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Organic carbon in the Humber rivers
the Science of the l&al Emhnment tiyryhlb.-.wa.m..l”-ad.II*YI*r ELSEVIER
The
Science
of the
Total Environment
194:195 (1997) 345-355
Organic carbon in the Humber rivers E. Tipping”, A.F.H. Markerb, C. Butterwick”, G.D. Collettb, P.A. Cranwell”, J.K.G. Ingram”, D.V. Leach”, J.P. Lishman”, A.C. Pinder”, E. Rigg”, B.M. Simon” hfn.ytitutr
“Institute qf Freshratrr ‘NERC
of’ Fresh~aier Ecology, Ecology, Eastern Rivers LOIS
Windermere Lahoruiorl; Ambleside, Cumhria, LA22 OLP, UK Laboratory, Monks Wood Experimental Station, Abbots Ripton, Camhridg~shirr, PEI 7 2LS, UK c/o Department of’Bio1og.v. University of York, York, YOI 5DD, UK
Lahoratorl:
Huntingdon,
Abstract Dissolved organic carbon (DOC), particulate organic carbon (POC), particulate organic nitrogen (PON), chlorophyll-u, and alkalinity were determined weekly or more frequently in samples from 11 rivers in the Humber catchment, between September 1993 and February 1995. [DOC] varied overall from 1 to 15 mg l_ ‘, [POC] from 0.2 to 67 mg 1~ ‘, [LOCI from 0.01 to 7.6 mg 1~ ’ and [DIC] f rom 1 to 52 mg 1~ ‘. For the rivers with predominantly rural catchments, the complex dependence of [DOC] on season and discharge can be interpreted in terms of soil humification and hydrological processes, whereas the inverse dependence of [DOC] on discharge in the more polluted industrial rivers indicates the dominance of point-source effluents. Concentrations of POC depend significantly upon discharge and on concentrations of suspended particulate matter, suggestive of particle mobilisation when physical thresholds are exceeded. During summer months, the ‘living organic carbon’ of algae accounts for much of the POC in all but the most polluted rivers. Fluxes of organic carbon were estimated by combining daily concentrations (measured or interpolated) with discharge data. The contribution of DOC to the annual flux of organic carbon in the different rivers varies from 51 to 80%. the overall contribution being 63%. For both DOC and POC, the main transport to the estuary takes place during the autumn-winter period. For the study rivers, the total flux of organic carbon during the period September 1993 to September 1994 was 73 x lo9 g a ~ ‘. However, these rivers represent only about two-thirds of the total catchment area of the Humber estuary, and so the true total flux is probably closer to 100 x 10’ g a- ‘. 0 1997 Elsevier Science B.V. Keywords: LOIS; Humber; Trent: Ouse
Carbon:
Dissolved
organic
carbon; Particulate
1. Introduction
8
PII SOO48-9697(96)05374-O
1997 Elsevier
Science
B.V.
All
rights
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coefficients;
Carbon
fluxes;
molecules, such as carbohydrates, amino acids, hydrocarbons, fatty acids and phenolics, together with natural macromolecules and colloids (humic substances), sewage and industrial particulates,
Dissolved and particulate organic matter in rivers is a complex mixture of chemically-identifiable 004%9697!97/$17.00
carbon;
reserved.
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Table 1 River characteristics and summary data for concentrations of dissolved organic carbon (DOC), particulate organic carbon (POC), ‘living organic carbon’ (LOC) and dissolved inorganic carbon (DIC) River
A
Q
DOC
POC
LOC
DIC
Upper Swale Lower Swale Ure Nidd Owe Wharfe Derwent Calder Aire Don Trent
381 1363 914 516 3315 159 1634 899 1932 1256 8231
16 24 26 10 58 15 20 26 42 22 155
1-12 2-12 2-12 4-11 3-10 2-7 2-7 3-15 2-13 3-7 4-9
0.2267 0.4-23 0.3-12 0.4-13 0.3-13 0.3-20 0.337 0.9-27 0.8-25 0.9-18 0.7711
0.01-0.9 0.03-1.4 0.01-7.6 0.02-1.3 0.02-3.1 0.02-2.1 0.02-1.2 0.01-1.0 0.01-0.7 0.03-4.5 0.02-6.4
7-42 11-52 13-46 l-26 10-47 14-38 22245 5530 12-45 13-39 25549
Catchment areas (A) are in km2, mean discharges (Q) in m3 s-’ and ranges of concentrations in mg 1-l. The discharge data were calculated from daily values for the period September 1993 to September 1994. The concentrations refer to between 79 and 88 values for each river for the period September 1993 to February 1994. Note that the discharges for the Aire, Calder, Ouse and Wharfe are in part estimated from correlations with discharges for the Don and Lower Swale (see Section 3). LOC is estimated from chlorophyll-a (see Section 3).
soil organic matter, and biological material, notably living phytoplankton and other plant material, (see e.g. Meybeck, 1993; Hope et al., 1994). These materials are of interest for several reasons. Their transport contributes significantly to the carbon cycle (Meybeck, 1993; Sedjo, 1993); according to Ittekkot and Laane (1991), the transport of soilderived organic matter by rivers and its subsequent burial in coastal sea sediments is an important global sink for carbon. The more reactive components, including dissolved compounds, phytoplankton and sewage particulates, may make a significant contribution to heterotrophic metabolism in estuaries and coastal seas. Fulvic acid and other humic substances play a significant role in the behaviour of metals, by complexing them and affecting transport and bioavailability. These compounds also interact with organic micropollutants, and adsorb to the surfaces of mineral solids affecting surface chemistry and rates of aggregation. Organic compounds, especially those in solution, also influence photochemistry. To understand and predict the behaviour and influence of riverine organic matter, basic information about concentrations and fluxes is required. The present paper concerns organic matter in rivers draining into the Humber estuary
in Northern England. We report determinations of dissolved organic carbon (DOC), particulate organic carbon (POC) and chlorophyll-a, from which ‘living organic carbon’ (LOC) was estimated. This work is part of LOIS RACS(R) river monitoring programme. The overall aims and scope of the LOIS study are presented by Wilkinson et al. (1997).
2. Field sites The Humber catchment is a major contributor of water and chemicals to the North Sea (Jarvie et al., 1997a,b). The estuary is fed by two main rivers, the Yorkshire Ouse and the Trent, which drain agricultural, industrial, urban and upland areas (Jarvie et al., 1997a). Water quality varies, the rivers in the north of the region being less polluted than those in the south (Jarvie et al., 1997b; Edwards et al., 1997; Robson and Neal, 1997). Table 1 gives information on catchment area and discharge. Sampling site locations are shown in Leeks et al. (1997). The sites on the Rivers Ouse, Wharfe, Derwent, Aire, Don and Trent are at or close to the tidal limit. In all but one case,
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the sites are within a few kilometres of flow gauging stations operated by the National Rivers Authority (now the Environment Agency); the exception is the Derwent, for which the gauge is 29 km upstream of the sampling site. The samples considered in the present work were collected at least weekly, throughout the period September 1993 to February 1995. In some cases, more frequent sampling was conducted, in order to improve information on high-discharge events.
3. Methods Dip samples for the determination of DOC were collected in 1 1 high-density polyethylene wide-necked bottles. They were filtered through Whatman GFjC filter discs within 1 week of collection, then transported to the IFE Windermere Laboratory where they were kept at 4°C prior to analysis, which was generally done within 14 days. The samples were analysed for DOC with a TOCSin II total carbon analyser, dissolved inorganic carbon being removed by sparging after acidification with HNO,. Recovery was checked by analysing solutions of potassium hydrogen phthalate of known concentration. The detection limit was 0.5 mgC 1-l and the precision was 5%. Suspended matter was collected by filtering known volumes (30-120 cm3) through ashed Whatman GF/F filter papers (15 mm diameter). The samples were fumed with 4 M HCl to remove inorganic carbonates (Hedges and Stern, 1984) then analysed for POC and PON with a CarloErba Elemental Analyser, Model 1106. The detection limits were 0.2 mg 1-i for both C and N, the precision for C was 5% and that for N was 10%. Quality control was by repeat analyses of the same lake sediment sample. The determination of concentrations of suspended particulate matter (SPM) within the LOIS rivers monitoring programme is described by Leeks et al. (1997) and Wass et al. (1997). To analyse for chlorophyll a, samples of river water (usually 1 1) were filtered through GFjC glass fibre filters (9 cm diameter) and extracted with ethanol, as described by Marker et al. (1980) and Marker (1994). Concentrations of ‘living organic carbon’ (LOC) were
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estimated from those of chlorophyll-u by assuming one gram of chlorophyll to represent 50 g of algal carbon, a relationship derived from data quoted by Reynolds (1984). Alkalinity was determined by Gran titration, as described by Mackereth et al. (1978). Flow data, usually recorded at 15 min intervals, were obtained from the National Rivers Authority and used to calculate mean daily discharges. There were some missing values, and some values of doubtful validity, in the original data sets, which resulted in gaps in the calculated mean daily discharges. These were infilled on the basis of correlations with similar nearby rivers. Thus, 20 missing flows of the Aire were estimated from those of the Don by assuming a constant discharge ratio between the two rivers of 1.84, established by regression analysis (545 points, r2 = 0.76, P < 0.001). Discharges of the Calder were also estimated from those of the Don (10 missing values, 536 points for regression, ratio = 1.11, r2 = 0.76, P < 0.001). Discharges of the Ouse were estimated from those of the Lower Swale (67 missing values, 563 points for regression, ratio = 2.16, r2 = 0.89, P < O.OOl), as were those of the Wharfe (337 missing values, 203 points for regression, ratio = 0.584, r2 = 0.81, P < 0.001). The high number of estimated discharges for the Wharfe mean that fluxes estimated for this river must be treated with caution. Daily fluxes (g d - ‘) were calculated by taking the products of mean daily discharges and instantaneous concentrations. The latter were assumed to hold for the whole of the day in question. Because samples were taken only approximately weekly, intermediate concentrations were estimated either by linear interpolation between measured values, or, in the case of POC, from [POC]-discharge relationships (see below). Yearly fluxes were obtained by summing the daily values, and export coefficients (g m -’ a ~ ‘) were estimated by dividing the annual flux by the catchment area of the river in question (Table 1). In making flux calculations from relatively sparse data, we acknowledge that appreciable errors may result, as demonstrated by Webb et al. (1997).
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4. Results
4.1. Concentrations Table 1 shows ranges of concentrations of DOC, POC, LOC and DIC for the 11 rivers. The ranges of values of [DOC] do not differ greatly among the rivers. Although the river with the smallest catchment area (Upper Swale) has the greatest range, and the river with the largest catchment area (Trent) the smallest, there is no general relationship. For all the rivers, the lowest [POC] values are less than 1 mg 1~ ‘, while the highest observed values vary from only 7 mg 1~ ’ for the Derwent to 67 mg l- ’ for the Upper Swale. Concentrations of LOC can reach values similar to those of DOC and POC, especially in the Ure, Don and Trent. Values of [DIC] are generally high, reflecting the widespread occurrence of calcareous sedimentary rocks in the catchments of the study rivers (cf. Jarvie et al., 1997a). Fig. 1 shows variations with time of DOC, POC and discharge for four rivers, the Ure, a relatively unpolluted northern tributary of the Ouse, the Aire, a river significantly impacted by sewage and industrial effluents, and for the Ouse and Trent, the two largest rivers draining into the Humber estuary. For the Ure, Ouse and Trent, both [DOC] and discharge increase during the autumn. Later in the winter, however, [DOC] tends to decrease while discharges remain high. The same patterns were observed for the Upper Swale, Lower Swale, Nidd, Wharfe and Derwent. As a result, when the whole study period is considered, the relationships between [DOC] and discharge for these rivers are weak. For the Upper Swale and Derwent, statistically significant positive correlations are found (u’= 0.19, P < 0.01; Y? = 0.21, P < 0.01 respectively), but for the other rivers the correlations were statistically insignificant (r2 = 0.01 - 0.07, P > 0.01). In contrast, [DOC] values in the Aire (Fig. l), Calder, and Don are at their highest during the summer, low-discharge, period, and significant negative correlations of [DOC] with discharge are found, with I.’ values of 0.40, 0.29 and 0.10 respectively (P < 0.01 in each case). The [DOC]-discharge relationship for the Aire is shown in Fig. 2.
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All of the rivers show a positive dependence of [POC] on discharge, as shown for the Aire in Fig. 2. Values of r2 vary from 0.28 (Calder) to 0.81 (Don), most being between 0.3 and 0.5 (Table 2). Fig. 2 shows that the largest increases in [POC] are not necessarily associated with the largest increases in discharge. A common feature is a major peak of POC associated with the first increase in discharge following the low-flow summer period. This is seen in September 1993 for all four rivers, in September 1994 for the Aire and Trent, and in November 1994 for the Ouse and Ure. Concentrations of POC correlate with concentrations of suspended particulate matter (SPM), as shown by Table 3 and Fig. 3. The ‘living organic carbon’ (LOC) is mostly due to phytoplankton (Pinder et al., 1997), but also includes contributions from benthic organisms carried by the flow. Preliminary examinations show that the spring phytoplankton maximum in the Trent was dominated by centric diatoms, particularly Cyclostephanos invisitatus, Cyclotella nzeneghiniana and Stephanodiscus hantzschii. Later in the summer, there were increasing proportions of a wide variety of Chlorophycae. Fig. 4 shows that, apart from the spring maximum in the Trent, variations of [LOCI in the rivers follow similar patterns. During the summer, LOC accounts for essentially all the POC in the Ouse, Trent and Ure, but not in the more polluted Aire. High [LOC]/[POC] ratios were also found for the Lower Swale, Nidd and Wharfe, the Calder gave similar results to the Aire, while the Upper Swale, Derwent and Don displayed intermediate behaviour. During the periods when LOC was dominant, the [PON]/[POC] ratios for the more productive rivers were in the range 0.15-0.20, close to the characteristic Redfield ratio (see e.g. Reynolds, 1984); at other times, the ratio was ca. 0.10. 4.2. Fluxes Annual fluxes for the period September 1993 to September 1994 are given in Table 4. In the case of POC, the linear regression of concentration on discharge (Table 1, Fig. 2) was used as an alternative method of estimating intermediate values,
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.Trent 25
"E fin k 5 s
I
200
100
"SONDJFMAMJJ, 1993YV-1994
Fig. I. Concentrations of dissolved organic carbon (continuous lines) and discharge, for the Rivers Ure, Ouse, Aire and Trent. OC = organic carbon.
although it should be borne in mind that the correlations are weak. The POC flux values estimated on this basis were higher than those derived from interpolated values, by factors varying from 1.04 for the Ouse to 1.63 for the Wharfe. The effect on the total flux from the Ouse-Trent system was to increase it by 12%, a relatively small difference.
--
I’
0
particulate organic Note the different
4SONDJF' --1995
carbon ordinate
(dotted scales.
lines),
and
of river
Fig. 5 shows plots of cumulative flux against time for the Ure and the Trent. The fluxes of both DOC and POC are greatest during the winter periods. In the case of DOC, the concentration of which varies relatively little with flow, this is principally due to the greater winter discharges. For POC, the positive dependence of [POC] upon discharge further strengthens the flux-discharge
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relationship, so that the difference between winter and summer fluxes is more pronounced. Because of the seasonal dependence of phytoplankton growth, the fluxes of LOC are not dominated by winter flows. This is very noticeable for the Trent (Fig. 5), for which the largest part of the flux comes in the low-flow period between April and June 1994. In contrast, the export of LOC in the Ure is more dependent upon changes in discharge.
DOC
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Table 2 Linear regressions carbon (mg 1-l) Humber rivers
Upper Swale Lower Swale Ure Nidd Ouse Wharfe Derwent Calder Aire Don Trent
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of the concentration against discharge
(m’
of particulate organic SK’) for the major
m
c
i-2
n
0.20 0.08 0.03 0.08 0.02 0.08 0.05 0.05 0.04 0.09 0.01
- 1.7 0.4 0.7 1.2 0.6 -0.1 0.4 2.0 1.4 1.3 1.0
0.60 0.37 0.45 0.35 0.49 0.48 0.37 0.28 0.42 0.81 0.47
80 87 84 84 84 84 83 85 88 82 79
The slope and intercept are denoted by m and c respectively, n is the number of observations. All the correlations are significant at the 0.1% level.
5. Discussion
0
50
100
flow
150
200
250
The results distinguish two kinds of DOC behaviour. For the Upper Swale, Lower Swale, Ure, Nidd, Ouse, Wharfe, Derwent and Trent, [DOC] is low during the summer, increases to maximum values during the autumn, and falls thereafter (Fig. 1). The cumulative flux of DOC, on the
m” s-’ Table 3 Linear regressions of the concentration carbon (mg I-‘) against the concentration ulate matter (mg 1-l) l
m Upper Lower Ure Nidd Ouse
0
50
100
flow
150
200
250
Wharfe Derwent Calder Aire Don Trent
Swale Swale
0.07 0.03 0.06 0.08 0.03 0.07 0.05 0.02 0.07 0.05 0.03
c 0.6 1.4 0.8 0.8 1.3 0.5 0.7 3.0 1.6 1.7 1.6
of particulate of suspended
organic partic-
9
n
0.60 0.47 0.62 0.67 0.44 0.78 0.49 0.10 0.66 0.65 0.33
77 82 79 79 81 80 83 83 88 81 78
m’ s-l
Fig. 2. Dependence of the concentrations carbon (DOC) and particulate organic charge for the River Aire.
of dissolved carbon (POC)
organic on dis-
The slope and intercept are denoted by m and c respectively, n is the number of observations. The correlations are significant at the 0.1% level, except that for the Calder, which is significant at the 1%) level
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351
Ouse
0
50
100
ISPMI
mg
150
200
0
50
100
I-’
150
[SPMI
200
mg
250
300
350
I-’
Aire .
50
100
[SPMI Fig. 3. Dependence of the concentration (SPM) for the Rivers Ure, Ouse, Aire
150
mg
200
250
0
I-’
of particulate and Trent.
50
100
[SPMI organic
other hand, increases approximately constantly during the autumn-winter period (Fig. 5). These observations can be interpreted in terms of soil humification and hydrology. It can be envisaged that, during the summer, the microbial processing of organic matter in the upper soil horizons produces ‘potential dissolved organic carbon’, i.e. carbon in organic compounds that are sufficiently soluble, or poorly adsorbed, to pass into the soil solution. At this time, percolating water penetrates relatively deeply into the soil, where removal of DOC by adsorption to mineral surfaces maintains low concentrations in drainage water,. and hence in river water. As the soils wet up in autumn, lateral flow becomes more significant.
carbon
(POC)
on the concentration
150
mg
200
250
I-’
of suspended
particulate
matter
The proportion of the drainage water that has passed through only surface horizons, and is therefore rich in DOC, increases, and so does river [DOC]. This process continues during the winter, but as the throughput of water increases, the rate at which potential dissolved organic carbon can transfer to the solution phase becomes limiting (because desorption and/or diffusion within soil peds are relatively slow). Thus at high infiltration rates DOC is diluted, while the absolute amount leached is maintained. As a result, riverine [DOC] decreases, but the flux does not. A different situation is found for the most polluted rivers (Calder, Aire, Don), in which [DOC] decreases with increases in discharge, and
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is highest during the summer (Figs. 1 and 2). Presumably, the DOC in these rivers is dominated by point-source effluents, industrial and from sewage treatment works. It is therefore likely to differ in physico-chemical properties and susceptibility to (bio)chemical transformations from the material transported by the less-polluted, rivers with more rural catchments. In common with [DOC], [POC] shows increases in autumn, although the responses are sharper,
15
Ure L
10 -
:
..
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Table 4 Fluxes of dissolved organic carbon (DOC), particulate organic carbon (POC), total organic carbon (TOC), ‘living organic carbon’ (LOC). and DIC for the vear 7 Sentember 1993 to 6 September 1994
Upper Swale Lower Swale Ure Nidd Ouse Wharfe Derwent Calder Aire Don Total Ouse Trent Total System
DOC
POC
TOC
LOC
DIC
2.0 3.7 3.3 1.9 8.7 3.0 4.0 4.9 7.9 3.3 26.9 19.0 45.9
0.5 2.9 1.4 0.9 5.5 0.7 1.2 2.9 5.0 3.2 15.6 11.4 27.0
2.1 6.6 4.6 2.8 14.2 3.7 5.3 7.7 12.9 6.4 42.4 30.4 72.8
0.03 0.1 0.2 0.05 0.4 0.06 0.1 0.1 0.2 0.3 1.0 2.1 3.1
10.9 23.5 20.1 5.1 48.6 11.6 22.5 11.3 27.3 14.7 124.7 141.5 266.2
The fluxes are given in IO9 g a-‘. The ‘Total Ouse’ flux is the sum of those for the Ouse. Wharfe, Derwent, Aire and Don. The ‘Total System’ flux is the sum of the ‘Total Ouse’ and Trent values
15
b F G 2
Aire j ;
10-l
,: : : ::; (.. ; ; ::: : .: :: _.. 5-i; ; i ;,; ; I:; .,.i ii :: j; ; ii' .": .; :.._ : :: ;.,,, :. ,:.- .,.,,,,.;.,..; L ; : .. ..".' ,;:7,.,.$'.,c: ; .: : .....,:'7 i'..'. j !__,,,,,;. :.I : : '.,. >. 0-w. ,*, , ‘I j: I]
15
Trent b
19931a1994
-1995
Fig. 4. Concentrations of particulate organic lines) and ‘living organic carbon’ (continuous Rivers Ure, Ouse, Aire and Trent.
carbon lines)
(dotted for the
being more closely related to discharge (Fig. 1). This behaviour is consistent with particle mobilisation when physical thresholds are exceeded. The especially high values of [POC] during the first one or two events of autumn probably arise from both catchment erosional processes and the entrainment of particulate material accumulated in the river bed over the long summer period of low flow (Tipping et al., 1993). POC differs from DOC in that, following the initial peaks, its concentrations do not decrease towards the end of the autumn-winter period (Fig. 1). This suggests that the rate of supply of material able to be mobilised is not limiting, in apparent contrast to DOC (see above). Concentrations of POC are positively correlated with [SPM] (Table 3, Fig. 3). The positive intercepts found for all the rivers reflect the fact that when [SPM] is low there is a higher proportion of carbon in the particulates. The same observation has been made by Cauwet (1985) for the River Loire (France). This observation may be explained, at least partly, by the presence of algae during the summer months, when [SPM] is low. The slopes of the regressions probably give an
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indication of the carbon contents of particulates eroded from the catchments. The results suggest a range between 2% (Calder) and So/o(Nidd), with an average of 5%. The results in Table 4 show that the relative contributions of DOC and POC to the total organic carbon flux vary among the rivers. For the Upper Swale, 80% is in the dissolved form, whereas for the Don the two fractions are almost equal. Overall, the DOC flux contributes 63%. Algal biomass comprises an appreciable fraction (12%) of the total POC flux to the Humber estuary. The contributions in the individual rivers vary from 4”/;, (Lower Swale) to 18% (Trent). The high value for the Trent means that it supplies 67% of the LOC flux to the Humber estuary (Table 4) compared to 42% of the POC. There is also a relatively high value of 11% for the Don, one of the most polluted rivers. 0 :
Ure
0 x m
6 -
DOC POC LOC X,0
_____
,_~,~_^ _ _._.I _.----
S 0 N D 1993*-
x
30 -
= =
zo-
DOC POC ‘JJC x,o
J
----
F M’A’M’J’J’A’S’O’N’D’J’F 1994 -1995
_____
Fig. 5. Cumulative fluxes of dissolved organic particulate organic carbon (POC) and ‘living (LOC) for the Rivers Ure and Trent.
carbon organic
(DOC). carbon’
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Table 5 Export coefficients particulate organic
Upper Swale Lower Swale Ure Nidd Ouse Wharfe Derwent Calder Aire Don Total Ouse Trent Total System
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of dissolved organic carbon carbon (POC,,,) in g m-’
(DO&,) a-’
DOG,
pocex,
5.3 2.7 3.6 2.4 2.6 4.0 2.5 5.4 4.1 2.6 3.0 2.3 2.1
1.3 2.1 1.5 1.2 1.7 0.9 0.7 3.2 2.6 2.5 1.8 1.4 1.6
and
Export coefficients (Table 5) express fluxes per unit catchment area. On this basis, the Upper Swale, Ure, Wharfe, Calder and Aire supply the most DOC, while the Calder, Aire and Don supply the most POC. For the first three of these rivers, the higher DOC export coefficients are presumably due to the leaching of upland peaty soils, for which conditions are most fdvourable for the production of ‘potential DOC’. For the more-polluted rivers (Calder, Aire and Don), the use of export coefficients is questionable, since both DOC and POC inputs are presumably due to point-source sewage and industrial discharges rather than whole-catchment characteristics. Concentrations of organic carbon may well be influenced by within-river processes, which might include addition due to the release of organic matter from algae, or desorption from particulates. and removal by decomposition or adsorption. If the organic carbon fractions behave conservatively, or if addition and removal balance, then downstream fluxes should exceed upstream ones. This is the case for the two sampling sites on the Swale (Table 4) for both DOC and POC. However, the sum of the DOC fluxes for the Lower Swale, Ure and Nidd, all of which flow into the Ouse, is 8.8 x 10” g a- ‘, a value slightly greater than the flux of 8.7 x lo9 g a-’ for the Ouse itself (Table 4). For POC, the Ouse value (5.5 x 10’ g a ‘) is greater than the sum of the
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tributary fluxes (5.2 x lo9 g a-‘). In principle, these data suggest a net removal of DOC in the stretch of river upstream of the Ouse sampling site, but it seems unlikely that the fluxes are accurately enough known to support this as a definite conclusion. Evidence that significant mineralisation (removal) of organic carbon does occur within the rivers, comes from the high partial pressures of carbon dioxide in the morepolluted rivers (Jarvie et al., 1997~).
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and Kohlmaier (1991) established relationships between export coefficients of DOC and POC and the annual catchment run-off for 16 (DOC) and 9 (POC) large world rivers. Their &rived slopes for DOC and POC are 6.4 and 2.2 g m -’ respectively; these values are discharge-weighted mean concentrations. For the Humber catchment, the values are similar, being 5.4 g m -3 for DOC and 3.1 g m - 3 for POC. 5.2. Implications for land-ocean interactions
5.1. Comparison with other data
There are few large data sets on organic carbon concentrations and fluxes in British rivers. For the River Severn, Mantoura and Woodward (1983) reported an export coefficient for DOC of 1.7-2.8 g m-* a-’ (quoted in Hope et al., 1994) which is similar to our value for the Humber (2.7 g me2 a-‘). The GEMS (1983) report (quoted by Kempe et al., 1991) gives a total organic carbon flux for the Trent at Nottingham of only 1.6 x lo9 g a-‘. Even though Nottingham is ca. 30 km upstream of the sampling site used in the present work (Cromwell Lock), this is considerably lower than the value of 3.04 x 10” g a-’ found in the present study (Table 3). The compilation of Hope et al. (1994) for rivers throughout the world shows that DOC export coefficients range from 0.01 to 44 g m -* a ~ ‘, the highest values referring to catchments with forests or bogs. Lowland rivers generally have values in the range l-10 g m-* a-‘. In this respect, therefore, the Humber value is typical. Reported POC export coefficients range from 0.07 to 50 g rn-* a- ‘, but again the range 1- 10 g m - ’ a - ’ covers most rivers, and contains our value of 1.6 g m -’ a - ’ for the Humber. The export coefficient, in g m - * a - ‘, for TOC for the Humber is 4.3 (Table 4) which is similar to those of large European rivers given by Kempe et al. (1991) e.g. Garonne (3.Q Seine (3.5) Rhone (6.1), Loire (4.8) and Rhine (5.3). In terms of specific carbon flux therefore, the Humber rivers appear typical of European rivers. Carbon export has also been considered in terms of annual catchment runoff. Thus, Esse
The total catchment area of the rivers which drain directly into the Humber is 17,127 km2 (cf. Table l), approximately two-thirds of the total catchment area of the Humber. The remaining third is contributed by smaller rivers, not sampled in the present study, and those areas of the catchment that drain into rivers and estuary below the tidal limits. Therefore the quoted fluxes for the ‘Total System’ (Table 4) are underestimates of the total fluxes from the rivers to the estuary; the values for TOC and DIC may therefore be close to 100 x lo9 g aa1 and 400 x lo9 g a ~’ respectively (cf. Table 4). The data in Table 4 show that organic carbon comprises approximately 20% of the total carbon flux to the Humber estuary, assuming the dissolved inorganic carbon (DIC) to dominate the total inorganic flux. Although the inorganic carbon flux contributes to the global carbon cycle, the organic material is more significant for processes in the estuarine and coastal zone, where it may be metabolised, participate in biogeochemical processes such as the binding of metal ions and organic micropollutants, and be buried in nearshore marine sediments. Within the context of LOIS, metabolically-labile organic material is important, as a substrate for heterotrophic metabolism in the estuary and coastal zone. This is shown by the significant oxygen sags in the low salinity regions of the Ouse and Trent branches of the Humber estuary (Gameson, 1982). To judge the significance of riverine fluxes of labile organic carbon in this respect, information on primary productivity in the coastal zone is needed.
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Acknowledgements We thank the Yorkshire and Severn-Trent Regions of the National Rivers Authority for supplying discharge data. The paper benefitted considerably from the critical comments of two anonymous referees.
References Cauwet, G., 1985. Dynamique de la mat&e organique dans le miheux marin et polyhahn. Son role dans les processus geochimiques aux interfaces. These de Doctorat d’Etat, Universite de Perpignan, 178 pp. Edwards, A.M.C., R.J. Freestone and C.P. Crockett, 1997. River management in the Humber catchment. Sci. Total Environ., 1941195: 2355246. Esse, G. and G.H. Kohlmaier, 1991. Modelling terrestrial sources of nitrogen, phosphorus, sulphur and organic carbon to rivers. In: E.T. Degens, S. Kempe and J.E. Richey (Eds.), Biogeochemistry of Major World Rivers, SCOPE. Wiley, Chichester, pp. 297.-322. Gameson, A.H. (Ed.), 1982. The Quality of the Humber Estuary, Yorkshire Water Authority, Leeds. GEMS, 1983. GEMS/WATER Data Summary, WHO Collaborating Centre for Inland Waters, Burlington, Ontario. Hedges, J.I. and J.H. Stern, 1984. Carbon and nitrogen determinations of carbonate-containing solids. Limnol. Oceanogr., 29: 6577663. Hope, D., M.F. Billet and M.S. Cresser, 1994. A review of the export of carbon in river water: fluxes and processes. Environ. Pollut., 84: 301-324. Ittekkot. V. and R.W.P.M. Laane, 1991. Fate of riverine particulate organic matter. In: E.T. Degens, S. Kempe and J.E. Richey (Eds.), Biogeochemistry of Major World Rivers, SCOPE, Wiley, Chichester, pp. 2333243. Jarvie, H.P., C. Neal and A.J. Robson, 1997a. The geography of the Humber catchment. Sci. Total Environ., 1941195: 87-99. Jarvie, H.P., C. Neal, G.P. Ryland, D.V. Leach and W.A. House, 1997~. Major ion concentrations and the inorganic carbon chemistry of the Humber rivers. Sci. Total Environ., 194/195: 285-302. Jarvie, H.P., C. Neal and A.D. Tappin, 1997b. European Land-based pollutant loads to the North Sea: an analysis of the paris Commission data and review of monitoring strategies. Sci. Total Environ.. 194/195: 39958.
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Kempe, S., M. Pettine, and G. Cauwet, 1991. Biogeochemistry of European rivers. In: E.T. Degens, S. Kempe and J.E. Richey (Eds.), Biogeochemistry of Major World Rivers, SCOPE, Wiley, Chichester, pp. 1699211. Leeks, G.J.L., C. Neal, H.P. Jarvie, H. Casey and D.V. Leach, 1997. The LOIS river monitoring network strategy and implementation. Sci. Total Environ., 1941195: 101 - 109. Mackereth, F.J.H.. J. Heron and J.F. Tailing, 1978. Water Analysis: Some Revised Methods for Limnologists, Freshwater Biological Association Special Publication No. 36. Mantoura, R.F.C. and E.M.S.Woodward, 1983. Conservative behaviour of riverine dissolved organic carbon in the Severn Estuary: chemical and geochemical implications. Geochim. Cosmochim. Acta, 47: 1293- 1309. Marker, A.F.H., 1994. Chlorophyll-cr, SCA method revision. Report to the National Rivers Authority. PR395/3/A, 64 PP. Marker, A.F.H., E.A. Nusch, H. Rai and B. Riemann, 1980. The measurement of photosynthetic pigments in freshwaters and standardization of methods: conclusions and recommendations. Ergebn. Limnol., 14: 91-106. Meybeck, M., 1993. Riverine transport of atmospheric carbon: sources, global typology and budget. Water. Air, and Soil Pollution, 70: 4433463. Pinder. L.C.V.. A.F.H. Marker, A.C. Pinder, J.K.G. Ingram, D.V. Leach and G.D. Collett, 1997. Concentrations of suspended chlorophyll-a in the Humber Rivers. Sci. Total Environ., 1941195: 3733378. Reynolds, C.S., 1984. The Ecology of Freshwater Phytoplankton. Cambridge University Press, Cambridge. Robson, A.J. and C. Neal, 1997. A summary of regional water quality for Eastern UK rivers. Sci. Total Environ,. 1941 195: 15537. Sedjo, R.A.. 1993. The carbon cycle and global forest ecosystem. Water, Air, and Soil Pollution, 70: 2955307. Tipping, E., C. Woof and K. Clarke, 1993. Deposition and resuspension of fine particles in a riverine ‘dead zone’. Hydrol. Proc., 7: 2633277. Wass, P.D., SD. Marks, J.W. Finch, G.J.L. Leeks and J.K.G. Ingram. 1997. Monitoring and preliminary interpretation of in-river turbidity and remote sensed imagery for suspended sediment transport studies in the Humber catchment. Sci. Total Environ., 194;‘195: 2633283. Webb, B.W., J.M. Phillips, D.E. Walling, I.G. Littlewood, C.D. Watts and G.J.L. Leeks, 1997. Load estimation methodologies for British rivers and their relevance to the LOIS RACS(R) programme. Sci. Total Environ.. 194/195: 3799389. Wilkinson, W.B.. G.J.L. Leeks, A. Morris and D.E. Walling, 1997. Rivers and coastal research in the Land-Ocean Interaction Study. Sci. Total Environ., 194/‘195: 5 -14.
The
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Organic carbon in the Humber rivers E. Tipping”, A.F.H. Markerb, C. Butterwick”, G.D. Collettb, P.A. Cranwell”, J.K.G. Ingram”, D.V. Leach”, J.P. Lishman”, A.C. Pinder”, E. Rigg”, B.M. Simon” hfn.ytitutr
“Institute qf Freshratrr ‘NERC
of’ Fresh~aier Ecology, Ecology, Eastern Rivers LOIS
Windermere Lahoruiorl; Ambleside, Cumhria, LA22 OLP, UK Laboratory, Monks Wood Experimental Station, Abbots Ripton, Camhridg~shirr, PEI 7 2LS, UK c/o Department of’Bio1og.v. University of York, York, YOI 5DD, UK
Lahoratorl:
Huntingdon,
Abstract Dissolved organic carbon (DOC), particulate organic carbon (POC), particulate organic nitrogen (PON), chlorophyll-u, and alkalinity were determined weekly or more frequently in samples from 11 rivers in the Humber catchment, between September 1993 and February 1995. [DOC] varied overall from 1 to 15 mg l_ ‘, [POC] from 0.2 to 67 mg 1~ ‘, [LOCI from 0.01 to 7.6 mg 1~ ’ and [DIC] f rom 1 to 52 mg 1~ ‘. For the rivers with predominantly rural catchments, the complex dependence of [DOC] on season and discharge can be interpreted in terms of soil humification and hydrological processes, whereas the inverse dependence of [DOC] on discharge in the more polluted industrial rivers indicates the dominance of point-source effluents. Concentrations of POC depend significantly upon discharge and on concentrations of suspended particulate matter, suggestive of particle mobilisation when physical thresholds are exceeded. During summer months, the ‘living organic carbon’ of algae accounts for much of the POC in all but the most polluted rivers. Fluxes of organic carbon were estimated by combining daily concentrations (measured or interpolated) with discharge data. The contribution of DOC to the annual flux of organic carbon in the different rivers varies from 51 to 80%. the overall contribution being 63%. For both DOC and POC, the main transport to the estuary takes place during the autumn-winter period. For the study rivers, the total flux of organic carbon during the period September 1993 to September 1994 was 73 x lo9 g a ~ ‘. However, these rivers represent only about two-thirds of the total catchment area of the Humber estuary, and so the true total flux is probably closer to 100 x 10’ g a- ‘. 0 1997 Elsevier Science B.V. Keywords: LOIS; Humber; Trent: Ouse
Carbon:
Dissolved
organic
carbon; Particulate
1. Introduction
8
PII SOO48-9697(96)05374-O
1997 Elsevier
Science
B.V.
All
rights
Export
coefficients;
Carbon
fluxes;
molecules, such as carbohydrates, amino acids, hydrocarbons, fatty acids and phenolics, together with natural macromolecules and colloids (humic substances), sewage and industrial particulates,
Dissolved and particulate organic matter in rivers is a complex mixture of chemically-identifiable 004%9697!97/$17.00
carbon;
reserved.
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Table 1 River characteristics and summary data for concentrations of dissolved organic carbon (DOC), particulate organic carbon (POC), ‘living organic carbon’ (LOC) and dissolved inorganic carbon (DIC) River
A
Q
DOC
POC
LOC
DIC
Upper Swale Lower Swale Ure Nidd Owe Wharfe Derwent Calder Aire Don Trent
381 1363 914 516 3315 159 1634 899 1932 1256 8231
16 24 26 10 58 15 20 26 42 22 155
1-12 2-12 2-12 4-11 3-10 2-7 2-7 3-15 2-13 3-7 4-9
0.2267 0.4-23 0.3-12 0.4-13 0.3-13 0.3-20 0.337 0.9-27 0.8-25 0.9-18 0.7711
0.01-0.9 0.03-1.4 0.01-7.6 0.02-1.3 0.02-3.1 0.02-2.1 0.02-1.2 0.01-1.0 0.01-0.7 0.03-4.5 0.02-6.4
7-42 11-52 13-46 l-26 10-47 14-38 22245 5530 12-45 13-39 25549
Catchment areas (A) are in km2, mean discharges (Q) in m3 s-’ and ranges of concentrations in mg 1-l. The discharge data were calculated from daily values for the period September 1993 to September 1994. The concentrations refer to between 79 and 88 values for each river for the period September 1993 to February 1994. Note that the discharges for the Aire, Calder, Ouse and Wharfe are in part estimated from correlations with discharges for the Don and Lower Swale (see Section 3). LOC is estimated from chlorophyll-a (see Section 3).
soil organic matter, and biological material, notably living phytoplankton and other plant material, (see e.g. Meybeck, 1993; Hope et al., 1994). These materials are of interest for several reasons. Their transport contributes significantly to the carbon cycle (Meybeck, 1993; Sedjo, 1993); according to Ittekkot and Laane (1991), the transport of soilderived organic matter by rivers and its subsequent burial in coastal sea sediments is an important global sink for carbon. The more reactive components, including dissolved compounds, phytoplankton and sewage particulates, may make a significant contribution to heterotrophic metabolism in estuaries and coastal seas. Fulvic acid and other humic substances play a significant role in the behaviour of metals, by complexing them and affecting transport and bioavailability. These compounds also interact with organic micropollutants, and adsorb to the surfaces of mineral solids affecting surface chemistry and rates of aggregation. Organic compounds, especially those in solution, also influence photochemistry. To understand and predict the behaviour and influence of riverine organic matter, basic information about concentrations and fluxes is required. The present paper concerns organic matter in rivers draining into the Humber estuary
in Northern England. We report determinations of dissolved organic carbon (DOC), particulate organic carbon (POC) and chlorophyll-a, from which ‘living organic carbon’ (LOC) was estimated. This work is part of LOIS RACS(R) river monitoring programme. The overall aims and scope of the LOIS study are presented by Wilkinson et al. (1997).
2. Field sites The Humber catchment is a major contributor of water and chemicals to the North Sea (Jarvie et al., 1997a,b). The estuary is fed by two main rivers, the Yorkshire Ouse and the Trent, which drain agricultural, industrial, urban and upland areas (Jarvie et al., 1997a). Water quality varies, the rivers in the north of the region being less polluted than those in the south (Jarvie et al., 1997b; Edwards et al., 1997; Robson and Neal, 1997). Table 1 gives information on catchment area and discharge. Sampling site locations are shown in Leeks et al. (1997). The sites on the Rivers Ouse, Wharfe, Derwent, Aire, Don and Trent are at or close to the tidal limit. In all but one case,
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the sites are within a few kilometres of flow gauging stations operated by the National Rivers Authority (now the Environment Agency); the exception is the Derwent, for which the gauge is 29 km upstream of the sampling site. The samples considered in the present work were collected at least weekly, throughout the period September 1993 to February 1995. In some cases, more frequent sampling was conducted, in order to improve information on high-discharge events.
3. Methods Dip samples for the determination of DOC were collected in 1 1 high-density polyethylene wide-necked bottles. They were filtered through Whatman GFjC filter discs within 1 week of collection, then transported to the IFE Windermere Laboratory where they were kept at 4°C prior to analysis, which was generally done within 14 days. The samples were analysed for DOC with a TOCSin II total carbon analyser, dissolved inorganic carbon being removed by sparging after acidification with HNO,. Recovery was checked by analysing solutions of potassium hydrogen phthalate of known concentration. The detection limit was 0.5 mgC 1-l and the precision was 5%. Suspended matter was collected by filtering known volumes (30-120 cm3) through ashed Whatman GF/F filter papers (15 mm diameter). The samples were fumed with 4 M HCl to remove inorganic carbonates (Hedges and Stern, 1984) then analysed for POC and PON with a CarloErba Elemental Analyser, Model 1106. The detection limits were 0.2 mg 1-i for both C and N, the precision for C was 5% and that for N was 10%. Quality control was by repeat analyses of the same lake sediment sample. The determination of concentrations of suspended particulate matter (SPM) within the LOIS rivers monitoring programme is described by Leeks et al. (1997) and Wass et al. (1997). To analyse for chlorophyll a, samples of river water (usually 1 1) were filtered through GFjC glass fibre filters (9 cm diameter) and extracted with ethanol, as described by Marker et al. (1980) and Marker (1994). Concentrations of ‘living organic carbon’ (LOC) were
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estimated from those of chlorophyll-u by assuming one gram of chlorophyll to represent 50 g of algal carbon, a relationship derived from data quoted by Reynolds (1984). Alkalinity was determined by Gran titration, as described by Mackereth et al. (1978). Flow data, usually recorded at 15 min intervals, were obtained from the National Rivers Authority and used to calculate mean daily discharges. There were some missing values, and some values of doubtful validity, in the original data sets, which resulted in gaps in the calculated mean daily discharges. These were infilled on the basis of correlations with similar nearby rivers. Thus, 20 missing flows of the Aire were estimated from those of the Don by assuming a constant discharge ratio between the two rivers of 1.84, established by regression analysis (545 points, r2 = 0.76, P < 0.001). Discharges of the Calder were also estimated from those of the Don (10 missing values, 536 points for regression, ratio = 1.11, r2 = 0.76, P < 0.001). Discharges of the Ouse were estimated from those of the Lower Swale (67 missing values, 563 points for regression, ratio = 2.16, r2 = 0.89, P < O.OOl), as were those of the Wharfe (337 missing values, 203 points for regression, ratio = 0.584, r2 = 0.81, P < 0.001). The high number of estimated discharges for the Wharfe mean that fluxes estimated for this river must be treated with caution. Daily fluxes (g d - ‘) were calculated by taking the products of mean daily discharges and instantaneous concentrations. The latter were assumed to hold for the whole of the day in question. Because samples were taken only approximately weekly, intermediate concentrations were estimated either by linear interpolation between measured values, or, in the case of POC, from [POC]-discharge relationships (see below). Yearly fluxes were obtained by summing the daily values, and export coefficients (g m -’ a ~ ‘) were estimated by dividing the annual flux by the catchment area of the river in question (Table 1). In making flux calculations from relatively sparse data, we acknowledge that appreciable errors may result, as demonstrated by Webb et al. (1997).
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4. Results
4.1. Concentrations Table 1 shows ranges of concentrations of DOC, POC, LOC and DIC for the 11 rivers. The ranges of values of [DOC] do not differ greatly among the rivers. Although the river with the smallest catchment area (Upper Swale) has the greatest range, and the river with the largest catchment area (Trent) the smallest, there is no general relationship. For all the rivers, the lowest [POC] values are less than 1 mg 1~ ‘, while the highest observed values vary from only 7 mg 1~ ’ for the Derwent to 67 mg l- ’ for the Upper Swale. Concentrations of LOC can reach values similar to those of DOC and POC, especially in the Ure, Don and Trent. Values of [DIC] are generally high, reflecting the widespread occurrence of calcareous sedimentary rocks in the catchments of the study rivers (cf. Jarvie et al., 1997a). Fig. 1 shows variations with time of DOC, POC and discharge for four rivers, the Ure, a relatively unpolluted northern tributary of the Ouse, the Aire, a river significantly impacted by sewage and industrial effluents, and for the Ouse and Trent, the two largest rivers draining into the Humber estuary. For the Ure, Ouse and Trent, both [DOC] and discharge increase during the autumn. Later in the winter, however, [DOC] tends to decrease while discharges remain high. The same patterns were observed for the Upper Swale, Lower Swale, Nidd, Wharfe and Derwent. As a result, when the whole study period is considered, the relationships between [DOC] and discharge for these rivers are weak. For the Upper Swale and Derwent, statistically significant positive correlations are found (u’= 0.19, P < 0.01; Y? = 0.21, P < 0.01 respectively), but for the other rivers the correlations were statistically insignificant (r2 = 0.01 - 0.07, P > 0.01). In contrast, [DOC] values in the Aire (Fig. l), Calder, and Don are at their highest during the summer, low-discharge, period, and significant negative correlations of [DOC] with discharge are found, with I.’ values of 0.40, 0.29 and 0.10 respectively (P < 0.01 in each case). The [DOC]-discharge relationship for the Aire is shown in Fig. 2.
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All of the rivers show a positive dependence of [POC] on discharge, as shown for the Aire in Fig. 2. Values of r2 vary from 0.28 (Calder) to 0.81 (Don), most being between 0.3 and 0.5 (Table 2). Fig. 2 shows that the largest increases in [POC] are not necessarily associated with the largest increases in discharge. A common feature is a major peak of POC associated with the first increase in discharge following the low-flow summer period. This is seen in September 1993 for all four rivers, in September 1994 for the Aire and Trent, and in November 1994 for the Ouse and Ure. Concentrations of POC correlate with concentrations of suspended particulate matter (SPM), as shown by Table 3 and Fig. 3. The ‘living organic carbon’ (LOC) is mostly due to phytoplankton (Pinder et al., 1997), but also includes contributions from benthic organisms carried by the flow. Preliminary examinations show that the spring phytoplankton maximum in the Trent was dominated by centric diatoms, particularly Cyclostephanos invisitatus, Cyclotella nzeneghiniana and Stephanodiscus hantzschii. Later in the summer, there were increasing proportions of a wide variety of Chlorophycae. Fig. 4 shows that, apart from the spring maximum in the Trent, variations of [LOCI in the rivers follow similar patterns. During the summer, LOC accounts for essentially all the POC in the Ouse, Trent and Ure, but not in the more polluted Aire. High [LOC]/[POC] ratios were also found for the Lower Swale, Nidd and Wharfe, the Calder gave similar results to the Aire, while the Upper Swale, Derwent and Don displayed intermediate behaviour. During the periods when LOC was dominant, the [PON]/[POC] ratios for the more productive rivers were in the range 0.15-0.20, close to the characteristic Redfield ratio (see e.g. Reynolds, 1984); at other times, the ratio was ca. 0.10. 4.2. Fluxes Annual fluxes for the period September 1993 to September 1994 are given in Table 4. In the case of POC, the linear regression of concentration on discharge (Table 1, Fig. 2) was used as an alternative method of estimating intermediate values,
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.Trent 25
"E fin k 5 s
I
200
100
"SONDJFMAMJJ, 1993YV-1994
Fig. I. Concentrations of dissolved organic carbon (continuous lines) and discharge, for the Rivers Ure, Ouse, Aire and Trent. OC = organic carbon.
although it should be borne in mind that the correlations are weak. The POC flux values estimated on this basis were higher than those derived from interpolated values, by factors varying from 1.04 for the Ouse to 1.63 for the Wharfe. The effect on the total flux from the Ouse-Trent system was to increase it by 12%, a relatively small difference.
--
I’
0
particulate organic Note the different
4SONDJF' --1995
carbon ordinate
(dotted scales.
lines),
and
of river
Fig. 5 shows plots of cumulative flux against time for the Ure and the Trent. The fluxes of both DOC and POC are greatest during the winter periods. In the case of DOC, the concentration of which varies relatively little with flow, this is principally due to the greater winter discharges. For POC, the positive dependence of [POC] upon discharge further strengthens the flux-discharge
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relationship, so that the difference between winter and summer fluxes is more pronounced. Because of the seasonal dependence of phytoplankton growth, the fluxes of LOC are not dominated by winter flows. This is very noticeable for the Trent (Fig. 5), for which the largest part of the flux comes in the low-flow period between April and June 1994. In contrast, the export of LOC in the Ure is more dependent upon changes in discharge.
DOC
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Table 2 Linear regressions carbon (mg 1-l) Humber rivers
Upper Swale Lower Swale Ure Nidd Ouse Wharfe Derwent Calder Aire Don Trent
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of the concentration against discharge
(m’
of particulate organic SK’) for the major
m
c
i-2
n
0.20 0.08 0.03 0.08 0.02 0.08 0.05 0.05 0.04 0.09 0.01
- 1.7 0.4 0.7 1.2 0.6 -0.1 0.4 2.0 1.4 1.3 1.0
0.60 0.37 0.45 0.35 0.49 0.48 0.37 0.28 0.42 0.81 0.47
80 87 84 84 84 84 83 85 88 82 79
The slope and intercept are denoted by m and c respectively, n is the number of observations. All the correlations are significant at the 0.1% level.
5. Discussion
0
50
100
flow
150
200
250
The results distinguish two kinds of DOC behaviour. For the Upper Swale, Lower Swale, Ure, Nidd, Ouse, Wharfe, Derwent and Trent, [DOC] is low during the summer, increases to maximum values during the autumn, and falls thereafter (Fig. 1). The cumulative flux of DOC, on the
m” s-’ Table 3 Linear regressions of the concentration carbon (mg I-‘) against the concentration ulate matter (mg 1-l) l
m Upper Lower Ure Nidd Ouse
0
50
100
flow
150
200
250
Wharfe Derwent Calder Aire Don Trent
Swale Swale
0.07 0.03 0.06 0.08 0.03 0.07 0.05 0.02 0.07 0.05 0.03
c 0.6 1.4 0.8 0.8 1.3 0.5 0.7 3.0 1.6 1.7 1.6
of particulate of suspended
organic partic-
9
n
0.60 0.47 0.62 0.67 0.44 0.78 0.49 0.10 0.66 0.65 0.33
77 82 79 79 81 80 83 83 88 81 78
m’ s-l
Fig. 2. Dependence of the concentrations carbon (DOC) and particulate organic charge for the River Aire.
of dissolved carbon (POC)
organic on dis-
The slope and intercept are denoted by m and c respectively, n is the number of observations. The correlations are significant at the 0.1% level, except that for the Calder, which is significant at the 1%) level
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Ouse
0
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ISPMI
mg
150
200
0
50
100
I-’
150
[SPMI
200
mg
250
300
350
I-’
Aire .
50
100
[SPMI Fig. 3. Dependence of the concentration (SPM) for the Rivers Ure, Ouse, Aire
150
mg
200
250
0
I-’
of particulate and Trent.
50
100
[SPMI organic
other hand, increases approximately constantly during the autumn-winter period (Fig. 5). These observations can be interpreted in terms of soil humification and hydrology. It can be envisaged that, during the summer, the microbial processing of organic matter in the upper soil horizons produces ‘potential dissolved organic carbon’, i.e. carbon in organic compounds that are sufficiently soluble, or poorly adsorbed, to pass into the soil solution. At this time, percolating water penetrates relatively deeply into the soil, where removal of DOC by adsorption to mineral surfaces maintains low concentrations in drainage water,. and hence in river water. As the soils wet up in autumn, lateral flow becomes more significant.
carbon
(POC)
on the concentration
150
mg
200
250
I-’
of suspended
particulate
matter
The proportion of the drainage water that has passed through only surface horizons, and is therefore rich in DOC, increases, and so does river [DOC]. This process continues during the winter, but as the throughput of water increases, the rate at which potential dissolved organic carbon can transfer to the solution phase becomes limiting (because desorption and/or diffusion within soil peds are relatively slow). Thus at high infiltration rates DOC is diluted, while the absolute amount leached is maintained. As a result, riverine [DOC] decreases, but the flux does not. A different situation is found for the most polluted rivers (Calder, Aire, Don), in which [DOC] decreases with increases in discharge, and
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is highest during the summer (Figs. 1 and 2). Presumably, the DOC in these rivers is dominated by point-source effluents, industrial and from sewage treatment works. It is therefore likely to differ in physico-chemical properties and susceptibility to (bio)chemical transformations from the material transported by the less-polluted, rivers with more rural catchments. In common with [DOC], [POC] shows increases in autumn, although the responses are sharper,
15
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Table 4 Fluxes of dissolved organic carbon (DOC), particulate organic carbon (POC), total organic carbon (TOC), ‘living organic carbon’ (LOC). and DIC for the vear 7 Sentember 1993 to 6 September 1994
Upper Swale Lower Swale Ure Nidd Ouse Wharfe Derwent Calder Aire Don Total Ouse Trent Total System
DOC
POC
TOC
LOC
DIC
2.0 3.7 3.3 1.9 8.7 3.0 4.0 4.9 7.9 3.3 26.9 19.0 45.9
0.5 2.9 1.4 0.9 5.5 0.7 1.2 2.9 5.0 3.2 15.6 11.4 27.0
2.1 6.6 4.6 2.8 14.2 3.7 5.3 7.7 12.9 6.4 42.4 30.4 72.8
0.03 0.1 0.2 0.05 0.4 0.06 0.1 0.1 0.2 0.3 1.0 2.1 3.1
10.9 23.5 20.1 5.1 48.6 11.6 22.5 11.3 27.3 14.7 124.7 141.5 266.2
The fluxes are given in IO9 g a-‘. The ‘Total Ouse’ flux is the sum of those for the Ouse. Wharfe, Derwent, Aire and Don. The ‘Total System’ flux is the sum of the ‘Total Ouse’ and Trent values
15
b F G 2
Aire j ;
10-l
,: : : ::; (.. ; ; ::: : .: :: _.. 5-i; ; i ;,; ; I:; .,.i ii :: j; ; ii' .": .; :.._ : :: ;.,,, :. ,:.- .,.,,,,.;.,..; L ; : .. ..".' ,;:7,.,.$'.,c: ; .: : .....,:'7 i'..'. j !__,,,,,;. :.I : : '.,. >. 0-w. ,*, , ‘I j: I]
15
Trent b
19931a1994
-1995
Fig. 4. Concentrations of particulate organic lines) and ‘living organic carbon’ (continuous Rivers Ure, Ouse, Aire and Trent.
carbon lines)
(dotted for the
being more closely related to discharge (Fig. 1). This behaviour is consistent with particle mobilisation when physical thresholds are exceeded. The especially high values of [POC] during the first one or two events of autumn probably arise from both catchment erosional processes and the entrainment of particulate material accumulated in the river bed over the long summer period of low flow (Tipping et al., 1993). POC differs from DOC in that, following the initial peaks, its concentrations do not decrease towards the end of the autumn-winter period (Fig. 1). This suggests that the rate of supply of material able to be mobilised is not limiting, in apparent contrast to DOC (see above). Concentrations of POC are positively correlated with [SPM] (Table 3, Fig. 3). The positive intercepts found for all the rivers reflect the fact that when [SPM] is low there is a higher proportion of carbon in the particulates. The same observation has been made by Cauwet (1985) for the River Loire (France). This observation may be explained, at least partly, by the presence of algae during the summer months, when [SPM] is low. The slopes of the regressions probably give an
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et al. I The Science
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indication of the carbon contents of particulates eroded from the catchments. The results suggest a range between 2% (Calder) and So/o(Nidd), with an average of 5%. The results in Table 4 show that the relative contributions of DOC and POC to the total organic carbon flux vary among the rivers. For the Upper Swale, 80% is in the dissolved form, whereas for the Don the two fractions are almost equal. Overall, the DOC flux contributes 63%. Algal biomass comprises an appreciable fraction (12%) of the total POC flux to the Humber estuary. The contributions in the individual rivers vary from 4”/;, (Lower Swale) to 18% (Trent). The high value for the Trent means that it supplies 67% of the LOC flux to the Humber estuary (Table 4) compared to 42% of the POC. There is also a relatively high value of 11% for the Don, one of the most polluted rivers. 0 :
Ure
0 x m
6 -
DOC POC LOC X,0
_____
,_~,~_^ _ _._.I _.----
S 0 N D 1993*-
x
30 -
= =
zo-
DOC POC ‘JJC x,o
J
----
F M’A’M’J’J’A’S’O’N’D’J’F 1994 -1995
_____
Fig. 5. Cumulative fluxes of dissolved organic particulate organic carbon (POC) and ‘living (LOC) for the Rivers Ure and Trent.
carbon organic
(DOC). carbon’
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Table 5 Export coefficients particulate organic
Upper Swale Lower Swale Ure Nidd Ouse Wharfe Derwent Calder Aire Don Total Ouse Trent Total System
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345-355
of dissolved organic carbon carbon (POC,,,) in g m-’
(DO&,) a-’
DOG,
pocex,
5.3 2.7 3.6 2.4 2.6 4.0 2.5 5.4 4.1 2.6 3.0 2.3 2.1
1.3 2.1 1.5 1.2 1.7 0.9 0.7 3.2 2.6 2.5 1.8 1.4 1.6
and
Export coefficients (Table 5) express fluxes per unit catchment area. On this basis, the Upper Swale, Ure, Wharfe, Calder and Aire supply the most DOC, while the Calder, Aire and Don supply the most POC. For the first three of these rivers, the higher DOC export coefficients are presumably due to the leaching of upland peaty soils, for which conditions are most fdvourable for the production of ‘potential DOC’. For the more-polluted rivers (Calder, Aire and Don), the use of export coefficients is questionable, since both DOC and POC inputs are presumably due to point-source sewage and industrial discharges rather than whole-catchment characteristics. Concentrations of organic carbon may well be influenced by within-river processes, which might include addition due to the release of organic matter from algae, or desorption from particulates. and removal by decomposition or adsorption. If the organic carbon fractions behave conservatively, or if addition and removal balance, then downstream fluxes should exceed upstream ones. This is the case for the two sampling sites on the Swale (Table 4) for both DOC and POC. However, the sum of the DOC fluxes for the Lower Swale, Ure and Nidd, all of which flow into the Ouse, is 8.8 x 10” g a- ‘, a value slightly greater than the flux of 8.7 x lo9 g a-’ for the Ouse itself (Table 4). For POC, the Ouse value (5.5 x 10’ g a ‘) is greater than the sum of the
354
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of the Total
tributary fluxes (5.2 x lo9 g a-‘). In principle, these data suggest a net removal of DOC in the stretch of river upstream of the Ouse sampling site, but it seems unlikely that the fluxes are accurately enough known to support this as a definite conclusion. Evidence that significant mineralisation (removal) of organic carbon does occur within the rivers, comes from the high partial pressures of carbon dioxide in the morepolluted rivers (Jarvie et al., 1997~).
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and Kohlmaier (1991) established relationships between export coefficients of DOC and POC and the annual catchment run-off for 16 (DOC) and 9 (POC) large world rivers. Their &rived slopes for DOC and POC are 6.4 and 2.2 g m -’ respectively; these values are discharge-weighted mean concentrations. For the Humber catchment, the values are similar, being 5.4 g m -3 for DOC and 3.1 g m - 3 for POC. 5.2. Implications for land-ocean interactions
5.1. Comparison with other data
There are few large data sets on organic carbon concentrations and fluxes in British rivers. For the River Severn, Mantoura and Woodward (1983) reported an export coefficient for DOC of 1.7-2.8 g m-* a-’ (quoted in Hope et al., 1994) which is similar to our value for the Humber (2.7 g me2 a-‘). The GEMS (1983) report (quoted by Kempe et al., 1991) gives a total organic carbon flux for the Trent at Nottingham of only 1.6 x lo9 g a-‘. Even though Nottingham is ca. 30 km upstream of the sampling site used in the present work (Cromwell Lock), this is considerably lower than the value of 3.04 x 10” g a-’ found in the present study (Table 3). The compilation of Hope et al. (1994) for rivers throughout the world shows that DOC export coefficients range from 0.01 to 44 g m -* a ~ ‘, the highest values referring to catchments with forests or bogs. Lowland rivers generally have values in the range l-10 g m-* a-‘. In this respect, therefore, the Humber value is typical. Reported POC export coefficients range from 0.07 to 50 g rn-* a- ‘, but again the range 1- 10 g m - ’ a - ’ covers most rivers, and contains our value of 1.6 g m -’ a - ’ for the Humber. The export coefficient, in g m - * a - ‘, for TOC for the Humber is 4.3 (Table 4) which is similar to those of large European rivers given by Kempe et al. (1991) e.g. Garonne (3.Q Seine (3.5) Rhone (6.1), Loire (4.8) and Rhine (5.3). In terms of specific carbon flux therefore, the Humber rivers appear typical of European rivers. Carbon export has also been considered in terms of annual catchment runoff. Thus, Esse
The total catchment area of the rivers which drain directly into the Humber is 17,127 km2 (cf. Table l), approximately two-thirds of the total catchment area of the Humber. The remaining third is contributed by smaller rivers, not sampled in the present study, and those areas of the catchment that drain into rivers and estuary below the tidal limits. Therefore the quoted fluxes for the ‘Total System’ (Table 4) are underestimates of the total fluxes from the rivers to the estuary; the values for TOC and DIC may therefore be close to 100 x lo9 g aa1 and 400 x lo9 g a ~’ respectively (cf. Table 4). The data in Table 4 show that organic carbon comprises approximately 20% of the total carbon flux to the Humber estuary, assuming the dissolved inorganic carbon (DIC) to dominate the total inorganic flux. Although the inorganic carbon flux contributes to the global carbon cycle, the organic material is more significant for processes in the estuarine and coastal zone, where it may be metabolised, participate in biogeochemical processes such as the binding of metal ions and organic micropollutants, and be buried in nearshore marine sediments. Within the context of LOIS, metabolically-labile organic material is important, as a substrate for heterotrophic metabolism in the estuary and coastal zone. This is shown by the significant oxygen sags in the low salinity regions of the Ouse and Trent branches of the Humber estuary (Gameson, 1982). To judge the significance of riverine fluxes of labile organic carbon in this respect, information on primary productivity in the coastal zone is needed.
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Acknowledgements We thank the Yorkshire and Severn-Trent Regions of the National Rivers Authority for supplying discharge data. The paper benefitted considerably from the critical comments of two anonymous referees.
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