Trends Organic Carbon

8/11/2019 Trends Organic Carbon

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SummaryScottish soils hold a large amount of carbon. Losses of that carbon can lead to emissions of the greenhouse gases carbondioxide (CO2) and methane. As well as direct gaseous emissions, organic matter can be transported from soils in water inthe form of Dissolved Organic Carbon (DOC) or Total Organic Carbon (TOC) fractions* and can subsequently be oxidised tocarbon dioxide.

Our data shows that the concentration of organic carbon in many Scottish rivers has approximately doubled over the lasttwenty years, with soils being the most likely source. This has implications for soil carbon budgets and greenhouse gasemissions.

The increased loss of TOC to rivers is likely to be driven by a combination of climate change (increased temperature andchanging rainfall patterns) and recovery from acidification.

The more limited data available for lochs seems to show that the concentration of organic carbon has decreased in suchsurface waters over the last five years.

IntroductionChanges in Scotlands climate will affect water quality in a number of ways. One possible impact of climate change is anincrease in organic carbon in water caused by an increased breakdown of soil organic matter to water soluble material, due inturn to warmer temperatures and drier summers which lower water tables and increase run-off during severe storm events.

Increases in water based organic carbon have been documented at a number of UK sites (Evans et al 2005, Worrall et al2004) although there may be decreasing trends at a few sites (Worrall and Burt 2007). Despite the high carbon content ofScotlands soils, information is limited on trends in organic carbon in surface water in Scotland.

Carbon lost from soils to water may be oxidised to produce carbon dioxide providing a positive feedback to climatechange. Scotland has large stocks of organic carbon held in peaty and organic soils, estimated at 2735 metric megatons(MtC) in total (ECOSSE Report 2007). If converted to carbon dioxide that is equal to 174 years of human emissions atcurrent rates.

Soil organic matter affects several other soil properties including water holding capacity and ability to retain pollutants

and nutrients. Loss of soil organic matter could increase run-off which in turn increases flood risk and pollutant content inwater. Therefore it is important to understand any trends in organic carbon concentration in Scottish rivers.

As well as affecting the environment, organic carbon compounds in water can form carcinogenic trihalomethanes duringwater treatment processes, so increasing organic carbon in water may cause additional costs for water treatment works.

Recovery from acidification and changes in land management and use have been suggested as causes of increased organiccarbon leaching from soils (Evans et al 2005, Worrall et al 2004) in addition to the climate related drivers of increasingtemperature and changes in rainfall patterns. In mountainous areas, reduction in snow coverage may increase temperatureand aeration in soils leading to increased biotic activity and hence organic carbon production and turnover. A combinationof pressures is likely responsible for the observed changes in water based organic carbon concentrations.

The aim of this work was to identify SEPA sites with long term data on organic carbon concentrations and to examine thisdata for trends.

* Published research generally refers to Dissolved Organic Carbon (DOC) rather than Total Organic Carbon (TOC). DOC comprises just that fraction of aqueous organic carbon which passesthrough a filter, whereas TOC includes particulate and purgeable organic carbon. Historically SEPA has generally monitored TOC in rivers and DOC in lochs. When rivers contain littlesuspended solid material DOC and TOC values are likely to be similar, but when sediment loadings are high, for example in high flows, TOC values will be higher than DOC as TOCmeasurements include organic material bound to sediments.


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Trends in TOC concentration in riversSEPA holds longterm (greater than 10 years) TOC data for 58 river sites (Figure 1) mainly rural catchments where inputsfrom point source discharges such as sewage treatment works and industrial sites are unlikely to contribute significantly toTOC loading. Although these sites do not give full geographical coverage of Scotland they do include many of the areaswith the highest soil carbon concentrations. Recent expansion of SEPAs monitoring network means that TOC wasmonitored at 490 river sites in 2008, which will improve geographical coverage in future.

The Seasonal Kendall trend test showed with very high confidence that 39 of 58 river sites with long term TOC data havesignificant upward trends in TOC concentration over the full time period for which data exists. The sites with significant

trends in TOC concentration are shown in red in Figure 1. The remaining 19 sites (shown in blue in Figure 1) showed notrend. Thirteen sites with shorter data sets also showed significant increasing trends in TOC. Although there were somestatistically significant changes in the data at some sites, they were small compared to the variability of the data and longterm increasing trends in TOC.

The rate of TOC increase averaged across all sites with increasing concentrations was 0.12 milligrams per litre per year(mg/l/y), giving an increase in TOC concentration of nearly 2.5 mg/l over a twenty year period. That amounts to a doublingof TOC concentration over twenty years at most of these sites. The rate of TOC increase ranged from 0.03 mg/l/y in theYthan upstream of Auchterlees to 0.543 mg/l/y in the Green Burn on Speyside downstream of Glenfarcas bioplant. An evenhigher rate of increase in TOC concentration of 2.28 mg/l/y was seen in the Tarf Water, Dumfriesshire, although that wasbased on only 6 years of data.

Decreases in TOC were only recorded at two sites: the Carron, which passes through an industrialised urban area of Falkirk, andthe Forth at Craigforth in Stirling, which also has urban areas in its catchment. These sites are both likely to have experienceddirect human influence from urban and industrial development on TOC inputs as well as changes related to indirect effectssuch as climate change and acidification recovery. Both sites with decreasing TOC trends had less than 10 years of data.

All the sites with long term data that showed increasing TOC weresouth of the Cromaty Firth, while 11 of the 18 long term sites whichdid not show a trend were north of Inverness. This latitudinalpattern could be linked to a gradient in the recovery from aciddeposition as the impacts of acidification were greater furthersouth, or to climatic factors: temperature increases have beengreater in southern Scotland (Barnett et al 2006) while higherrainfall in northern Scotland may maintain a high water table innorthern soils even during drier spells, plus TOC is only produced inaerobic soil. Topography may also have an influence on rainfall andrun-off characteristics. Another possibility is that proportionallysmall changes in TOC concentrations in northern Scotland aremasked by natural fluctuations in the higher background levels.Mean TOC concentrations at the northern sites over the monitoringperiod were 715 mg/l compared to 17 mg/l further south.

Most of the sites with increasing TOC were in the east of Scotland,

which reflects the distribution of sites sampled, but may also reflectdifferent changes in rainfall patterns between eastern and westernScotland (Barnett et al 2006).

At eight sites there was very high confidence that TOCconcentration displayed a seasonal pattern, with highestconcentrations occurring in late summer and autumn. This couldreflect reduced dilution of TOC in low summer flows, or increases ininputs caused by higher temperatures and lower water tables.Figure 1: Changes in TOC concentration in rivers with long

term data.

The Seasonal Kendall test is a statistical test to detect trends in data after the effect of any season cycles has been removed.

Very high confidence means at least a 9 out of ten chance of being correct. Intergovernmental Panel on Climate Change Guidance Notes for Lead Authors of the IPCC Fourth

Assessment Report on Addressing Uncertainties. 2005


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As well as increasing mean concentration, the square root of adjacent pairs test has shown increasing variability of thedata at some sites due to larger peak concentrations. These sites are clustered around and to the east of the Cairngormmountains (Figure 2) and may reflect changing rainfall patterns in this area which has experienced drier winters andwetter autumns (Figure 3) or, in higher areas, reflect reductions in percolation of melting snow pack during the summer.

For sites where a long term trend in TOC was observed, the rate of increase in TOC concentration is significantly correlatedwith the mean TOC concentration over the measurement period (Figure 4). This suggests that south of Inverness TOCincrease was proportional to the background TOC concentration at the site. Background TOC concentration depends on anumber of factors including the amount of soil organic carbon as well as catchment area (Aitkenhead-Peterson et al2007). However this correlation does not extend to northern Scotland where rivers with high TOC concentrations do notshow any increasing trends.

Where increasing trends are evident these were seen from thestart of the measurement period which was in the early to mid1980s in many cases. This is before significant reductions inacid or sulphate inputs occurred, and in any case the effects ofthis recovery on soil chemistry would be expected to be slow.This suggests that while recovery from acidification may havesome influence on TOC increases, it is not the only driver.

TOC concentration increased with flow at 45 sites, perhapsbecause at those sites rainfall infiltration helps to transportorganic matter from soil. Sites which did not show thiscorrelation were mainly in Dumfriesshire and north of the GreatGlen, which suggests that in these areas other factors such as

acidification or TOC production in soil may be the main driversof TOC release. Alternatively sites in drier locations in easternScotland may be more sensitive to erosion and flushing oforganic matter linked to more extreme weather events.

Although TOC and DOC are different measures of aquatic organic matter, in most flows particulate levels in rivers are likelyto be low and so TOC will be made up mainly of dissolved material. However it is possible that if sediment loading in rivershas increased it may have contributed to an increase in TOC. SEPA does not hold long term data on DOC in rivers.

Since 2002 there have been significant decreases in TOC at a few sites: the Shiel, Beauly, Ness, Ewe and Allt Darrie. All fiveof these sites showed increasing or no trends over longer periods. TOC decreases are only seen at a few sites over a

relatively short period and so it is not clear whether they are driven by climate-related factors or changes inenvironmental management such as improvements to sewage treatment works, changes in forestry practices or other landuse change. Since 2002 TOC concentration has increased at one site: the Ythan downstream of Fyvie. No trend in TOC since2002 was found at 21 sites. The remaining sites did not have enough data to assess trends over this period.

Figure 2: Sites showing significant increases in TOC concentration variability. Figure 3: Patterns of Precipitation Change (%) 19612004(Taken from Barnett et al 2006).

Figure 4: Rate of TOC increase against mean TOC concentration forsites with increasing TOC.


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Trends in TOC loading in riversTo find out whether the higher TOC concentrations observed are due to higher input of organic material or simply reduceddilution in lower flows, TOC loadings were calculated using measured flows where available, or otherwise modelled flows.

The pattern of trends in TOC concentration (Figure 1) and TOC loading (Figure 5) were very similar. There was very highconfidence that TOC loadings showed increasing trends at 35 of the 61 sites for which flow data was available. Theincreasing trends in TOC loadings suggest that increases in TOC concentration are driven largely by increasing inputs ratherthan reducing flow.

As with concentration there was very high confidence that somesites showed increasing variability over time, and again these siteswere predominantly located in the north-east. There was not anexact match between sites showing increases in TOC concentrationand those showing increases in TOC loading. Sites showingincreases in concentration but not loading may be more affectedby decreasing flow, whereas at those showing increasing loadingbut no trend in concentration the increased input may be balancedby increasing flow.

The extra amount of TOC carried by a river depends on its flow, with

larger increases occurring in larger rivers. Rates of increase in TOCloading ranged from 256 tonnes per year (t/y) for the Dee atMilltimber to 1.8 t/y for the Ythan upstream of Auchterlees.

TOC loadings increased sharply with flow at all sites, which isunsurprising given that TOC concentration was correlated withflow at most sites, and this is multiplied by flow to calculateloading.

It is difficult to estimate annual fluxes of TOC in a catchmentbecause the measurement frequency for TOC is low compared to the

variability of concentration and flow, but integrating the area underthe curve of spot loadings gives an estimated mean annual flux.Estimated mean annual fluxes of two large catchments, the Deemeasured at Milltimber and the Findhorn at the A96 road bridge are5,700 and 6,600 t/y respectively. Given these annual fluxes, the rateof increase amounts to an approximate doubling over 20 years,which is similar to the increased rate of TOC concentration.

Trends in DOC concentration in lochs219 lochs were monitored for TOC and/or DOC (Figure 6) in 2008. In most cases only one of these parameters wasmeasured, so comparison of trends in DOC and TOC is not possible. In contrast to rivers, there was more data on DOC for

lochs than TOC: 91 sites had five or more years of DOC data while 30 sites had five of more years of TOC data. Few siteshad records for either parameter going back more than ten years.

Water samples from lochs are taken at the bank, and therefore for stratified lochs only reflect the water quality in thetopmost layer.

Decreasing trends in DOC concentration were found with very high confidence at 23 of the loch sites with five or moreyears of data, and increases at two sites. There were no significant trends at the other sites with more than five years ofDOC data. TOC showed a decreasing trend at one of the sites with long term data, and no trend at the remaining sites. Thedata on these trends is limited and therefore certainty about them is fairly low. Where no trends were detected it may bebecause conditions have not changed or because there is not enough data to detect changes.

There are several factors that may explain why DOC concentration in lochs appears to be steady or declining, while inrivers TOC is increasing. One may be the different length of record, but even when river data was restricted to the post-2002 period few decreases in TOC were found. Possible reasons for DOC decreases in lochs include increasing watertemperature or increasing pH which could be increasing the rate at which organic carbon in lochs is mineralised.Alternatively changing climate may be affecting mixing process within lochs or there may be changes in loch ecology.

Figure 5: Changes in TOC loading in rivers.


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Lochs showing decreases in DOC concentration tend to be close tothe west coast with another cluster in the Trossachs (Figure 6).However, not all lochs close to the west coast show a trend in DOC,and the two lochs exhibiting DOC increases are also located in thewest. The distribution of lochs showing DOC decreases could belinked to climatic factors such as wind, rain or marine salt input,but the effect may also depend on characteristics of the loch suchas volume, depth or residence time. Further investigation wouldhelp to explain this pattern. In the east and south of Scotland

organic carbon concentrations in lochs generally did not change.

The DOC decreases in lochs have happened over a relatively shortperiod and it is possible that they may not represent a true longterm trend. However decreases in DOC have also been recorded atsome other sites in the last few years (Worrall and Burt 2007).Further monitoring is needed to find out whether the decreasingDOC trends continue, and the potential causes. A betterunderstanding of the relationship between DOC and TOC would alsobe useful.

Environmental impacts of increasing TOC run-offIncreasing TOC concentrations have a range of environmentalimpacts. The most obvious of these is darkening of water colourwhich reduces available light and energy, particularly in deeperlakes. This is likely to impact flora in these waterbodies and possiblythe organisms that feed on it.

Loss of organic matter from soils will increase surface water run-off, which in turn increases flood risk. Retention of pollutants may also be reduced, increasing their concentrations in thereceiving water.

Increasing TOC levels in water bodies stemming from soil organic matter losses may indicate changes in soil chemical and/ormicrobial processes, and could suggest that exchange rates of carbon dioxide and methane between soils and the atmosphereare also changing. This could further affect soil carbon stocks.

The loss of soil organic matter is also likely to reduce soil function by decreasing water and nutrient storage, leading to areduction in biodiversity and increasing soil erosion.

Future workSEPAs ongoing monitoring programme will ensure fuller coverage of TOC trends across Scotland, although it will be a fewyears before enough data is collected to detect trends at sites where monitoring began in 2007. Ongoing monitoring of DOC inlochs will give a more complete dataset with which to assess long term trends.

It may be possible to extend the length of records and improve geographical coverage of long term data by correlating TOC orDOC with other parameters such as Chemical Oxygen Demand (COD) or water colour. Past COD or water colour measurementscould then be converted to TOC or DOC concentrations. That approach has been adopted successfully elsewhere (Worral andBurt 2007).

To gain a better understanding of the relationship between TOC and DOC in rivers and lochs it would be beneficial tomonitor both parameters at selected sites for a few years.

A better understanding is needed of the factors affecting TOC and DOC production. Although SEPA is not well positioned toresearch the processes involved, our monitoring data may be able to help identify which processes are important in the field.

A budget needs to be developed to assess carbon losses from Scottish soils, taking into account losses as aqueous organiccarbon and gaseous fluxes. Again SEPA may not be the best body to do this, but may hold data which could contribute tothis assessment. The environmental impacts of increasing TOC concentration in Scottish rivers have not yet been assessed,but SEPA may hold data which enables some assessment of that to be made.

Figure 6: Trends in DOC/TOC in Lochs for sites with at least 5 yearsof data.


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ReferencesAitkenhead-Peterson JE, Smart RP, Aitkenhead MJ, Cresser MS, McDowell WH. Spatial and temporal variation of dissolvedorganic carbon export from gauged and ungauged watersheds of Dee Valley, Scotland: Effect of land cover and C:N. WaterResources Research 43. 2007.

Barnett C, Hossell J, Perry M, Procter C and Hughes G. A handbook of climate trends across Scotland. SNIFFER projectCC03, Scotland & Northern Ireland Forum for Environmental Research. 2006.

ECOSSE Estimating Carbon and Organic Soils Sequestration and Emissions. Scottish Executive. 2007.

Evans, CD, Monteith DT, Cooper DM. Long-term increases in surface water dissolved organic carbon: observations, possiblecauses and environmental impacts. Environmental Pollution, 137 (1) 55 -71. 2005.

Gosling, R. Changes in flow variability in snow-influenced catchments in Scotland SEPA Factsheet, 2009.

Worrall F, Harriman R, Evans CD, Watts CD, Adamson J, Neal C et al. Trends in dissolved organic carbon in UK rivers andlakes. Biogeochemistry 70 (3) 369 402. 2004.

Worrall F and Burt TP. Trends in DOC concentration in Great Britain. J. Hydrol, 346 81 92. 2007.

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Trends Organic Carbon