Riverine nitrogen and carbon exports from the Canadianlandmass to estuaries

Thomas A. Clair • Ian F. Dennis • Simon Belanger

Received: 3 August 2012 / Accepted: 27 January 2013

� Her Majesty the Queen in Right of Canada 2013

Abstract Dissolved total nitrogen (Nt) and total

organic carbon (TOC) exports were measured from 30

catchments and regions draining 76 % of the Canadian

landscape in order to estimate reactive N and organic

C runoff losses to estuaries and the conditions that

control them. N exports from the catchments were

lower than measured in most of Europe and the United

States due to significantly less agricultural activity

and atmospheric deposition, especially in northern

Canada. We produce statistical models using a number

of geographical, climatic, agricultural, and population

factors in order to predict N and C losses from the

remaining regions. Using measured and extrapolated

data, we estimated that the Canadian landscape

exports 884 and 18,210 ktons of Nt and OC per year.

Area normalized exports ranged from 29.4 kg km-2

for the northern Mackenzie River to 299 kg km-2 for

the semi-agricultural Saint John. Area normalized OC

exports ranged from 495 kg km-2 in the high Arctic to

7,295 to the wetland dominated Broadback River in

northern Quebec. N exports were best predicted by the

latitude of the catchment centroid, mean slope,

population density, runoff and % of the catchment as

agricultural land. The best model for predicting TOC

exports needed only slope and runoff. The Nt/OC ratio

in the rivers unsurprisingly was highest in the southern

portion of the country where anthropogenic activities

were concentrated.

Keywords Nitrogen � Carbon � Rivers �Exports � Arctic � Canada

Introduction

The increased use of fertilizers in agriculture, the need

for fossil fuels for transportation and power genera-

tion, as well as increases in waste products from

animal and human populations have caused major

shifts in the way nitrogen cycles globally (Galloway

et al. 2008). The changes in the N cycle are reflected in

atmospheric reactive nitrogen (Nr) increases which

cause the formation of smog and particulate matter

which affect human health, and as eutrophication of

terrestrial and aquatic ecosystems (Sutton et al. 2011).

One of the ways of better understanding the

movement and potential effect of Nr on the planet is

T. A. Clair (&)

Department of Earth Science, Dalhousie University,

Halifax, NS B3H 4R2, Canada

e-mail: t.clair@dal.ca

T. A. Clair

Environment Canada, Water Science and Technology

Directorate, 45 Alderney Dr, Dartmouth, NS B2Y 3N6,

Canada

I. F. Dennis

Environment Canada, Water Science and Technology

Directorate, 6226, Sackville, NB E4L 1G6, Canada

S. Belanger

Groupe BOREAS, Departement de Biologie, Chimie et

Geographie, Universite du Quebec a Rimouski, 300, allee

des Ursulines, Rimouski, QC G5L 3A1, Canada

123

Biogeochemistry

DOI 10.1007/s10533-013-9828-2

to trace its path using a budget approach. Large scale

global, as well as national, regional and ecosystem N

budgets have been constructed and reported in the

literature (e.g. Gruber and Galloway 2008) which

allow scientists and policy makers an appreciation of

the importance of various sources and ultimate fates of

N on the planet and to assess how best to reduce

leakages into the environment. These large scale

budgets in turn are dependent on in-depth sectoral

studies which describe N pathways and pools which

quantify interactions between agricultural, urban and

industrial activities to the terrestrial, atmospheric and

marine sub-components of the planet (Howarth et al.

2012).

An important component of Nr global budgets is its

transfer from terrestrial systems to oceans. Howarth

et al. (2006) described freshwater N export as a

function of total terrestrial ecosystem processes which

also included climate and deposition. They estimated

that an average of 20–25 % of anthropogenic N

deposition to landscapes was exported in rivers, with

the remaining 75–80 % retained in plants and soils or

denitrified and returned to the atmosphere as N2 or

N2O gases.

The present-day flux of nitrogen from large rivers in

North America and Europe to the North Atlantic was

calculated by Boyer et al. (2006a), Boyer and Howarth

(2008) to be 4.1 Tg year-1, which they estimated to be

a quadrupling of pre-industrial values. These increases

in riverine N exports have created important ramifi-

cations for estuaries causing eutrophication and

anoxia in heavily affected areas, not only in North

America and Europe, but also elsewhere on the planet

(Diaz and Rosenberg 2008; Swaney et al. 2012).

In recent years, more work has been done to improve

models designed to estimate terrestrial nitrogen (Boyer

et al. 2006b; Kroeze et al. 2012) and carbon (Schlunz

and Schneider 2000) losses to estuaries and to attempt

to develop models which link these fluxes to landscape

and ecosystem processes (Vorosmarty et al. 2000;

Schlesinger et al. 2006; Wollheim et al. 2008). This is

needed in order to better understand how to reduce the

influence of society in modifying the nitrogen cycle

which is causing increasing eutrophication in fresh

waters and estuaries, as well as the carbon cycle which

is the main driver of climate change. A consistent

theme in these papers (e.g. Howarth et al. 1996; Green

et al. 2004; Kroeze et al. 2012) is the need to obtain

better high quality, well documented river N export

data with which to test the models, especially from

more isolated, less polluted regions. Data from these

environments are important as they can provide an

estimate of background or pre-pollution conditions

which are necessary in order to understand what

controls anthropogenic contributions (Howarth et al.

2006; Kroeze et al. 2012).

Until recently, the amount of nitrogen data avail-

able from Canadian rivers was relatively scarce due to

the isolated nature of many of its northern rivers, as

well as due to the difficulty of accessing databases

scattered in government, hydropower companies and

research papers. As the northern regions of Canada

drain significant amounts of land and are relatively

unpolluted by nitrogen, we therefore felt it important

to assemble and interpret newly available data from

this region, as well as the more affected southern

regions in order better understand terrestrial-aquatic N

cycling interactions, as well as its contribution to

oceanic nitrogen budgets in the northern portion of

North America.

Another important element exported by rivers from

the landscape to estuaries is organic carbon (OC).

Approximately 430 9 1012 g of OC is lost globally

from terrestrial ecosystems to rivers and eventually

estuaries (Schlunz and Schneider 2000). Though this

transfer is small compared to direct atmospheric fluxes

between land and the atmosphere, it is nevertheless

important as it is modified by temperature and

hydrology (i.e. climate) as well as anthropogenic

activities such as land use. Much of the exported OC is

remineralized to CO2 in both lakes and estuaries, with

the remaining amounts precipitating in sediments

which affects their geochemical and ecosystem func-

tioning (Cole et al. 2007). The problems with data

availability and quality which we faced with Nr were

also reflected with OC. Clair et al. (1999) estimated a

freshwater OC budget for Canada, but they were

limited in their conclusions and accuracy by the

sparseness of the data available to them at the time. For

the earlier work, Clair et al. (1999) attempted to

incorporate N in their study of river exports, but there

was then not enough data available to produce even

rudimentary estimates.

For this study, we assembled river N and OC

concentration data which were collected mostly since

1990, from catchments draining the Canadian land-

scape and which empty into marine environments,

with the goal of quantifying N and C exports from the

Biogeochemistry

123

landscape to estuaries. With the measured estimates

available, we were then able to develop simple

statistical models which included geographical,

hydrological and land use characteristics which we

used to predict export estimates for regions where no

data existed. We then assumed that analysis of the

model inputs would allow us to identify the catchment

characteristics which were most important in explain-

ing the controls of N and C export in Canadian rivers.

We then combined the results from measured and

estimated catchments to produce estimates of total

freshwater N and C exports from the terrestrial to

marine ecosystems for all of Canada.

The relationship between dissolved N and C in

rivers should provide an indication of water pathway

and soil ecosystem conditions which prevail in

catchments. Higher N inputs generated from agricul-

tural or urban runoff or from precipitation in more

populated and agricultural areas should be reflected in

higher N/C ratios of dissolved elements (Khalili et al.

2010). We therefore tested for the hypothesis that the

ratios of exported N and C would be greater in rivers

draining the catchments in the southern portions of the

country where more N pollution occurs.

Study area

Canada occupies 10,216,590 km2 of the northern part

of North America (Fig. 1). Because of its great size,

geographical diversity and location, Canada has a

large number of ecosystem types, from Arctic Tundra

in the north, to wet, cool, temperate forests on both

coasts, as well as semi-arid regions in the Prairie

region located in the center of the country. The country

is ideally situated to allow a better understanding of N

exports from rivers and thus terrestrial N cycling, as it

contains a wide range of atmospheric deposition

amounts, population densities, as well as agricultural

and industrial activities.

The country’s large, northern watersheds receive

low N deposition and have little human activity, while

the large southern rivers drain the Great Lakes region

and southern British Columbia catchments which are

influenced by relatively high population densities and

significant agricultural activity. All Canadian rivers

studied in this work eventually discharge into either

the Atlantic, Arctic or Pacific Oceans. Only one river,

the Milk, located in southern Alberta eventually

discharges into the Mississippi drainage. However,

as this river originates in the US state of Montana and

eventually returns there after traversing a small part of

Canada, we decided not to include it in our Canadian

estimate.

N deposition in Canada is highest in the south

eastern part of the country, with values exceeding

1,000 kgN km-2year-1 in the southern Ontario and

Quebec region, much of which originates in the US

Midwest and Eastern Seaboard (Vet et al. 2005). There

is also a region of higher N deposition in the petroleum

industry rich central Alberta (*300 kgN km-2year-1,

southern portion of #2). The far northern portion of

Canada receives\100 kgN km-2year-1 (Moran et al.

2008). There is intensive agriculture in southern

Ontario and Quebec, the southern portions of the

Prairies Provinces (Manitoba, Saskatchewan and

Alberta, catchment #30), and in the lower Fraser Valley

near Vancouver on the west coast.

Methods

Data acquisition and export calculations

Overall, the catchments for which we had water

chemistry values drained approximately 7,868,251

km2 or almost 76 % of the Canadian landmass.

Aquatic chemistry monitoring has been done on a

regular basis for most of the large southern rivers in

Canada, with water sampling usually co-located at

gauge sites allowing easy calculation of N and C

exports. We identified 30 catchments or regions where

water chemistry data were collected by Environment

Canada and Provincial government partners, as well as

university and industry collaborators over varying

periods of time (Fig. 1; Table 1).

Due to the isolated nature of many of northern

catchments and the reduced information demands for

ecosystem management or protection, sampling in

northern Canada is spotty and what little information

exists can be difficult to access. An important source of

water chemistry data we used originated with hydro

power generation companies who maintain facilities in

the Boreal Forest or have generated data in the course of

fulfilling environmental impacts evaluations. Another

source came from individual university researchers who

conducted studies in the region. Most of the data we used

were collected at river mouths above estuarine effects,

Biogeochemistry

123

though the Churchill and Nelson Rivers in Manitoba

(Fig. 1; Map #’s 21 and 30) were sampled at power dam

sites several hundred km from Hudson Bay, though

these sites nevertheless included more than 90 % of the

catchments’ surface areas (Fig. 1).

We only had rudimentary water chemistry and

runoff estimates for the Arctic Archipelago (Fig. 1;

#3) as the only water chemistry information available

was collected in the ice-free season in 2007 and 2008,

at 66 Arctic streams and rivers which drained catch-

ments ranging in size from 0.6 to 7,434 km2

(Dr. J. Culp, Environment Canada, pers. Com.). As

there are no gauged rivers in this large region, we used

modeled runoff values from Spence and Burke (2008)

for the Canadian far north which we combined with

the available concentration data to provide estimates

of N and C exports.

As the Arctic archipelago estimate only used

summer water chemistry values collected in a large

range of catchment sizes, it did not take into account

the seasonality inherent in the snow and ice melt

period in late spring and early summer (Thomas et al.

2011), so it is difficult to accurately evaluate annual

export values or the potential error in the estimate.

However, as will be seen below, the range of TN and

TOC values measured was relatively narrow and we

think that the estimate is useful as a first approximation

to situate potential values for the far north.

Table 1 shows values from the catchments charac-

teristics where exports were measured from water

chemistry data, as well as the number of samples to

generate N and C exports. Because of irregularity in

sampling frequencies between studies, we used a basic

method for estimating annual elemental exports.

Fig. 1 Location of Canada and its major catchments. Light colored areas are where water chemistry data existed and darker one where

exports were estimated. Numbers for the measured catchments are related to Table 1 where names and other characteristics are listed

Biogeochemistry

123

Where both water chemistry and discharge were

available, mean monthly N and C exports were

calculated for the period of record available by

multiplying concentration with mean flow for each

month of the record and monthly values were averaged

over the record period. Monthly averaged data were

then summed for each catchment to produce an annual

export estimate. Sampling on the larger rivers

(St Lawrence, Fraser, Columbia, Mackenzie) and the

smaller ones in the southern part of the country was

usually done monthly or at least seasonally as can be

seen by the sampling frequency (Table 1), so that the

export estimates we present usually take into account

the seasonality of river flows.

There were a number of catchments for which data

quality was not as high as hoped for. The Saint John

Table 1 Characteristics and locations of river catchments with measured values

Map # River Drainag. Area

(km2)

Mean lat.

�N

Pop.

# (km2)

Agric.

(% area)

Mean slope

(m km-2)

Runoff

(m)

Years

sampled

Number of

samples

23 Churchill Atl 80,924 53.4 0.1 0.2 0.46 0.621 1990–2007 64

48 Labrador1 Atl 167,531 54.8 0.1 0 1.88 0.622 2001–2008 93

49 Maritimes2 Atl 105,506 46.4 11.8 12.5 1.11 0.75 1990–2008 617

47 Newfoundland3 Atl 108,900 48.7 4.5 0.4 1.12 1.045 1990–2008 209

45 Saint John Atl 56,735 46.6 7.2 9.2 1.18 0.625 2001–2010 34

33 St Lawrence StL 1,113,128 45.7 32.7 22 0.64 0.384 2000–2007 154

77 Betsiamites StL 18,831 49.8 2.2 0.8 0.95 0.631 2010–2012 12

53 Natashquan StL 15,682 51.8 0.2 0.2 1.01 0.765 2010–2012 13

60 Moisie StL 18,770 51.5 0.3 0.7 1.3 0.689 2010–2012 10

56 Romaine StL 14,187 51.7 0.2 1.7 1.76 0.778 2000–2005 7

57 Outardes StL 18,841 50.9 1 0.9 0.85 0.657 2010–2012 13

78 Manicouagan StL 45,322 51.5 0.3 0.7 1.3 0.689 2010–2012 14

21 Churchill H B 284,505 56.3 0.3 6.1 0.31 0.144 2000–2009 41

30 Nelson H B 1,124,592 51.4 4.4 52 0.5 0.112 2001–2009 132

31 Hayes H B 105,440 54.9 0.2 0.2 0.14 0.189 2008–2009 6

26 Gr. Baleine H B 38,061 55 0 0 0.3 0.506 1989–1990 6

38 Broadback H B 20,874 51.3 0 0.1 0.42 0.564 1977–1979 14

29 La Grande H B 94,623 53.7 0 0 0.39 0.526 1974–1978 36

41 Rupert H B 42,367 50.8 0 0.1 0.37 0.494 1977–1978 9

16 Koksoak H B 130,212 55.7 0 0 0.61 0.566 1980–1981 19

37 Eastmain H B 44,860 52.4 0 0.1 0.4 0.517 1978–1979 23

42 Nottaway H B 66,434 49.5 0.1 1.6 0.33 0.539 1977–1979 11

44 Harricanaw H B 36,101 49.7 0.1 2.9 0.27 0.418 1991 6

2 Mackenzie Arc 1,703,303 61.1 0.3 2.9 1.86 0.203 2000–2009 47

1 Yukon Pac 255,374 62.3 0.1 0 4.71 0.255 1986–1994 181

19 Fraser Pac 226,948 52.4 5.2 0.9 5.4 0.512 1991–2006 405

36 Columbia Pac 101,908 50.3 3.6 1.2 9.16 0.888 1990–2006 851

15 Skeena Pac 50,797 55.2 0.4 0.2 7 0.99 1990–2006 431

13 Stikine Pac 54,294 57.4 0.3 0.2 7.42 0.729 1982–1994 76

51 Porcupine Pac 58,013 67.1 0.1 0 2.16 0.194 1993–2001 165

3 Arctic islands Arc 1,665,185 72.3 [0.01 0 1.9 0.3 2007–2008 66

Map # relates to Fig. 1. Mean latitude is estimated from the catchment centroid point, population, % agricultural area are estimated

from the Canadian Atlas (NRCan 20xx). St Law refers to the Gulf of St Lawrence which includes a number of rivers draining into the

Gulf, HB is Hudson Bay.The Arctic region is roughly north of the 63rd parallel

Biogeochemistry

123

River for example, had only 34 samples over an

11 year period collected near the river mouth, and

runoff had to be estimated from regional runoff maps

because the closest gauge site was 100 km upriver and

only took in only 75 % of the catchment. Many of the

rivers draining into Hudson Bay (Grande Baleine,

Broadback, Rupert, Koksoak, Eastmain, Nottaway)

were sampled monthly in the late 1970s or early 1980s

for only 1 year. This frequency did not allow for any

estimation of inter-annual variability though they were

sampled over a complete seasonal cycle.

Sample analytical methods

For quality control reasons, we preferred using data

collected since the year 2000, but where only earlier

data was available, such as in northern Quebec, these

were nevertheless used to provide spatial coverage

(see Table 1 for sampling dates). The total nitrogen

(Nt) values found were almost always measured using

persulphate/UV digestion or similar wet oxidation

methods. Because of this consistency between the

various sampling programs, the discussion will be

focused on this parameter as we have the greatest

confidence in its comparability throughout Canada.

NO3- and NH4

? are usually measured in Canadian

freshwaters, though analytical methods and detection

limits have changed over time. Since 1990, most

NO3- and NH4

? were measured using ion chroma-

tography, while pre-1990 samples were mostly done

using colorimetric methods. In waters draining non-

urban and/or non-agricultural catchments, NH4? and

NO3- are usually very low and often at undetectable

levels which we report as ‘no data’ (ND). Though we

show inorganic N mean concentrations where they are

available to give a relative idea of its importance, we

only calculated Nt exports with the understanding that

they include both inorganic and organic N.

Dissolved organic or total organic carbon (DOC,

TOC) analytical approaches suffer from many of the

same issues as for N. Samples analyzed before the

mid-1990s were usually analyzed using wet oxidation

methods which were later superseded by high tem-

perature combustion methods which provided results

approximately 25 % higher than wet oxidation

(Koprivnjak et al. 1995). However, as we mostly kept

to post-2000 data (except for northern Quebec), this

issue is not an important factor in our overall analysis.

Organic nitrogen and carbon analyses are usually

reported either as dissolved from filtered samples or as

total from unfiltered. Though water samples collected

from urban or agricultural areas are usually filtered to

remove particulate matter, samples from remote or

undisturbed catchments rarely need to be, so that both

total and dissolved values are commonly found in

databases. We assume that sample collection and

processing by individual programs or researchers was

suited to local conditions and will assume that they

provide equivalent results. We therefore report all

organic N and C data as total nitrogen (Nt) and total

organic carbon (TOC), even though not all samples

were unfiltered.

Spatial analysis and regression models

Data sources for spatial analyses of population,

agriculture, elevation came in a variety of formats

including image, raster and vector types. All were

converted into 1 km raster data and analyzed using

ESRI Zonal statistics tool with a drainage basin vector

layer. Centroid values were calculated for each

drainage basin polygon using ETGeoWizards Tools

in conjunction with ESRI ArcView. Data sources for

GIS analysis are listed in ‘‘Appendix’’.

Nitrogen deposition measurements have not been

done for the whole of Canada, though eastern Canada

has been well covered by the Canadian Air and

Precipitation Monitoring Network (CAPMoN)

(Vet et al. 2005). The only estimates of N deposition

for the whole country are modeled using the ‘‘A Unified

Regional Air-quality Modeling System’’ (AURAMS)

(Moran et al. 2008). A shortcoming of this model, from

the point of view of our study, is that it only predicts total

N deposition to approximately 60�N, so that it is not

possible to quantitatively compare N inputs to outputs

for a large portion of the country. We therefore use the

information from the AURAMS output in our discus-

sion of results, but did not use it in statistical model

development.

In order to fill catchment export calculation gaps for

the 24 % of the country which wasn’t covered by

sampling, we explored a number of linear and non-

linear statistical models using catchment centroid

latitude (a surrogate for anthropogenic influences) and

longitude, runoff, percent area under agriculture,

population and catchment sizes and mean slopes as

Biogeochemistry

123

input variables to develop predictive equations which

could then be used to estimate export values for the

catchments where no data existed.

Results and discussion

Nt exports

Nitrogen concentrations in all Canadian rivers we

studied were relatively low (Table 2; Fig. 2) compared

to more polluted water courses such as the Mississippi

(1,500 to more than 3,100 kg N km2year-1 in the

upper portions of the catchment, Goolsby & Battaglin

2001) or rivers draining the western European land-

mass ([1,000 kg N km2year-1; Billen et al. 2011).

Moreover unlike from more heavily populated regions,

organic nitrogen dominated Nt concentrations at all of

our rivers (Table 2), a phenomenon also noted by Scott

et al. (2007) for a number of rivers in the US. The

highest mean Nt concentrations in the study were from

the Nelson River (0.76 mg l-1) and the St Lawrence

(0.65 mg l-1), followed by the Hayes (0.49 mg l-1),

Harricanaw (0.49 mg l-1) and Saint John (0.41 mg l-1)

Rivers (Table 2). The Nelson, St Lawrence and Saint

John all drain agricultural regions, while the Hayes and

Harricanaw have extensive wetlands within their

drainages.

The largest catchment, the St Lawrence River

(Fig. 2; Map # 33) exported 235.6 kt year-1 Nt,

3.59 as much as the next highest river, the Nelson

which empties into Hudson Bay (Table 2; Fig. 2). As

the St Lawrence catchment contains the largest

population centers in Canada and a number of

significant ones on the US side of the Great Lakes,

as well as a great deal of agriculture in both countries,

this finding is not surprising.

The mean area normalized export value for the St

Lawrence (211 kg km-2year-1) was low compared to

estimates from eastern US rivers (Boyer et al. 2006a;

Goolsby & Battaglin 2001; Howarth et al. 2006) and

Europe (Billen et al. 2011), especially considering the

concentration of population, industry and agriculture

in this watershed. Though the heavily populated and

farmed portion of the St Lawrence catchment can

receive [1,000 kg km-2year-1 N deposition plus

agricultural and urban runoff, the undeveloped and

forested western and northern portions only receive

*200 kg km-2year-1 N (Moran et al. 2008), much of

which is either taken up by catchment plants or

denitrified in wet soils and lake sediments.

Our measured St Lawrence export value is

1/3 of that estimated by Howarth et al. (1996)

(660 kt year-1) and 70 % of that modeled by Boyer

et al. (2006a) (340 kt year-1). Howarth et al. (1996)

estimates were derived from a 1978 report which

suffered from the inadequate data which was available

at the time and we feel that our database is more

trustworthy than the earlier unpublished work. The

Boyer et al. (2006a) value is modeled, which we

suspect underestimates denitrification in Great Lake

sediments and littoral wetland areas. Moreover, bio-

logical uptake of N by algae with subsequent precip-

itation into sediments of the Great Lakes may be other

reasons for the lower than expected area normalized

exports from this watershed.

The reason for the large differences between our

data and the other two studies are not likely due to

improvements in sewage treatment as Holeton et al.

(2011) show a slight increase in N discharges from

Canadian sewage treatment plants in the last decade.

Legislated changes in both Canada and the US have

managed to reduce atmospheric reactive N emissions

to the Great Lakes region (Zbieranowski and Aherne

2011), but those small reductions cannot be the source

of the large discrepancies between our values and

previous ones.

The second highest N exporting river in Canada is

the Nelson (Fig. 2; Map # 30), which produces

66 kt year-1 though it’s N export rate of 58.4 kg

km-2year-1 is 27 % of the St Lawrence’s (Fig. 2;

Table 2) and much lower than the values reported for

the upper Mississippi watersheds which abut on this

drainage (Goolsby and Battaglin 2001). The Nelson

drains a large portion of Canada’s Prairie Provinces

which are the source of much of Canada’s grain

growing and animal husbandry, as well as portions of

North Dakota and Minnesota in the USA. Though N

deposition in this catchment is low, (between 200–400

kg km-2year-1 N deposition) we feel that the intense

agricultural activity in the southern half should never-

theless cause larger N normalized export rates.

The Mackenzie River (Fig. 2; #2) has the largest

catchment of any Canadian river and exports the third

largest N amount (50 kt year-1) from the landscape

into the Arctic Ocean. Much of the oil and gas

production and as well as associated petrochemical

industries in Canada are found in the catchment’s

Biogeochemistry

123

upstream portion, locally causing Nr deposition

amounts of up to 340 kg N km-2year-1 (Moran

et al. 2008). The deposition levels in the south are

not translated into high N exports at its Arctic Ocean

estuary however, as the catchment shows the lowest

area normalized export value (29 kg km-2year-1) of

any other drainage area measured, even lower than the

Arctic Archipelago (39.4 kg km2 year-1) which we

found surprising. Our export estimate is lower than the

60 kt year-1 reported by Holmes et al. (2012), though

the difference is not great considering the difficulties

and inaccuracies involved in such calculations. Our

study used a similar sampling frequency to theirs

(*4.2X year-1), though we had data from a longer

sampling period (10 vs 4 years).

One consistent factor which seems to be affecting

export rates from the St Lawrence, Nelson and

Mackenzie catchments seems to be the role of

denitrification. Though we have not quantified N

loadings into these catchments, it is evident that a

Table 2 Mean NH4–N, NO3–N and Nt concentrations, area normalized measured N and C exports and Nt and TOC exports from the

measured catchments

River Drain NH4–N

(mg l-1)

NO3–N

(mg l-1)

Nt

(mg l-1)

TOC

(mg l-1)

N area exp

(kg km-2year-1)

C area exp

(kg km-2year-1)

N total

103 (t)

C total

103 (t)

Churchill Atl ND 0.04 0.18 3.7 103.8 2,200.8 8.4 178.1

Labrador Atl ND 0.02 0.19 5 133.1 3,463.4 22.3 580.2

Maritimes Atl ND 0.13 0.19 3.2 226 3,759.9 23.8 396.7

Newfoundland Atl ND 0.09 0.26 5.7 265.2 5,745.0 28.9 625.6

Saint John Atl 0.02 0.19 0.41 7.6 299.6 5,539.8 17 314.3

St Lawrence StL 0.04 0.06 0.65 3.7 211.8 1,202.1 235.7 1,338.1

Betsiamites StL ND 0.05 0.22 7 122.1 3,928.7 2.3 74

Natashquan StL 0.01 0.02 0.15 7.1 127.4 5,875.5 2 92.1

Moisie StL 0.01 0.05 0.21 7.1 121.9 4,182.7 2.3 78.5

Romaine Stl 0.09 0.04 0.21 4.1 140.2 2,699.2 2 38.3

Outardes StL ND 0.05 0.21 6.8 143.8 4,706.5 2.7 88.7

Manicouagan StL ND 0.07 0.21 5.7 145.3 3,935.8 6.6 178.4

Churchill HB 0.01 0.02 0.31 6.6 37.8 803.4 10.8 228.6

Nelson HB 0.05 0.06 0.76 7.6 58.4 584.9 65.7 657.8

Hayes HB 0.02 0 0.49 10.2 60.9 1,277.6 6.4 134.7

Gr. Baleine HB 0.01 0 0.16 3.9 64.2 1,543.2 2.4 58.7

Broadback HB 0.01 0.03 0.13 12.5 74.3 7,394.9 1.6 154.4

La Grande HB 0 0.02 0.1 6.2 62.7 3,802.7 5.9 359.8

Rupert HB 0.03 0.08 0.1 7.9 70.5 5,551.3 3 235.2

Koksoak HB ND 0.01 0.11 4.2 112.8 4,323.4 14.7 563

Eastmain HB 0.01 0.02 0.14 7.4 107.4 5,762.3 4.8 258.5

Nottaway HB 0.02 0.08 0.19 14.6 108.1 8,223 7.2 546.3

Harricanaw HB 0.02 0.13 0.49 13.3 229.5 6,237.8 8.3 225.2

Mackenzie Arc 0.03 0.06 0.16 7.6 29.4 1,384.5 50.1 2,358.2

Yukon Pac ND 0.07 0.19 6.4 52.6 1,812.4 13.4 462.8

Fraser Pac 0.02 0.09 0.25 3.5 92.6 1,284.5 21 291.5

Columbia Pac ND 0.11 0.17 1.4 96.7 771.7 9.8 78.6

Skeena Pac ND 0.06 0.16 2.8 109.3 2,094.7 5.6 106.4

Stikine Pac ND 0.07 0.12 2.8 120.8 2,838.2 6.6 154.1

Porcupine Pac ND 0.04 0.29 9.7 50.3 1,695.3 2.9 98.4

Arctic islands Arc ND ND 0.19 1.8 39 495 64 824.3

ND is for no data or below detection limits

Biogeochemistry

123

densely populated and agriculturally intense catch-

ment such as the Great Lakes should be exporting

higher amounts than are measured at its outlet. The

heavily agricultural Nelson catchment should also be

exporting more than 58.4 kg km2 year-1. We would

also expect that the Mackenzie catchment which has

major petrochemical industries in its southern portion

would generate more N per unit area than the high

Arctic region.

We suspect denitrification has a major influence

reducing N exports from these catchments for a

number of reasons which have been discussed by

Boyer et al. (2006b); Seitzinger et al. (2006); Behrendt

& Opitz (2000). These studies suggests that denitri-

fication is enhanced with increased residence time of

waters in lakes and rivers due to a number of factors

related to hydrology and watercourse physical char-

acteristics. Residence time of the Great Lakes, the

largest freshwater system in the world allows much

greater opportunities for contact with sediments and

anaerobic conditions which can lead to denitrification.

Much of the Nelson River flow passes through Lake

Winnipeg. This lake is 24,514 km2 in area with a

shallow mean depth of *12 m and has been identified

as major source of N loss in the catchment due to

probable denitrification and loss to sediments (Scott

et al. 2011).

The Mackenzie River also contains large lakes in its

drainage, and Emmerton et al. (2007) show that over

11,200 km2 of the catchment is composed of flooded

vegetation surfaces in the spring flood period which

should provide good conditions for denitrification

processes. Moreover, the location of the industrial

areas are near the headwaters of the Mackenzie, so that

waters receiving the greater N deposition have a

higher residence time in the catchment, leading to

Fig. 2 Measured and estimated Nt export rates from the Canadian landscape

Biogeochemistry

123

greater denitrification opportunities. The physical and

hydrological characteristics of these large catchments

therefore clearly point to the potential of denitrifica-

tion being an important player in N dynamics and

show a need for further research.

Our data that the Yukon River exports 52.6

kg N km-2year-1 in its Canadian portion, which is

lower than the value of 81 kgN km-2year-1 reported

by Holmes et al. (2012) for the whole catchment. A

number of reasons may be responsible for the

discrepancy. First, the Canadian upstream portion of

the Yukon catchment occupies 38 % of the total

catchment and receives less precipitation and gener-

ates lower runoff than the downstream USA portion.

Moreover, our sampling frequency was semi-monthly

for eight years, while the Holmes et al. (2012)

information was collected seasonally for only 3 years.

So the difference in estimates may be due a combi-

nation of difference in data quality and climatic

conditions.

The Saint John River catchment in the Maritime

Provinces of eastern Canada (Map #45) had the highest

unit area Nt export of any Canadian river studied at

300 kg km-2year-1. This river receives [200 kgN

km-2year-1 in deposition (Moran et al. 2008) and is

almost 10 % agricultural land (potato cultivation in

large part) which contributes N runoff to the catch-

ment. This export value compares well with rivers from

the nearby US state of Maine where Howarth et al.

(2006) reported values between 320–400 KgN

km-2year-1, while Cronan (2012) showed normalized

export values near 200 kgN km-2year-1. These two

USA estimates, though somewhat different, at least

provide us with some certitude that our data are not

unreasonable.

The next highest normalized export values are from

the Maritime Provinces (Map #49, N deposition

*90 kgN km-2year-1) and the Island of Newfound-

land (NF) (Map # 70, N deposition *50 kgN km-2

year-1) both regions emptying into the Atlantic

Ocean. The Maritime catchments are small, near

higher sources of N emissions in the US and Canada

and contain agriculture which explain the higher Nt

export. Newfoundland catchments on the other hand,

receive low N deposition amounts, being further

downwind of eastern North American emission

sources and support little agriculture. These high

export values are affected by the contribution of

organic matter from wetlands which are controlled by

terrain conditions and high runoff amounts (Clair et al.

1994).

The rivers of Labrador which drain into the Atlantic

Ocean (Map #48) and those from northern Quebec

which drain into southern Hudson Bay and the Gulf of

St Lawrence have N export rates as high as for the St

Lawrence despite having much lower atmospheric

deposition and low population or agricultural pres-

sures. However, they are also relatively flat, short and

contain high wetland amounts which then produce

organic N which is exported. The very high N export

into Hudson Bay by the Harricanaw River (Fig. 2; #68)

is also most likely explained by the high prevalence of

peatlands in its catchment (Barnett et al. 2011).

On the west coast of Canada, the Fraser River

which empties into the Pacific Ocean (Map #19) and

the Columbia which drains into Washington State of

the US (Map #36) are influenced by agricultural

activities, but because of the large portion of their

catchments which are in wilderness areas with low N

deposition (up to 150 kgN km-2year-1; Table 1), Nt

export rates are low at 72 and 97 kgN km-2year-1

(Table 2). A number of other coastal catchments

export high N amounts mostly because of high runoff

in these coastal regions. Not surprisingly, these Nt

export values are between 50 to 100 % lower than

those reported by Schaefer et al. (2009) for the US

portion of the west coast which receive greater

atmospheric and land use inputs.

The Arctic Archipelago stream Nt concentrations

were low (inorganic N was undetectable using stan-

dard EC methods), with a mean value from 66 samples

of 0.19 mg l-1 Nt (0.86 max, 0.03 min, 0.13 median).

As the average annual runoff for the Arctic Archipelago

was estimated at 0.3 m (Spence and Burke 2008), we

calculated normalized export for this region to be

approximately 39 kg km-2year-1. As our estimate

does not take into account the seasonality inherent in

this region’s snow and ice melt period in late spring

and early summer (Thomas et al. 2011), it is difficult to

accurately evaluate annual export values, but this

estimate can be used as a first approximation to situate

potential N export values for the far north of Canada.

Our Arctic value was 20 % of the model prediction

of Green et al. (2004) who suggested that current Nt

exports from polar regions should be in the range of

200 kgN km-2year-1, a value which seems to be

mostly dependent on theoretical N fixation estimates.

Our data show that this assumption needs to be

Biogeochemistry

123

adjusted to better reflect the reality of N fixation by the

impoverished lichen fauna in this region.

TOC exports

Measured TOC export rates ranged from 8,223

kg km-2year-1 in the Nottaway River (Map # 42;

Table 2), to a low of 1,812 kg km-2year-1 from the

Yukon River (Map # 1), a dry, mountainous region

which drains into Alaska and eventually into the

Pacific Ocean (Fig. 3). These values all fall within the

range of observations assembled by Alvarez-Cobelas

et al. (2012) from 550 catchments located across the

globe.

Mean annual TOC concentrations were highest

from four rivers draining into Hudson Bay (Table 2).

As mentioned above, this region is characterized

by very shallow catchment slopes which generate

extensive wetlands. The high concentrations lead to the

highest area normalized TOC export rates, especially

in the southern portion of Hudson Bay. Interestingly,

the more northern and western HB catchments

(Nelson, Churchill and Hayes) which produce lower

runoff, export some of the lowest TOC amounts.

The relatively high TOC exports from catchments

located in the eastern and southern shores of Hudson

Bay and were compared to TOC export rates estimated

by Rosa et al. (2012) as part of a study on regional

cation weathering potential. For the seven catchments

our study had in common with theirs (Grande Baleine,

Broadback, La Grande, Rupert, Koksoak, Harricanaw,

Nelson), differences in export values ranged between

60 and 380 %. However, their values were only

estimated from a spring and autumn sampling for

1 year, so that their sampling regime was not adequate

for dealing with these highly seasonal systems.

Fig. 3 Measured and estimated TOC export rates from the Canadian landscape

Biogeochemistry

123

Our TOC export estimate for the Yukon is similar to

that of Holmes et al. (2012) (1,812 vs. 1,771 kg km-2

year-1) and Raymond et al. (2007; 1,770 kg km-2

year-1) but somewhat higher than Striegl et al.

(2007, 1,388 kg km-2year-1). Our Mackenzie value

(1,384.5 kg km-2year-1) is very similar to that of

Raymond et al. (2007, 1,400 kg km-2year-1) but

considerably higher than the Holmes et al. (2012)

value of 820 kg km-2year-1. All of these studies

report on data collected seasonally, though ours used

data collected over a 10 year period compared to two

and four years for theirs. We suspect that our data set

was able to take in a larger range of conditions which

included more of the natural variability which could be

expected over this large area. Striegl et al. (2007) also

estimated an export rate of 1,623 which was close to

ours (1,695 kg km-2year-1) for the Porcupine River

which drains into the Yukon.

Our study’s mean TOC concentration for the

St Lawrence River was 3.7 mg l-1 for data collected

between 2000 and 2007 which was close to that of

Pocklington & Tan (1987) who estimated a value of

3.8 mg l-1 DOC with a 3–14 % addition for partic-

ulate C from samples collected at our site from 1981 to

1985. Though our sampling frequency for the Saint

John River catchment was not optimal, our TOC

export estimate of 5,540 kg km-2year-1 was near to

that from the nearby Penobscot River in Maine

(Cronan 2012) which has a similar distribution of

forestry-agricultural usage (5,830 kg km-2year-1).

We applied the same approach as for Nt, to estimate

TOC exports from the Arctic Archipelago region using

measured stream chemistry data and estimated runoff.

The mean TOC value measured in the Arctic Archipelago

from 66 stream samples was 1.8 mg l-1 (5.8 max, 0.3

min, 1.7 median). Multiplying the mean value with the

estimated runoff of 0.3 m for this region provided an

annual export estimate of 495 kg km-2year-1 which

we then applied to all catchments north of the 63rd

latitude.

Estimating Nt and TOC exports for all of Canada

In order to estimate exports for the 24 % of the country

for which we had no data we used a simplified,

statistical approach which only needed rudimentary

data available from geographical and hydrological

databases. We considered using the more sophisticated

‘‘net anthropogenic nitrogen inputs’’ (NANI) approach

described by Howarth et al. (2012), however as all the

unsampled catchments were in the northern portion of

the Canadian mainland, there was little agricultural or

other human activity and there was no atmospheric

deposition data available, so we felt that the approach

was not suitable for the conditions we dealt with.

Based in part on the observation that Nt exports

were roughly highest in the south and in wetter regions

of the coasts (Fig. 4 mid and bottom), we developed a

Fig. 4 Relationship between Nt export and size (top), latitude

(mid) and runoff (bottom) of study catchments

Biogeochemistry

123

statistical model using geographical parameters

(catchment centroid latitudes and longitudes, surface

area, mean slope) as well as hydrology and population

and agricultural intensities. We attempted linear and

non-linear regression approaches but found that the

best approach was the use of a simple multilinear

regression which explained 58 % of the variability:

Nt exp kg km�2y�1� �

¼ 259� 3:9 latð Þ þ 2:5 popð Þþ 0:02 % agricð Þ� 5:7 slopeð Þ þ 133 roð Þ r2

¼ 0:58; n ¼ 30; p\0:001

ð1Þ

where latitude (lat) is the catchment centroid value,

population (pop) is in average people km-2, %agric is

the percent of the catchment under agriculture, slope is

the catchment median slope in m km-1, and runoff

(ro) is in meters. Latitude in this case is not indicative

of any particular process or group of processes, but is a

surrogate for a combination of anthropogenic activi-

ties causing high Nr emissions and nutrient runoff.

Runoff is an indication of water flow through the

system and in smaller systems has a positive influence

on N export which has also been noted by Lewis et al.

(1999).

The negative sign on the catchment slope factor

suggests flatter catchments have reduced exports more

than could be expected, which confirms the concept

that denitrification is greater in low slope conditions

than in steeper regions. The Maritimes, Newfoundland

and Harricanaw regions, are outliers to this trend due

to high wetland contributions of organic N. The

interactions between competing catchment and hydro-

logical factors fit in well with the synthesis produced

by Seitzinger et al. (2006) in describing the factors

controlling N dynamics in aquatic ecosystems.

When we assessed the TOC data from our measured

catchments using the same approach as for Nt, we

found that our best export prediction came from an

equation which only took into account runoff and

catchment slope:

TOC Export kgkm�2y�1� �

¼ 1706� 507:6 slopeð Þþ 4865 roð Þr2

¼ 0:42; n ¼ 30; p\0:001

ð2Þ

where slope is in m km-1, and runoff (ro) is in

m year-1. The regression which could only explain

42 % of the variability could not be significantly

improved by the addition of other variables or by log-

normalizing the data, and is probably the best outcome

that could be achieved, as Alvarez-Cobelas et al.

(2012) concluded that there could not be a single

approach for developing OC export models due to the

wide range of local conditions which can affect the

export outcome. Our result is similar to that of

Lauerwald et al. (2012) who also included land cover

and wetland factors in their models, though our

correlation coefficients were worse than theirs (0.42

this study, 0.55 and 0.60 for their regression from

small and large catchments), most likely because we

had fewer catchments in our study (30 vs their 246

small and 207 large catchments) as well as fewer sites

with very low TOC exports.

Clair et al. (1994) have shown that flat landscapes

correspond well with the occurrence of wetlands and

thus high TOC exports in Canada. Under relatively

high precipitation and suitable temperature conditions

for the formation and subsequent senescence of

mosses and other wetland species, more water flow

will cause greater export of decayed organic plant

matter into receiving water courses. On the other hand,

catchments with low runoff, such as the Yukon,

Nelson, Mackenzie, Fraser and Columbia (Table 1)

had considerably lower TOC exports, due to more

oxidation of soil plant matter under drier conditions.

We combined the calculated export values with the

measured ones to produce estimates of N and C

riverine exports from the whole of the Canadian

landmass (Table 3). Our combined measured and

estimated results (Table 3) suggest that the Canadian

landscape exports 884.1 kton Nt per year, most of it in

organic form, with 65 % of this amount being captured

by our data sources and the remaining 35 % being

estimated.

Compared to the N budgets done by Howarth et al.

(1996) and Boyer et al. (2006a), our total value

(Table 3) which includes exports from the Arctic

Ocean, Hudson Bay, the St Lawrence River, as well as

the Atlantic coast of Canada (801 kton year-1) is

lower than their estimate of 960 kton year-1 for the

east coast of Canada. The most important difference

comes from our Gulf of St Lawrence-Atlantic coast

contribution which is lower than their estimate. This is

discussed above and the evidence seems to support the

fact that the main cause for the change is improved data

quality especially with the St Lawrence River exports.

Biogeochemistry

123

Our TOC export value for all of Canada (18,210

kton year-1) was compared to Clair et al. (1999)

estimate of 14,250 kton year-1 though the earlier value

did not include the Arctic Archipelago. The difference

between the two (3,960 kton year-1) is due to the

addition of the archipelago data, so that we feel

confident that the newer value is relatively accurate.

Nitrogen/carbon relationships

We hypothesized that the N/C ratio of the exported

organic matter would decrease with increases in latitude

as atmospheric deposition and runoff from agriculture

and urban influences would also decrease as shown by

Khalili et al. (2010). Our data supported this hypothesis

(Fig. 5a) and pointed out another interesting phenom-

enon. The highest N/C ratios were from the Columbia

(0.13), the Nelson (0.1) and the St Lawrence (0.09) with

the next highest being Maritime Province catchments

(0.06), the Fraser (0.055) and the St John (0.054), all of

which have significant agricultural activity. The catch-

ments with the lowest N/C values on the other hand, are

all heavily influenced by wetlands. We plotted %

agricultural area against N/C ratios (Fig. 5b) and

estimated a relationship described by a linear equation:

N=C ratio ¼ 0:0373þ 0:0014 % agric:ð Þ r2 ¼ 0:33; p¼ 0:001

ð3Þ

The relationship only predicts 33 % of the vari-

ability in the data, but is nevertheless highly signif-

icant. The higher N/C ratio in agricultural catchments

is not surprising and is due to leakage from fertilizer

use or manure storage piles which are more likely to be

leached into catchment streams. Atmospheric deposi-

tion seems less important to the N/C ratio as it will be

more evenly distributed across the landscape and thus

more likely to be incorporated into plant and soil

matter. The poor predictability of this equation is not

surprising as a number of factors will influence the

N/C ratio at the point of sampling, including the

presence of wetlands in the catchment (Gergel et al.

1999), as well as the location of the agricultural land in

relation to the water sampling site.

Conclusions

In this study, we produced river N and C export data

from pristine as well as from polluted catchments

Table 3 Summary of measured and estimated carbon and

nitrogen exports from the major Canadian regions

Surface area

(km2)

Nt export

103 (t)

TOC export

103 (t)

Measured

Atlantic Ocean 414,089 76.6 1,698.3

St Lawrence Gulf 1,244,762 253.6 1,888.1

Hudson Bay 1,988,071 130.7 3,422.1

Arctic 3,368,488 131.4 1,667.4

Pacific Ocean 747,334 59.3 1,191.8

Total measured 7,762,744 570.3 10,558.5

Estimated

Atlantic Ocean 145,888 0 0

St Lawrence Gulf 206,335 8.9 153.0

Hudson Bay 1,889,060 291.6 7,123.9

Pacific Ocean 212,564 13.2 374.7

Total estimated 2,453,847 313.7 7,651.6

Total Canada 10,216,590 884.1 18,210.2

Fig. 5 Relationship between aquatic N/C ratios and catchment

centroid latitude (top) and % area under agriculture (bottom)

Biogeochemistry

123

located in the northern portion of North America. We

have improved N export estimates from the

St Lawrence River, the second largest N exporting

river in North America as well as from a number of

other rivers in southern Canada and have identified the

Nelson River, a previously ignored catchment, as an

important contributor of reactive N to Hudson Bay and

thus the eastern Arctic Ocean.

We found that catchments with shallow slopes and

high runoff ([0.5 m year-1), such as are found in

Newfoundland and eastern Hudson Bay export higher

than expected levels of N due to the influence of wetland

organic matter, though they are composed of organic

matter with low N/C ratios. High runoff and steep slopes

in low N deposition areas such as in the Pacific Coast

rainforest also export larger than expected N amounts

due to high water volumes through the systems.

However, shallow catchments where wetlands are

not as prevalent, such as the St Lawrence, or where

runoff is lower than 0.5 m year-1 such as the

Mackenzie, show lower than expected N exports most

likely due to in-catchment denitrification. Slow mov-

ing rivers such as the Mackenzie underscore the

importance of denitrification in reducing catchment N

exports, as values from this site which experiences

higher N deposition levels in its southern portion, are

even lower than exports from high Arctic catchments

which are far from anthropogenic sources.

Organic carbon exports are in large part determined

by the presence or absence of wetlands and runoff

levels, as flatter catchments with high runoff will

produce the greatest TOC export. Though TOC fluxes

on a local level are obviously affected by land-use and

water control factors, our data show that on a large

scale and in relatively undisturbed regions, catchment

characteristics and the hydrological cycle are the main

determinants of organic carbon as was also found by

Alvarez-Cobelas et al. (2012). Finally our data show

that unsurprisingly, N/C ratios are strongly determined

by the importance of agriculture in catchments.

Acknowledgments The authors thank Roger Shetagne of

Hydro Quebec, Allison Zacharias of the Manitoba/Manitoba

Hydro Coordinated Aquatic Monitoring Program and Elaine

Page of Manitoba Water Stewardship for providing data from

rivers draining into Hudson Bay. Thomas Jaegler of the

University of Quebec assisted with St Lawrence basin database

contributions. From Environment Canada, Joseph Culp provided

data from the Arctic, Denis Parent from the Atlantic Region,

Myriam Rondeau for the St Lawrence, and Nancy Glozier for the

Prairie and northern rivers. Finally, we thank the Associate

Editor and two reviewers whose patience and constructive

comments allowed us to improve our original submission.

Appendix

GIS and other data sources used in the analysis

Agricultural areas were defined using the Simple

Biosphere model of the North American Land Cover

Characteristics Data Base, http://edcsns17.cr.usgs.

gov/glcc/.

Catchment slope was calculated from USGS 30 arc-

second DEM for North America corrected for hydro-

logical features: http://eros.usgs.gov/#/Find_Data/

Products_and_Data_Available/gtopo30/README.

The runoff data was mostly from the 1978 hydro-

logical atlas of Canada http://atlas.nrcan.gc.ca/site/

english/maps/archives/hydrological_atlas_1978/

water_quantity_general/24_Annual_Runoff_1978.

River Drainage area was modified from the Canada

National Atlas Major River Basin layer http://atlas.

nrcan.gc.ca/site/english/maps/reference/national/

drainbasins/referencemap_image_view and NRCan

2001, Atlas of Canada (rivers).

http://atlas.nrcan.gc.ca/site/english/learningresources/

facts/rivers.html/#bay (Accessed Oct 27, 2011).

http://atlas.nrcan.gc.ca/site/english/maps/archives/

5thedition/environment/water/mcr4055.

Population distribution was quantified from the

Canadian Atlas (NRCan 2001): http://atlas.nrcan.gc.

ca/site/english/maps/peopleandsociety/population/

population2001/density2001 (accessed Oct. 19, 2011).

Water chemistry data from British Columbia and

the Yukon were accessed at: http://waterquality.ec.gc.

ca/waterqualityweb/searchtext.aspx?lang=EN.

Catchment biological characteristics were extracted

from the Atlas of Canada (biomes) (NRCan 2001)

http://atlas.nrcan.gc.ca/site/english/learningresources/

theme_modules/borealforest/forest_regions.jpg/image_

view (accessed Oct. 19, 2011).

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