
-125.0
-16.21
435
62.7 ± 1.8
1250
1663
14 Apr 13
GRO000935
2579
5.7
13.0
121.7
7.77
30.9 ± 0.7
49.1 ± 1.1
-127.3
-16.64
445
49.0 ± 1.2
1339
1594
15 Apr 13
GRO000936
2479
6.0
13.0
127.4
7.90
81 ± 4
133 ± 6
-128.0
-16.77
458
89 ± 4
929
1710
16 Apr 13
GRO000182
2475
6.6
12.9
124.1
7.81
44.1 ± 0.9
71.0 ± 1.5
-129.1
-16.89
558
62.9 ± 1.4
1303
1776
17 Apr 13 09:00
GRO000937
2385
6.1
13.2
125.8
7.81
39.6 ± 1.1
63.5 ± 1.8
-130.2
-16.98
526
59.3 ± 1.7
1421
1856
17 Apr 13 11:45
GRO000938
2357
6.6
13.3
126.2
7.81
40.6 ± 1.2
65.2 ± 1.9
-131.3
-17.12
522
1396
1836
17 Apr 13 14:30
GRO000939
2349
6.8
13.4
126.3
7.79
40.9 ± 1.7
65.7 ± 2.8
-130.9
-17.13
506
17 Apr 13 17:45
GRO000940
2355
7.0
13.1
126.3
7.77
-130.3
-16.90
505
18 Apr 13
GRO000941
2405
6.2
13.2
125.6
7.81
36.5 ± 1.6
58.3 ± 2.5
-131.4
-17.14
497
61 ± 3
19 Apr 13
GRO000942
2356
6.5
12.9
125.7
7.80
31.2 ± 1.1
49.6 ± 1.7
-129.6
-16.80
493
51.1 ± 2.1
20 Apr 13
GRO000183
2430
6.8
12.8
124.8
7.74
27.0 ± 1.1
42.5 ± 1.7
-127.8
-16.82
492
52.9 ± 2.4
21 Apr 13
GRO000943
2906
7.3
12.6
125.4
7.79
27.6 ± 1.9
43.5 ± 2.9
-127.5
-16.87
469
47 ± 3
22 Apr 13
GRO000945
3078
6.9
12.7
122.5
7.82
25.9 ± 0.4
40.7 ± 0.6
-128.3
-16.74
477
Continued.
NO2+ NO3
PO4
NH4
SiO4
Ca
Mg
Na
K
Cl
SO4
SPM %OC
SPM C / N
SPM δ13C
SPM δ15N
TDHg
THg
SPM THg
(µmol L-1)
(µmol L-1)
(µmol L-1)
(µmol L-1)
(µmol L-1)
(µmol L-1)
(µmol L-1)
(µmol L-1)
(µmol L-1)
(µmol L-1)
(‰)
(‰)
(pmol L-1)
(pmol L-1)
(pmol mg-1)
15.10
0.36
1.46
95
414
161
215
28.3
80.7
100
3.3 ± 0.1
9.7 ± 0.3
-27.7 ± 0.3
1.6 ± 0.7
11.75
0.33
1.86
104
433
167
198
25.0
61.6
105
2.8 ± 0.1
10.5 ± 0.9
-28.1 ± 0.4
1.1 ± 0.4
19.40
0.32
1.85
107
421
165
211
26.8
75.5
103
6.87
0.34
2.29
92
434
172
222
28.0
75.2
104
3.0 ± 0.1
9.61 ± 0.07
-27.1 ± 0.3
2.2 ± 0.4
13.82
0.24
1.48
102
431
164
198
25.6
70.4
104
3.1 ± 0.1
9.0 ± 0.4
-27.3 ± 0.3
2.0 ± 0.5
10.07 ± 0.10
14.5 ± 1.6
0.44
11.22
0.23
1.23
101
428
173
211
25.5
60.0
104
4.5 ± 0.1
10.3 ± 0.7
-27.2 ± 0.3
2.0 ± 0.4
9.96
0.22
0.38
97
433
175
210
24.9
56.6
102
4.5 ± 0.2
8.9 ± 0.4
-27.3 ± 0.3
1.4 ± 0.4
6.55
0.23
1.79
88
406
154
187
22.5
54.3
97
3.6 ± 0.1
8.5 ± 0.4
-27.2 ± 0.3
2.0 ± 0.4
9.64
0.29
1.33
90
401
150
170
22.0
51.6
97
2.9 ± 0.1
9.27 ± 0.13
-26.4 ± 0.3
2.1 ± 0.4
13.3 ± 0.8
16.4 ± 1.0
0.46
4.24
0.35
0.98
83
390
151
181
22.0
52.1
92
2.8 ± 0.1
9.4 ± 0.9
-27.6 ± 0.6
2.0 ± 0.4
9.64
0.34
1.71
95
396
153
187
23.6
55.4
90
2.9 ± 0.1
9.3 ± 0.2
-26.1 ± 0.3
2.4 ± 0.4
7.44
0.31
1.10
92
385
146
174
23.9
52.2
85
1.8 ± 0.1
9.9 ± 0.8
-27.3 ± 0.4
2.3 ± 0.5
12.23
0.29
0.98
93
317
122
143
20.0
45.6
73
1.4 ± 0.1
10.0 ± 0.6
-26.9 ± 0.3
2.3 ± 0.6
5.9 ± 0.5
41.7 ± 1.6
0.46
10.43
0.40
2.12
88
371
140
158
25.1
45.8
71
1.2 ± 0.1
10.5 ± 1.2
-26.9 ± 0.3
2.1 ± 0.6
7.60
0.34
0.82
96
393
150
161
24.4
47.9
75
1.2 ± 0.1
10.7 ± 0.9
-26.8 ± 0.3
2.0 ± 0.4
13.85
0.48
1.15
107
408
158
165
27.1
51.1
77
1.0 ± 0.1
10.3 ± 0.8
-27.0 ± 0.5
2.7 ± 0.4
7.74
0.37
0.87
100
398
159
164
26.5
48.6
78
1.1 ± 0.1
11.0 ± 1.0
-27.1 ± 0.4
1.9 ± 1.0
12.17
0.38
0.92
104
391
148
151
24.9
45.1
74
0.9 ± 0.1
10.9 ± 1.1
-26.9 ± 0.3
1.7 ± 0.7
10.6 ± 1.2
44.3 ± 2.7
0.43
11.85
0.45
1.15
103
383
154
155
26.7
49.9
75
1.2 ± 0.1
10.7 ± 0.9
-27.1 ± 0.3
1.8 ± 0.5
7.10
0.28
0.70
94
400
163
158
27.3
46.9
76
1.2 ± 0.1
10.8 ± 1.2
-26.8 ± 0.3
1.9 ± 0.5
11.18
0.38
0.87
108
425
163
156
26.5
45.6
76
0.8 ± 0.1
10.9 ± 0.7
-26.4 ± 0.3
1.9 ± 0.6
11.69
0.38
0.95
107
416
162
154
27.0
58.9
73
1.1 ± 0.1
10.9 ± 1.1
-26.9 ± 0.3
2.3 ± 0.8
11.8 ± 0.3
50.1 ± 1.8
0.43
11.64
0.36
0.79
113
423
172
165
28.4
46.2
73
1.1 ± 0.1
10.7 ± 0.7
-26.7 ± 0.3
2.1 ± 0.6
10.19
0.36
1.21
111
426
176
168
28.8
65.7
78
7.77
0.35
0.87
110
426
175
167
28.2
49.1
77
8.24
0.33
0.88
108
422
172
163
28.5
47.3
75
7.83
0.34
1.24
84
427
171
158
26.8
47.8
78
1.2 ± 0.1
11.1 ± 1.3
-26.8 ± 0.3
2.2 ± 0.8
11.22
0.38
0.91
108
410
165
160
27.8
47.0
73
1.2 ± 0.1
10.8 ± 1.2
-27.5 ± 0.4
2.2 ± 0.4
12.01
0.65
0.88
111
411
169
162
28.3
49.7
77
1.5 ± 0.1
10.9 ± 0.8
-27.6 ± 0.3
1.9 ± 0.4
15.2 ± 1.7
69 ± 13
0.46
6.78
0.36
0.64
105
421
170
162
28.1
49.8
74
1.3 ± 0.1
11.1 ± 0.9
-27.7 ± 0.4
2.0 ± 0.6
10.05
0.25
0.79
105
409
164
154
25.9
43.6
72
Two-year record of DOC concentration and optical properties of the
Fraser River main stem at Fort Langley. Sampling in 2012 was not at
sufficiently high frequency to capture the freshet pulse of DOC. IGSN codes
refer to International GeoSample Numbers in the System for Earth Sample
Registration (SESAR) database; sample metadata can be accessed at www.geosamples.org.
Date
IGSN
DOC
a254
SUVA254
a250 : a365
SR
(µmol L-1)
(m-1)
(L mgC-1 m-1)
21 Apr 2011
GRO000128
532
3 May 2011
GRO000129
733
10 May 2011
GRO000130
830
13 May 2011
GRO000131
671
25 May 2011
GRO000133
601
3 Jun 2011
GRO000135
457
7 Jun 2011
GRO000076
386
28 Jun 2011
GRO000140
265
19.37
2.64
6.45
0.98
8 Jul 2011
GRO000142
282
20.90
2.68
6.38
0.95
15 Jul 2011
GRO000143
364
28.18
2.80
5.92
0.87
19 Jul 2011
GRO000144
339
28.32
3.02
6.06
0.95
29 Jul 2011
GRO000145
335
26.91
2.90
5.91
0.95
26 Sep 2011
GRO000155
193
12.65
2.37
6.60
1.11
14 Oct 2011
GRO000156
242
17.43
2.61
6.03
1.02
25 Oct 2011
GRO000157
256
16.69
2.36
6.89
0.97
26 Oct 2011
GRO000158
250
16.87
2.44
6.58
1.04
31 Oct 2011
GRO000159
275
17.15
2.25
7.91
0.86
15 Nov 2011
GRO000160
318
18.28
2.08
8.12
0.87
28 Nov 2011
GRO000161
254
19.88
2.83
5.98
0.92
11 Jan 2012
GRO000163
237
22.52
3.43
6.38
0.93
10 Feb 2012
GRO000165
217
20.13
3.35
6.53
1.01
18 May 2012
GRO000168
352
35.67
3.67
5.50
0.92
13 Jun 2012
GRO000220
299
28.89
3.49
5.84
0.89
22 Jun 2012
GRO000207
280
26.92
3.47
5.78
0.88
29 Jun 2012
GRO000169
207
19.26
3.37
6.45
0.94
5 Jul 2012
GRO000170
200
18.28
3.30
6.54
0.96
13 Jul 2012
GRO000171
176
16.33
3.35
6.56
0.98
27 Jul 2012
GRO000222
14.87
6.15
1.10
17 Aug 2012
GRO000239
13.34
7.05
1.11
7 Sep 2012
GRO000172
14.48
6.46
1.17
21 Sep 2012
GRO000221
14.97
6.64
1.11
5 Oct 2012
GRO000223
13.33
6.67
1.11
18 Oct 2012
GRO000173
17.40
5.72
1.00
12 Jan 2013
GRO000218
23.58
5.49
0.85
2 Feb 2013
GRO000174
266
21.02
2.85
5.82
0.92
9 Feb 2013
GRO000175
248
22.55
3.28
5.46
0.89
16 Mar 2013
GRO000176
242
23.97
3.59
5.21
0.86
23 Mar 2013
GRO000177
209
17.69
3.06
5.88
0.92
27 Mar 2013
GRO000921
240
17.02
2.56
5.98
1.04
28 Mar 2013
GRO000922
206
18.76
3.29
5.80
0.94
29 Mar 2013
GRO000923
198
19.56
3.57
5.62
0.96
30 Mar 2013
GRO000178
205
18.66
3.29
5.73
0.99
31 Mar 2013
GRO000924
198
18.47
3.37
5.72
0.98
1 Apr 2013
GRO000925
205
17.70
3.13
5.98
0.98
2 Apr 2013
GRO000926
189
20.60
3.94
5.34
1.07
3 Apr 2013
GRO000179
199
19.92
3.62
5.58
1.04
4 Apr 2013
GRO000927
205
25.96
4.58
5.49
1.11
5 Apr 2013
GRO000928
217
21.23
3.54
5.51
0.99
6 Apr 2013
GRO000929
234
25.40
3.93
5.38
1.02
7 Apr 2013
GRO000180
266
32.65
4.44
5.30
1.06
8 Apr 2013
GRO000930
286
29.12
3.68
5.48
0.89
9 Apr 2013
GRO000931
343
34.35
3.62
5.52
0.88
10 Apr 2013
GRO000932
415
41.05
3.58
5.64
0.87
11 Apr 2013
GRO000933
413
52.53
4.60
4.94
0.99
12 Apr 2013
GRO000181
419
47.00
4.05
5.18
0.93
13 Apr 2013
GRO000934
435
51.81
4.31
5.08
0.94
14 Apr 2013
GRO000935
445
44.52
3.62
5.42
0.86
15 Apr 2013
GRO000936
458
49.14
3.88
5.28
0.90
16 Apr 2013
GRO000182
558
49.82
3.23
5.48
0.85
17 Apr 2013
GRO000937
526
54.12
3.72
5.36
0.84
17 Apr 2013
GRO000938
522
52.39
3.63
5.44
0.83
17 Apr 2013
GRO000939
506
53.43
3.81
5.39
0.84
17 Apr 2013
GRO000940
505
52.23
3.74
5.41
0.83
18 Apr 2013
GRO000941
497
48.68
3.54
5.51
0.83
19 Apr 2013
GRO000942
493
51.40
3.77
5.40
0.83
20 Apr 2013
GRO000183
492
49.91
3.66
5.27
0.96
21 Apr 2013
GRO000943
469
48.29
3.73
5.30
0.90
22 Apr 2013
GRO000945
477
51.46
3.90
5.31
0.85
1 May 2013
GRO000184
79.94
5.46
0.81
17 May 2013
GRO000946
38.33
5.70
0.84
7 Jun 2013
GRO000949
25.20
5.87
0.91
22 Jun 2013
GRO000947
20.10
5.92
0.95
24 Jul 2013
GRO000948
15.37
6.56
1.10
24 Jul 2013
GRO000948
14.14
6.71
1.05
20 Sep 2013
GRO000950
9.26
7.11
1.07
Basic water properties were determined with a handheld multiparameter probe
(YSI Professional Plus). The probe was equipped with sensors for water
temperature (∘C), conductivity (µS cm-1), pH, and
dissolved oxygen (DO, mg O2 L-1). DO and pH probes were calibrated
according to manufacturer specifications approximately every 5 days.
Samples were collected from a floating dock, ∼ 5 m from the
river bank, where the water depth is ∼ 6 m. All samples for
concentrations of dissolved species were collected by in-line filtration
(Pall AcroPak 500 Supor Membrane, 0.2 µm pore size with 0.8 µm
pre-filter) of surface water directly into pre-cleaned vials, which were
rinsed three times with filtered sample water before filling. Therefore, all
results for “dissolved” constituents represent material that is < 0.2 µm. Sampling and analytical methods for most types of samples are
described in detail by Voss et al. (2014), therefore the
following methods descriptions are abbreviated.
Nutrient samples were collected in pre-cleaned 20 mL polyethylene
scintillation vials and stored frozen until analysis. Analyses for dissolved
NO3+NO2, NH4, PO4, and SiO2 were performed on an
AutoAnalyzer (Lachat QuickChem 8000) with standard US Environmental
Protection Agency-certified spectrophotometric methods and calibrated using
standard reference material MOOS-2 (National Research Council Canada).
Instrumental detection was < 0.05 µmol L-1 for all
nutrients.
Samples for dissolved major cations (Ca, Mg, Na, K) and anions (Cl,
SO4) were collected in pre-cleaned 125 mL high-density polyethylene
bottles. Cation concentrations were determined using a Thermo Scientific
Element2 single collector inductively coupled plasma mass spectrometer,
based on a standard curve of natural river water standard reference material
SLRS-5 (National Research Council Canada). Anion concentrations were
determined using a Dionex ion chromatograph with an anion column (AS15, 4 mm, with ASRS suppressor), based on a standard curve of a mixture of
SpecPure ion chromatography standards (Alfa Aesar). No preservative was
added to major cation and anion samples after collection, and samples were
stored at room temperature until analysis. Instrumental precisions and
accuracies for cation and anion concentrations were < 5 %
(with the exception of accuracy for Ca, which was 5.3 %;
Voss et al., 2014).
Samples for stable isotope compositions of water were collected by filling 4
mL glass vials with filtered water without head space. Hydrogen (δD)
and oxygen (δ18O) isotope compositions were measured on a Picarro
L2120-I cavity ring-down spectrometer. Measured values were calibrated using
secondary standards (mean ± 1 s.d.: Mediterranean Sea water, δD
8.12 ± 0.30 ‰, δ18O 0.95 ± 0.05 ‰;
Jungfrau water, δD - 160.28 ± 0.21 ‰,
δ18O -22.50 ± 0.06 ‰; Zürich water, δD
-75.57 ± 0.19 ‰,
δ18O -10.62 ± 0.04 ‰), which were calibrated against standard
reference materials SLAP2, GISP, and VSMOW2 (International Atomic Energy
Agency). Precisions for δD and δ18O were
0.3 and 0.03 ‰, respectively;
accuracies were 1.0 and 0.07 ‰ .
Turbidity and suspended sediment concentration
An optical nephelometer (LaMotte, 2020-WE) was used to determine turbidity
(measured in normalized turbidity units, NTU). The nephelometer was
calibrated before each measurement with solutions of known turbidity (0,
1.0, and 10.0 NTU). A 20 mL surface water grab sample was collected in a
glass vial and allowed to equilibrate to ambient air temperature. Vial walls
were dried and wiped thoroughly with a Kimwipe, and the vial was rolled and
gently inverted to resuspend particles before analysis. At least six readings
were averaged for each sample to account for measurement variability.
To transform turbidity measurements into concentrations of suspended
particulate matter (SPM), nephelometer readings were complemented with
weighed sediment masses from filtered water samples. Large volume surface
water grab samples (4–20 L) were filtered with specially designed
filtration units onto 90 mm polyethersulfone membrane filters (Millipore,
pore size 0.22 µm). The resulting sediment was rinsed from filters with
purified water (Millipore, 18.2 MΩ cm-1), freeze dried, and
weighed. As there is some loss of sediment in sample processing due to
retention of a small amount of sediment within the filters, rather than use
these measured concentrations directly, we have used the linear correlation
between turbidity and filtered-mass sediment concentration (Fig. 2) to
calculate SPM concentration based on nephelometer measurements. The SPM
concentration values presented throughout the text are turbidity
measurements converted to SPM concentration using this relationship.
Organic matter analyses
Fluorescent dissolved organic matter (FDOM) was measured in the field using
a handheld probe (TurnerDesigns Cyclops-7 with DataBank). The probe was
blank-calibrated with deionized water every 5 days and values are reported
as blanked relative fluorescence units (RFUB). In situ FDOM was measured by
lowering the probe from the dock at least 3 m below the river surface to
eliminate possible interference from sunlight. Because changes in sediment
concentration can significantly impact FDOM measurements
(Saraceno et al., 2009), for each sample, a second FDOM
measurement (“ex situ FDOM”) was performed on filtered water (filtered with 0.2 µm
pore size membrane filters as described above) in a shaded vessel.
Since fluorescence is temperature-sensitive, ex situ FDOM samples were filtered and
analyzed as quickly as possible, with the FDOM measurement typically
completed within 30 min of sample collection. The probe measurement
frequency was 30 s, and a minimum of 20 values were averaged to
account for measurement variability.
Samples for the determination of DOC concentration were collected as 0.2 µm-filtered
water (as described above) in pre-combusted 40 mL amber
glass vials. Concentrated HCl was added immediately to achieve a pH of 2 and
hinder biological activity. Samples were stored in the dark at 4 ∘C
until analysis. DOC concentrations were determined by high-temperature
catalytic oxidation on a Shimadzu TOC/TN-V instrument combined with a
nitrogen chemiluminescence detection unit (TNM-1). Concentrations are
reported as the mean of 3–5 replicate injections with a coefficient of
variation < 2 % (Mann et al., 2012).
Samples for the determination of DOM optical properties were collected in
20 mL polyethylene scintillation vials. Samples were stored at 4 ∘C
in the dark until analysis. UV-visible absorbance spectra were measured at
room temperature on a Shimadzu UV1800 dual-beam spectrophotometer using a
10 mm path length quartz cuvette. All samples were analyzed in triplicate and
referenced to purified laboratory water (MilliQ, 18.2 MΩ cm-1;
Mann et al., 2012). Naperian absorption coefficients
(a(λ)) were calculated at integer wavelengths between 200–800 nm from
absorbance as follows:
α(λ)=2.303×A(λ)/l,
where A(λ) is the measured absorbance and l is the cell path length in
meters (Del Vecchio and Blough, 2002). Absorbance at specific
wavelengths can be diagnostic of certain DOM properties
(e.g., Spencer et al., 2012). Normalizing DOC concentrations
to wavelength-specific absorbance (e.g., SUVA254= absorbance at 254 nm
divided by DOC concentration in mg L-1) allows the chromophoric
character of DOM in different settings to be compared, and the value of
SUVA254 has been previously shown to be positively correlated with bulk
aromaticity (Weishaar et al., 2003). The absorbance ratio
(a250/a365) represents the ratio of absorbance coefficients at two
wavelengths (250 and 365 nm), and has previously been shown to be negatively
correlated with molecular weight and aromaticity of DOM (Peuravuori and
Pihlaja, 1997). The slope ratio (SR) represents the ratio of the slopes
of the absorbance-wavelength curve over two wavelength ranges (275–295 and
350–400 nm) and is also negatively correlated with DOM molecular weight and
aromaticity, as well as relative vascular plant content (cf.
Spencer et al. (2012) and sources therein). The use of multiple optical
proxies provides support from multiple metrics for interpretations of the
data.
For bulk carbon and nitrogen content and stable isotope analysis, suspended
sediment samples were weighed in triplicate into combusted silver capsules.
Samples were then exposed to concentrated HCl vapor under partial vacuum at
65 ∘C for 3 days to remove carbonate. Organic carbon and nitrogen
concentrations %OC and %N, weight percent of total sediment mass) and
stable isotope values (δ13C and δ15N) were
measured on an Elemental Analyzer (Carlo Erba 1107) coupled via a
Finnigan-MAT Conflo II open split interface to a DeltaPlus stable
isotope ratio mass spectrometer for measurement of 13C / 12C
(referenced to Vienna Pee Dee Belemnite) and 15N / 14N
(referenced to N2 air; IAEA, 1995). Sample %OC, %N, δ13C,
and δ15N values were determined from standard reference
materials NBS-19 limestone (Coplen et al., 2006), IAEA-N-1
ammonium sulfate (Böhlke et al., 1993), USGS-40 glutamic
acid (Qi et al., 2003), and an internal glycine standard.
Analytical accuracy and precision of these measurements (1 s.d.) are
0.1 wt. % for C and N abundance, 0.3 ‰ for δ13C, and 0.4 ‰ for δ15N.
Mercury analyses
Water samples for Hg concentrations were collected in pre-cleaned 250 mL
glass bottles, that were double-bagged and handled using “clean hands–dirty
hands” approaches (Patterson and Settle, 1976), and stored in
the dark after collection. Bottles were prepared following procedures
outlined in Hammerschmidt et al. (2011); all materials were
prepared in an ISO 5 cleanroom. Following established procedures
(US EPA, 2002), samples were preserved with BrCl (final
concentration of 0.5 % w/w) within 30 days of collection, then stored in
the dark at 4 ∘C until analysis. Total dissolved Hg (TDHg)
concentration represents water samples filtered to 0.2 µm (as described
above), while total Hg (THg) concentration represents unfiltered water
samples. In a clean laboratory, samples were oxidized on the day before
analysis with 100 µL saturated BrCl solution to convert all Hg species
to Hg2+, then reduced with 100 µL each
NH2OH⋅HCl and SnCl2 immediately before analysis to convert all species to
elemental gaseous Hg0. Mercury concentrations were determined using a
purge and trap/cold vapor atomic fluorescence spectrometry total mercury
analyzer (Tekran 2600). A MilliQ water (18.2 MΩ cm-1) blank was
analyzed at the beginning of each day of analysis to ensure that background
signal was sufficiently low (∼ 0.5 pmol). A standard curve was
generated from analyses of varying quantities of aqueous standard (NIST
SRM-3133), which was checked against a vapor Hg0 saturated air
standard held at 15 ∘C (Tekran 2505) and introduced to the
detection system using a gas-tight syringe. Samples were analyzed at least
three times until standard deviations were < 10 % of the measured
value, or until sample material was exhausted.
Total Hg concentration in suspended particulate matter (SPM THg) was
analyzed on material recovered from membrane filters. SPM samples were
analyzed on a Milestone Direct Mercury Analyzer (DMA-80) following
established methods (US EPA, 2007). Concentrations were
calibrated using standards MESS-3 and PACS-2 (marine sediments; National
Research Council Canada) and IAEA-SL-1 (lake sediment; International Atomic
Energy Agency). Samples were analyzed in pre-combusted (500 ∘C,
1 h) nickel boats.
Results
Early freshet water chemistry trends
During the period 26 March–22 April 2013, hereafter referred to as the
“early freshet,” discharge (at Hope) increased from 950 to 3000 m3 s-1.
SPM concentrations were very low (3–9 mg L-1) for the first
∼ 10 days, then increased rapidly to higher, but variable
values (40–150 mg L-1; Fig. 2c).
Nutrient concentrations varied significantly during the early freshet period
(NO3+NO2: 4–19 µmol L-1, PO4:
0.2–0.7 µmol L-1; NH4: 0.4–2.3 µmol L-1;
SiO2: 83–113 µmol L-1; Fig. S2). Nitrate/nitrite and NH4 concentrations showed
slight decreasing trends, continuing the decline from peak winter values
that is typical of the seasonal cycle in the Fraser River and other
temperate to high-latitude fluvial systems (Cameron, 1996; Voss et al.,
2014; Whitfield and Schreier, 1981). Phosphate and dissolved SiO2
concentrations did not show clear trends. Nutrient concentrations during the
early freshet exhibited a substantial portion of the total annual variation.
Such large day-to-day variability was not anticipated based on lower
frequency (i.e., once to twice monthly) time series sampling presented by
Voss et al. (2014).
Concentrations of some major dissolved species (Na, Cl, SO4) decreased
as discharge increased during the early freshet (Fig. S3). Others decreased
for the first 2 weeks, then increased (Mg, K) or remained relatively
stable (Ca). Those elements which exhibited a systematic decrease in
concentration throughout the early freshet are more significantly influenced
by sea salt aerosols in precipitation relative to chemical weathering of
rocks than those characterized by more variable behavior. All major species
except SiO2 (which is also a nutrient) showed significant linear
correlations (p < 0.01, 95 % confidence interval) with conductivity
(a proxy for total dissolved solids).
The stable isotope composition of water (δD, δ18O) showed a
steady shift to lighter compositions over the 2013 early freshet (Fig. S4).
The change (> 15 ‰ in δD over 27 days) is large
and rapid in the context of the total annual variability in this parameter
(∼ 40 ‰ in δD; Voss et al., 2014). Deuterium excess
(δD - 8×δ18O; values not shown) decreased during
this period from ∼ 8.4 to ∼ 4.7 ‰.
Early freshet changes in DOM concentration were monitored in the field by
proxy from FDOM probe measurements. Filtered FDOM values (539–1856 RFUB)
correlated strongly with measured DOC concentrations, with an apparent
approach towards a plateau in FDOM (∼ 1800 RFUB) at high DOC
(> ∼ 500 µmol L-1; Fig. S5a), likely due
to light attenuation from increasing chromophoric DOM (Downing et al.,
2012; Pereira et al., 2014). The suspended sediment concentration was found
to account for most of the difference between in situ and filtered ex situ FDOM
measurements (Fig. S5b).
Seasonal changes in OC concentration and OM composition
In both 2011 and 2013, DOC concentrations rose rapidly during the early
freshet period from fall/winter levels of ∼ 200 µmol L-1
to a peak of 700–900 µmol L-1 (Fig. 3). This “pulse” of
DOC occurs at the very onset of the rise in discharge, with peak DOC
concentrations achieved when discharge had only reached 40 % of its
maximum. DOC concentration is positively correlated with wavelength-specific
absorption coefficients (Fig. 4).
Concentrations of dissolved organic carbon (DOC) peak during the
early stages of the spring freshet and decrease over the course of the
summer.
DOC concentration is strongly correlated with absorption
coefficients (shown here at 254 and 350 nm).
Optical properties of DOM reveal changes in the composition of the DOM pool
during the early freshet period, as well as at other times of the
year (Fig. 5). During the early freshet of the Fraser River in 2013, values of
a250/a365 and SR decrease, while SUVA254 increases. These
early freshet changes are part of a larger seasonal cycle, shown in Fig. 5
for a250/a365 and SR. While the values of these parameters
decrease during the early freshet DOC pulse, they gradually rise throughout
the summer, peaking in early fall. In winter, values drop again before
rising rather abruptly prior to the early freshet. For the ∼ 2-year
record of these DOM optical properties, both SR and
a250/a365 show a consistent twice-yearly cycle between higher values
in fall and early spring and lower values in winter and during the early
freshet (and the reversed trends for SUVA254), exhibiting a hysteresis
cycle with discharge similar to DOC concentration.
(a) Spectral slope ratio (SR) and (b) absorbance ratio
(a250/a365) show rapid changes in DOM composition during the early
freshet DOC pulse. A more gradual return to pre-freshet composition follows
throughout the summer. A second cycle occurs in late winter.
In addition to DOC concentration, the POC concentration and particulate
organic matter (POM) composition change rapidly during the early freshet
(Fig. 6). Although the OC content of suspended sediments decreases during
this time (from 4.5 to 0.8 %), the increase in SPM concentration is so
large that the POC concentration rises by an order of magnitude (from 9 to
89 µmol L-1). The relative change in POC concentration is greater
than that of DOC concentration, as the DOC : POC ratio decreases from a value
of 30 on 26 March to a minimum of 4.6 on 8 April, followed by
values of ∼ 10 over the following 2 weeks. The C : N
composition of POM also changes, with values before 6 April varying
between 8.5 and 9.9 (excepting the values of 10.5 on 27 March and
10.3 on 31 March), and then rising to values between 10–11 for the
remainder of the sampling period. The δ13C of POC also varied
(-27.7 to -26.1 ‰); however, no clear trends are
apparent.
The abundance and composition of POC shifted during the early
freshet period towards (a) relatively lower %OC, (b) higher POC
concentration, and (c) higher C : N. (d) No distinct temporal trend is evident
in δ13C. (e) POC concentration and (f) SPM OC content are
tightly coupled with SPM concentration. Error bars represent 1 s.d. of
triplicate measurements. Discharge is shown as gray lines in (a)–(d).
Early freshet changes in mercury concentration
Total dissolved Hg (TDHg) concentrations varied between 5.9 and 15.2 pmol L-1,
with no distinct temporal trend during the early freshet period
(Fig. 7). In contrast, unfiltered THg concentrations increased significantly
in a matter of days during this period, with concentrations before 7 April
of ∼ 15 pmol L-1 and those on and after this
date of ∼ 50 pmol L-1. The portion of the total Hg load
composed of dissolved Hg correspondingly decreased from ∼ 75 %
before 7 April to < 25 % afterwards.
Total mercury concentrations in filtered (TDHg) and unfiltered
(THg) water samples during the 2013 early freshet period. Error bars
represent 1 s.d. of repeated measurements.
Results of suspended sediment Hg concentrations are presented with
consideration of analytical detection limits. Empty combusted nickel boats
were analyzed to determine the analytical blank of the sediment Hg analysis:
0.37 ± 0.20 ng Hg (avg. ± 1 SD). The total amount of Hg analyzed
in suspended samples ranged from 1.2–4.9 ng. The very low amounts of Hg
reported here are above the detection limit (3× standard deviation of blank = 0.6 ng),
but in two of six samples were not quantifiable (10× standard
deviation of blank = 2.0 ng). This limitation does not, however, affect
the conclusions drawn from the data. Concentrations of Hg in SPM were
relatively constant, varying between 0.43–0.46 pmol mg-1.
Discussion
Rapid geochemical changes in the early freshet
The geochemical data presented here highlight the importance of the very
early portion of the spring freshet to biogeochemical dynamics in the Fraser
River. While discharge and basic water properties (temperature, DO, pH,
conductivity, and major element concentrations) record only modest changes
during this interval, the initial melting of snowpack is reflected in a
suite of geochemical shifts, including water δD and δ18O composition, SPM concentration, and OM concentration and
composition.
The sudden change in water sources from different portions of the basin is
demonstrated by the rapid decrease in δD and δ18O
values. As runoff from headwater areas and snowmelt with signatures more
depleted in heavy isotopes begins to contribute a greater portion of the
total discharge, the lower Fraser main stem quickly records this transition
at the whole basin-scale. Quantification of the proportional contributions
of various water sources (particular snowpacks and tributaries) based on the
observed changes in stable isotope composition is complicated due to
insufficient knowledge of source water compositions. The isotope composition
of precipitation in the Fraser basin is poorly characterized and the δD and δ18O values of individual tributaries are highly
variable across the year (Voss et al., 2014). Furthermore,
the isotope composition of snow and ice likely varies with elevation and
water vapor source, causing the composition of the snow within a single
tributary basin to vary as seasonal melting progresses. The magnitude of the
change in isotope composition of the Fraser main stem, however, is
sufficiently large that the transition to greater headwater and snowmelt
influence during the early freshet is unequivocal.
Assuming that the deuterium excess of Pacific Ocean source moisture does not
change seasonally, the shift towards lower deuterium excess during the early
freshet indicates a change in the continental transport pathways and/or
evaporation processes affecting spring vs. winter precipitation in the
Fraser basin (Froehlich et al., 2008; Riche et al., 2014). In the absence
of spatially resolved precipitation isotopic data, we can surmise that the
apparently lower deuterium excess of winter snowpack relative to spring
rainfall is due to more evaporative recycling over land en route to the site
of precipitation for the latter than for winter snowfall. This proposition
should be further tested with time series observations of stable isotopes in
precipitation, snow and ice, and small streams across the basin.
A pronounced geochemical shift is recorded by the change in concentration of
DOC. DOM dynamics are driven by a combination of hydrological and biological
processes. When discharge is low at base flow levels, hydrologic flow paths
through soil are relatively deep and slow, drawing modest amounts of DOM
from deeper soil layers (Townsend-Small et al., 2011). This DOM is
likely older and more degraded from its parent plant source material as a
result of preferential remineralization of younger carbon sources by soil
microbiota (Gangloff et al., 2014), although a portion of soil
DOM may also derive from soil biota. Preferential sorption of DOM to soil
particles may also influence soil DOC concentration and DOM composition,
enhancing DOM removal during base flow periods. As discharge and overland
flow increase and soils become inundated across the basin, hydrologic flow
paths draw DOM from greater distances from stream channels and shallower
soil horizons. Despite potential dilution from increased discharge, this DOM
is likely more concentrated, and exported to stream channels more quickly
than base flow DOM (Michalzik et al., 2001, 2003), leading to a pulse of DOM into streams.
Discharge-weighted fluxes and concentrations of DOC in the Fraser
River. Environment Canada data were accessed online at
http://aquatic.pyr.ec.gc.ca. Sampling for both Environment Canada and this
study was performed at approximately twice monthly frequency, with the
exception of our 2013 freshet sampling, for which sampling was daily.
Environment Canada record
This study (2011–2013)
at Hope (1998–2013)
DOC flux (mol C a-1)
2.7 ± 0.4 × 1010
2.8 ± 1.0 × 1010
DOC yield (mol C km-2 a-1)
1.2 × 105
1.2 × 105
Discharge-weighted average DOC concentration (µmol L-1)
277 ± 14
270 ± 71
The average total DOC flux of the Fraser River can be estimated from our 3-year record, as well as from the 16-year record from Environment
Canada in the city of Hope. Hope is ∼ 100 km upstream of the
sampling site used in our study, and notably excludes a large portion of the
agricultural Fraser Valley and a large, DOC-poor Coast Range tributary, the
Harrison River. Despite these differences, the estimates of total Fraser
River DOC flux based on these two records (Table 3) agree within their
uncertainties (2.8 ± 1.0 × 1010 mol C a-1 for the
record in this study, 2.7 ± 0.4 × 1010 mol C a-1
for the Environment Canada record), with the longer Environment Canada
record showing smaller uncertainty, as expected. The DOC yield of the Fraser
River (1.2 × 105 mol C km-2 a-1) is significantly
greater than that of the nearby Columbia River (0.5 × 105 mol C km-2 a-1),
and comparable to that of the Yukon River to the
north (1.4 × 105 mol C km-2 a-1;
Spencer et al., 2013). Absent quantitative estimates of headwater fluxes of
DOC to the main stem of each of these rivers, these differences in DOC
yields suggest that DOC mobilized in the Fraser River is retained more
effectively during downstream transport than in the Columbia River, while
DOC in the Yukon River is transported roughly as efficiently as in the
Fraser. The extensive reservoir impoundment of the Columbia River may be an
important factor in the apparently greater loss of terrestrial DOC from this
system, while lower terrestrial productivity in the vast arid portions of
the Columbia basin likely also generate smaller inputs of soil DOC than in
the forested Fraser basin. The more natural hydrologic regime of the Yukon
River, and land cover composed largely of forests and wetlands, likely
contribute to its similar DOC yield compared to that of the Fraser River.
Annual cycles of DOM composition and sources
Considering the full annual records of DOC concentration and DOM
composition, it is evident that DOM composition varies at times outside the
spring freshet. Throughout the summer, as main stem DOC concentration
decreases toward base flow levels, the DOM optical properties gradually
return to values similar to those of pre-freshet DOM (higher
a250/a365 and SR, lower SUVA254). In the late fall, while
DOM concentrations remain low, optical properties once again shift to
freshet-like compositions (lower a250/a365 and SR, higher
SUVA254), and then return to pre-freshet composition in late
winter/early spring, just before the early freshet DOC pulse. Although daily
records of DOC concentration in the Fraser River are not available for the
fall period, the Environment Canada record (beginning in 1998, approximately
twice monthly sampling) indicates that a secondary peak in DOC concentration
(300–500 µmol L-1) often occurs between September and December.
The twice-annual cycle in DOM composition appears to be driven by hydrologic
changes. The early freshet shift represents more rapid export of shallow
soil DOM driven primarily by melting snowpack across the basin. The
compositional change in the fall is likely driven by a similar flushing of
shallow soil DOM derived from biomass accumulated over the growth season and
mobilized by large rain events, as has been observed in the Yukon River
(Wickland et al., 2012), as well as in non-snowmelt-driven
systems such as the Mississippi River (Bianchi et al., 2013).
The Fraser basin, particularly in areas east of the Coast Range, generally
receives very little precipitation in the late summer months, which allows
litter from fresh vegetation to accumulate and DOM export to revert to
slower, deeper flow paths (Oswald and Branfireun, 2014). Warmer
temperatures in late summer relative to spring may also promote more rapid
microbial degradation of soil DOM, thus diminishing the potential amount of
DOM that can enter streams. In the winter, precipitation in much of the
basin falls primarily as snow, which limits surface runoff. Fall
precipitation (rain), however, is capable of flushing shallow soil DOM into
streams. The more freshet-like composition of this fall DOM suggests that it
has a similar soil residence time and limited degradation history.
The smaller quantity of DOC mobilized by fall soil flushing compared to that
of the spring freshet (evident in the smaller fall pulses of DOC exhibited
by the long-term Environment Canada DOC record) is likely due to incomplete
recovery of the soil DOC pool to its pre-freshet size, in addition to the
fact that the amount of runoff generated by fall rain storms is much less
than that from spring snowmelt. The relative magnitude and composition of
fall DOC pulses may also be affected by spatial differences in vegetation
types (e.g., between coniferous forests which dominate the mountainous
portions of the basin and the bunchgrass and dryland vegetation covering
significant portions of the interior basin), which also likely exhibit
variations in the timing of growing seasons. Growing season varies
significantly across the Fraser basin, from > 170 days per year
in the Fraser Valley area to < 100 days in the Coast Range and parts
of the Rocky Mountains (Agriculture and Agri-Food Canada, 2014).
Consequently, this relatively small input of compositionally distinct DOM to
the base flow DOM load of the Fraser causes a change in DOM optical
properties of a similar magnitude to that seen during the early freshet. It
should be noted, however, that optical parameters are not necessarily a
linear function of the relative quantity of the functional components
responsible for them (Stedmon and Markager, 2003; Yang and
Hur, 2014). The spatial extent and magnitude of fall rain events is also
highly variable from year-to-year and across different tributary catchments;
hence the fall and winter DOM composition changes are likely to be more
variable than those during the freshet.
The fact that DOC concentrations peak and begin to fall before discharge
reaches its freshet zenith indicates that this hydrologic flushing of DOM is
limited by the size of the shallow soil DOM pool, and/or by the differences
in snowmelt timing across the basin. Disentangling these effects would
require extensive knowledge of seasonal changes in DOM flux and composition
from individual tributary basins. If the spring freshet effectively flushes
the shallow soil DOM pool across the basin, this implies a decoupling from
the deep soil DOM pool, as negligible shallow DOM is able to persist during
high flow conditions, and transfer to deeper soil layers only occurs between
late summer and the following spring. A stronger understanding of the nature
of interaction between shallow and deep soil DOM pools would further inform
the results of this study. An investigation of seasonal changes in soil DOM
properties (e.g., concentration, optical properties, biolability, 14C
age) with depth could build on the results presented here by identifying
whether hydrologic flushing imparts a “fresh” DOM signature on deep soil DOM
and how long such a signature persists.
The contributions of deep and shallow soil DOC to total Fraser DOC
were estimated based on the observed maximum and minimum values of DOC
optical properties (spectral slope ratio, SR, and absorbance ratio,
a250/a365). These fractions are related to the DOC concentration
(a and b), and change throughout the year (c and d,
shown here for 2013; e and
f detail the 2013 early freshet period).
The brief rise in SR at the beginning of the freshet DOC pulse may
indicate an initial release of highly soluble and less aromatic, lower
molecular weight or non-chromophoric DOM that is quickly overwhelmed by more
aromatic, higher molecular weight DOM for the remainder of the pulse as
SR drops (Ward et al., 2012). Such an initial shift is less
clear in the a250/a365 and SUVA254 records at the onset of the
2013 freshet, thus an initial shift in DOM composition during the freshet
DOC pulse requires further investigation. In addition, the magnitude of
anthropogenic contributions to the dissolved and particulate OC load of the
Fraser basin could potentially be investigated through measurement of
tracers such as mammalian fecal markers (e.g., coprostanol) or phenolic
flavor compounds (e.g., Writer et al., 1995; Keil et al., 2011).
The average depth of the shallow soil horizon responsible for the freshet
pulse of DOC can be estimated from the volume of water above base flow
discharged during this pulse. In 2013, the freshet DOC pulse – from the
point when DOC concentration began to rise rapidly until it returned to
nearly pre-freshet values – lasted approximately from 5 April to 7 June.
The cumulative discharge of the Fraser River during this time was
33 km3, or 28 km3 subtracting a constant base flow of 950 m3 s-1.
If this volume of water constitutes the shallow soil end
member, we estimate that the maximum shallow soil DOC concentration during
this time period is 900–950 µmol L-1, assuming a base flow DOC
concentration of 100–200 µmol L-1. Given an average forest soil
porosity of ∼ 0.43 in British Columbia (Zhao et
al., 2008), the actual soil volume represented by 28 km3 of water is
∼ 65 km3. If this volume is distributed equally across
the entire basin (228 776 km2), it corresponds to a soil depth of
∼ 28 cm. Given that soil porosity varies considerably across
the basin (as low as 0.15 in highly compacted fine-grained and agricultural
soils, and as high as 0.70 in coarse-grained soils), depths of ca. 20–80 cm
could be flushed in different localities. This estimate represents a
basin-wide average soil depth; it is likely that the majority of the DOC
pulse derives from soil water in the top centimeters with DOM concentrations
much higher than that observed in the river, due to dilution by low-DOM snow
melt, low-DOM soil water from deeper layers, and respiration between the
point of mobilization into the stream and the sampling location. Total soil
depth (above bedrock) is also highly variable across the basin, ranging from
< 0.5 m in rapidly eroding mountainous areas to > 3 m in
flatter areas that have accumulated significant glaciofluvial sediments
(Valentine et al., 1978; Vold, 1979). An estimate of a
surface horizon of 0.2–0.8 m flushed during the freshet DOC pulse is
therefore a reasonable first-order approximation of the maximum spatial
extent of this event.
In addition to hydrologic and soil microbial controls on DOM dynamics,
in-stream and lacustrine biological activity may play a role in the observed
changes in DOC concentration and DOM composition. Throughout the spring
freshet and summer, nutrient concentrations in the Fraser main stem also
decrease. Similar to DOC, this trend is likely due in part to changes in the
sources of nutrients from soil flushing. However, nutrients are also
consumed by autotrophic activity in some portions of the basin, particularly
in tributary basins containing lakes. In these basins, such as the Thompson,
Nechako, and Quesnel rivers, lakes function as suspended sediment filters,
allowing sunlight to penetrate more deeply. Such basins are likely
especially depleted in nutrients in spring and summer, and may also
contribute some DOM derived from autotrophic production. This DOM would have
optical properties reflecting a less aromatic, lower molecular weight
composition (Rochelle-Newall and Fisher, 2002). Aquatic autotrophic
and heterotrophic DOM input is likely strongest during summer, when water
temperatures are higher and discharge and river sediment concentrations are
lower than during the peak freshet. More detailed data on seasonal and
spatial variability in DOC concentration and DOM composition within these
tributary basins would be needed to better constrain this potential source
of DOM to the main stem Fraser.
We used the concentrations and optical properties of DOC to estimate the
contributions of “shallow” vs. “deep” soil-derived DOC to the total
DOC load of the Fraser River (Fig. 8). Assuming that the maximum and minimum
values of the optical properties observed in our time series represent these
hypothetical end-members, we determined the proportion of deep and shallow
soil-derived DOC for each point in the time series, and generated
discharge-weighted average values using LoadEst (Table 4). As derived
optical properties such as a250/a350 and SR may not vary
linearly as a function of end-member inputs (Yang and Hur,
2014; Stedmon and Markager, 2003), the results of this mixing model are
intended to show seasonal trends and differences, while the absolute
fractions of end-member contributions are necessarily approximate. With this
in mind, both SR and a250/a365 indicate that, on average,
shallow soil-derived DOC constitutes ∼ 60 % of the total DOC
flux. The estimated fractions of total DOC derived from deep and shallow
soils varies non-linearly with DOC concentration (Fig. 8). This may reflect
the observation from the time series record that DOM optical properties can
change on the basin scale not only during the spring freshet, but also in
response to relatively small hydrologic changes while the total DOC load is
less variable.
Estimates of soil DOC sources calculated using LoadEst
(Runkel et al., 2004). Uncertainties represent 1 s.d. of the
average for the 3 years of the record (2011–2013). Shallow and deep
soil DOC fractions indicate the fraction of the total DOC load estimated to
derive from shallow and deep soil DOC pools, respectively. Calculations were
made based on shallow and deep soil DOM end-members defined by observed
maximum and minimum SR and a250/a365 values.
SR
a250/a365
shallow soil end-member
0.81
4.94
deep soil end-member
1.17
8.12
shallow soil DOC fraction
0.611 ± 0.018
0.638 ± 0.014
deep soil DOC fraction
0.459 ± 0.036
0.364 ± 0.015
The role of the Fraser River in transferring terrestrial OC to the coastal
ocean can be assessed by comparing the fluxes of DOC and POC with total
carbon fixation by land plants. The global compilation of riverine OC fluxes
of Ludwig et al. (1996) reports net primary productivity (NPP) in the
Fraser basin of 585 g C m-2 a-1 (however no OC fluxes are
reported), corresponding to total terrestrial carbon fixation in the basin
of ∼ 4.0 × 1012 mol C a-1. Based on the
measured DOC flux of 2.8 × 1010 mol C a-1 (Table 3), this
accounts for 0.25 % of NPP. Quantifying the annual POC flux is complicated
by the very limited data set available (n=29, with 26 values from the
2013 early freshet period and 3 values during low discharge in 2010 and
2011); however, a first-order estimate of POC flux based on these data using
LoadEst is ∼ 1 × 1010 mol C a-1, i.e.,
∼ 0.1 % of NPP. This is likely an overestimate, as the
limited POC training data set is biased towards the early freshet period,
when POC concentrations are relatively high while discharge is not yet at
peak values; thus, extrapolating to peak discharge may overestimate high
discharge POC concentrations. Furthermore, some portion of POC derives from
petrogenic OC (Voss, 2014), not recently living vegetation, thus a
correction could be applied to the calculated POC flux based on POC
radiocarbon ages, which would further reduce the flux of POC derived from
recent NPP (Galy et al., 2015). In addition, widely varying values
of NPP in the Fraser basin are available in the literature. For instance,
Liu et al. (2002) report NPP of 189 and 215 g C m-2
a-1 for the Pacific Maritime and Montane Cordillera ecozones,
respectively, within the basin, corresponding to < 50 % of the
value of Ludwig et al. (1996) used for our calculations. Though beyond
the scope of this study, the issue of basin-scale NPP estimates clearly
warrants careful reassessment.
Accepting these uncertainties, it is clear that DOC dominates the export of
terrestrial NPP in the Fraser basin, and annual DOC+POC fluxes transfer
0.25–0.35 % of total NPP in the Fraser basin to the coastal ocean. Based
on NPP and DOC and POC flux data presented by Ludwig et al. (1996), such
a proportion of basin NPP exported as DOC+POC is typical, with most large
rivers exporting 0.3–0.5 % of NPP (e.g., Columbia: 0.2 %; Rhine: 0.3 %,
Mississippi: 0.3 %, St. Lawrence: 0.4 %, Mackenzie: 0.5 %, Yukon:
0.5 %, Congo: 0.5 %). The DOC yield of the Fraser River is also not
exceptional for its runoff relative to other North American rivers
(Spencer et al., 2013). Subtle differences in DOC export
efficiency are likely obscured within this broad assessment; however, it is
possible that the limited lake and reservoir area in the Fraser basin, which
shortens DOC residence time and therefore limits the opportunity for
heterotrophic consumption, may result in relatively efficient DOC export in
this basin.
In light of anticipated future changes in regional climate and basin
hydrology including a shift towards relatively more rain and less snow, an
earlier onset of spring melting producing the freshet, and a rise in annual
average air temperature, it is possible that DOM export from the Fraser
River may change. Higher temperatures throughout the year may cause higher
microbial activity in soils, leading to decreased inputs of soil DOM to
streams. A shift towards more rain-dominated precipitation will cause river
water temperatures to increase, which may promote increased microbial
consumption of DOM in the aquatic realm. The dampening of the onset of the
spring freshet may have the most significant impact, by drawing out the
flushing of soil DOM over a longer period of time. This will increase the
residence time of DOM in the river, providing greater opportunity for
consumption of DOM before it reaches the coastal ocean. Thus future changes
in climate are likely to decrease the total flux of DOM in the Fraser River.
Early freshet mercury dynamics
Changes in dissolved and total Hg concentrations were examined as a possible
consequence of the significant changes in DOC concentration during the early
freshet period. Although sampling for dissolved, total, and sedimentary Hg
was not as comprehensive as that for OM and other dissolved species, this
contemporaneous data set allows for an initial assessment of Hg dynamics in
an understudied watershed. Previous studies have identified a strong
correlation between DOC concentration and TDHg concentration (e.g.,
Dittman et al., 2010; Schuster et al., 2011; Riscassi and Scanlon, 2011;
Shanley et al., 2008), including during snowmelt (Shanley et al., 2002;
Schuster et al., 2008; Demers et al., 2010), resulting from the association
of Hg with DOM functional groups, particularly reduced sulfur moieties
(Gerbig et al., 2011). The TDHg concentrations in the Fraser River
during the early freshet period, however, are not clearly correlated with
DOC concentration (Fig. 9a). In small headwater tributaries of the Hudson
River (Burns et al., 2012), streams in northern New England
(Dittman et al., 2010), and the Yukon River
(Schuster et al., 2011), the TDHg concentration observed for a
given DOC concentration is generally lower than our observations for the
Fraser. In particular, TDHg concentrations on 30 March (10.1 pmol L-1)
and 3 April (13.3 pmol L-1) are significantly higher
than those predicted based on these previous studies. While the size of this
data set is limited, the concentration changes are sufficiently large to
indicate that processes in addition to changes in DOC concentration are
likely required to explain the data.
(a) Total dissolved mercury and DOC concentrations across the early
freshet in 2013 were weakly correlated (r2= 0.15). (b) Total Hg
concentration (unfiltered) is positively correlated with SPM concentration
(r2= 0.51).
The DOM optical property data demonstrated that the composition of DOM
varies during the early freshet period; thus it is possible that distinct
types of DOM bind dissolved Hg more or less strongly (Haitzer et
al., 2003). While the relevant compositional characteristics may not be
reflected in the optical property data, the lack of correlation between TDHg
concentration and DOM optical properties does not support this explanation.
To properly investigate this hypothesis, the bulk sulfur content of DOM
should be analyzed, or specific sulfur-bearing functional groups quantified
via high-resolution mass spectrometry or X-ray spectroscopy.
The concentrations of total Hg in both dissolved and suspended material may
provide further insight. The marked increase in THg concentrations over the
early freshet period corresponds to an increase in the suspended sediment
concentration (Fig. 9b), demonstrating that when suspended sediment
concentrations rise above base flow levels, sediments contribute the
majority of the THg load.
Regarding the TDHg concentrations, it is possible that the exchange of Hg
between solid and dissolved phases changes with changes in SPM
concentration. The apparent distribution coefficient (Kd′ in
units of L kg-1) of Hg is defined as
Kd′=THg(SPM)/TDHg
and quantifies the proportion of Hg present in solid vs. dissolved form.
The calculated log(Kd′) values for the six time points sampled
during the early freshet vary between 4.5 and 4.9, which are within the
range of observations in other rivers (e.g., Hurley et al., 1998; Babiarz
et al., 2012; Naik and Hammerschmidt, 2011). The significant decrease in SPM
OC content during the early freshet may affect the affinity of Hg for the
solid phase; i.e., higher SPM OC content may enhance sorption of dissolved
Hg. However, we do not observe a correlation between Hg Kd′ and
SPM %OC. Thus it appears that other processes (such as changes in the
chemical composition of DOM or POM) or synergistic effects are responsible
for the trends observed in dissolved and particulate Hg during the Fraser
River early freshet period. The most practical first step towards better
understanding of Hg dynamics in this system is to generate a larger data set
of TDHg concentrations.
Mercury-assisted gold mining in British Columbia, which involved the
mobilization of ∼ 58 × 106 m3 of sediment in the
central Fraser basin (Nelson and Church, 2012), has been
proposed as the source of elevated Hg concentrations found in delta and lake
sediments across the basin (Hales, 2000; Gallagher et al., 2003;
Johannessen et al., 2005). Although the majority of the Hg contamination was
most likely removed from the basin within decades due to its preferential
association with rapidly mobilized fine-grained (OC-rich) sediments, the
ongoing transport of mining-mobilized sediment through the basin
(Nelson and Church, 2012) creates the potential for continued
contamination.
In order to assess whether the Hg load of the Fraser River is elevated from
possible legacy gold mining contamination or other pollution, we have
estimated the expected Hg deposition flux assuming Hg is entirely derived
from deposition of atmospheric aerosols and plant material. Soluble
Hg2+ in precipitation constitutes wet deposition, while Hg derived from
vegetation can be characterized as a combination of throughfall (adsorbed Hg
on the surfaces of plant tissues) and litterfall (Hg
within plant tissues; Graydon et al., 2008). Total deposition is the sum of
wet and dry deposition. Dry deposition can be expressed as the sum of
throughfall and litterfall, less open wet deposition if throughfall is
determined by direct measurement of Hg on plant surfaces, which includes Hg
from precipitation. Wet deposition was estimated using precipitation data
from the Mercury Deposition Network (http://nadp.sws.uiuc.edu/mdn), which
includes six stations near the Fraser River watershed: the Olympic Peninsula
in Washington (WA03; 48.2892, -124.6519), Seattle, Washington (WA18;
47.6843, -122.2588), Glacier National Park (MT05; 48.5102, -113.9970), near
Edmonton, Alberta (AB14; 53.3016, -114.2016), Vancouver, British Columbia
(BC06; 49.1000, -123.1700), and the Strait of Georgia, British Columbia
(BC16; 48.7753, -123.1281). The precipitation Hg concentration records at
these sites were weighted by precipitation amount to determine the average
concentration at each site, which ranged from 2.8–8.7 ng Hg L-1. The
mean for all six sites was 4.8 ± 0.3 ng Hg L-1. Adopting this
value as the Hg concentration of precipitation in the Fraser basin, which
receives 742 mm of precipitation annually, results in wet deposition of 3.5 µg Hg m2 a-1.
A recent study by Graydon et al. (2008) in a remote area of northwestern Ontario found
throughfall in forested areas to be 2–4× greater than wet deposition, and
litterfall of a similar magnitude to throughfall (i.e., a total dry
deposition rate 3–7× greater than wet deposition). Assuming similar behavior
in the mostly forested Fraser basin, we estimate a total deposition flux of
3.2–6.5 t Hg a-1.
The total Hg flux of the Fraser River can be roughly estimated based on our
observed SPM Hg concentrations during the 2013 early freshet, which showed
little variability (mean ± 1 s.d.: 0.447 ± 0.015 pmol mg-1).
Given an average sediment flux in the Fraser River of 17 Mt a-1
(Peucker-Ehrenbrink, 2009), we estimate a Fraser River Hg
flux of 1.5 t Hg a-1. This is necessarily a first-order estimate, as
SPM concentration and THg are not perfectly correlated in our data set,
indicating that other factors play an important role in Hg export. However,
this value is 24–47 % of total Hg deposition, which represents a watershed
delivery efficiency that is similar to or somewhat higher than other
temperate watersheds (Brigham et al., 2009; Swain et
al., 1992). It is therefore difficult to determine from
these data whether there is a significant source of Hg beyond atmospheric
deposition to the Fraser River. As the potential additional sources of Hg to
the Fraser River, including natural weathering, legacy mining contamination,
and contemporary pollution, do not elevate the Hg load beyond what is
deposited by the atmosphere, it appears that soils and sediments within the
basin are accumulating Hg and/or releasing a portion of the deposited Hg
back to the atmosphere.
Conclusions
This study has demonstrated for the first time the rapid shift in DOC and
POC concentration and DOM and POM composition during the rising limb of the
spring freshet of the Fraser River, as well as full annual trends in DOC
concentration and DOM composition. DOM optical properties demonstrate that
during the early spring freshet, as well as during the fall, DOM shifts to a
composition consistent with increasing proportions of fresh plant-derived
DOM (higher molecular weight, higher aromaticity; e.g., Fellman et al.,
2009, 2010; Wickland et al., 2007), as well as highly
aromatic black carbon (Jaffé et al., 2013), relative to
microbially degraded sources of DOM (lower molecular weight, lower
aromaticity). These trends demonstrate the hydrologic control of OM dynamics
in a snowmelt-dominated river basin and suggests the importance of limited
terrestrial water storage to DOM export. The rapid changes in DOM dynamics
observed in the Fraser River underscore the utility of optical sensors,
which could be deployed across a basin to generate time series of
spatially resolved DOM behavior for process studies and flux estimates.
Concurrent dissolved and particulate Hg samples during the early freshet
suggest DOM-Hg dynamics in the Fraser River that are distinct from what has
been observed in other rivers, with apparently weaker control of dissolved
Hg by DOC concentration. Future work on Hg in the Fraser River should focus
on multiple metrics of DOM composition (e.g., molecular and elemental) as
well as a data set covering a wider range of DOC concentrations and
hydrologic conditions.
The characterization of time-varying DOM dynamics in this study adds to the
understanding of hydrologic vs. biogeochemical controls on aquatic DOM
cycling by revealing seasonal trends in an intermediate-sized temperate,
forested river basin. Previous work has focused primarily on small headwater
and mountain streams and large tropical and Arctic basins. The distinct
climatic and hydrologic conditions of the Fraser basin (notably its
snowmelt-dominated hydrology and minimal natural or anthropogenic
impoundments) result in a DOC yield similar to that of the large Arctic
Yukon River, despite its less extreme hydrograph. The high yield of DOC from
the Fraser River relative to the highly impounded nearby Columbia River
suggests that the relative lack of lakes and artificial reservoirs in the
Fraser basin may be an important factor in transmitting terrestrial DOM to
the coastal ocean.
Compared to large rivers globally, the Fraser River exports a typical
proportion (0.25–0.35 %) of annual basin net primary productivity as DOC
and POC, predominantly as DOC. This proportion is a fundamental metric for
quantifying the relative efficiency of terrestrial OC export to the coastal
ocean by diverse watersheds. At present, disagreement among published values
of basin-scale NPP complicates a global assessment of fluvial NPP export;
hence a careful reassessment of these values is critical.
Finally, as a relatively pristine river basin, the Fraser provides an
important reference point for natural biogeochemical conditions that no
longer exist in most mid- and low-latitude watersheds of comparable size.
Anthropogenic impacts from population growth and associated pollution,
channel modification, and impoundment have significantly altered flows of
carbon, nutrients, and sediment in large and small river basins globally
over the past century, and in some areas continue at an accelerating rate.
Studies such as this on a largely free-flowing temperate basin have broad
implications for the understanding of natural fluvial processes which is not
accessible elsewhere. A system such as the Fraser therefore provides a rare
window into the impacts of large-scale processes, such as global climatic
change, and sustained research on biogeochemical cycling in the Fraser River
thus has the potential to demonstrate the response of natural aquatic
systems to long-term changes in ecological conditions.