Kim J. Rattan 1 * , Patricia A. Chambers 1 1 Environment Canada and - - PowerPoint PPT Presentation

N:P stoichiometry in Canadian prairie streams: effects of land cover and hydrologic variability

Kim J. Rattan 1*,

Patricia A. Chambers1

1 Environment Canada and Climate Change, Canada Centre for Inland Waters,

Burlington, Ontario, L7S 1A1, Canada * Corresponding author: kim.rattan@canada.ca; 1-905-336-4451

• Prairie streams are an important freshwater

resource in North America Great Plains.

• Runoff/seepage from agricultural land

introduces nutrients, resulting in eutrophication and loss of valuable ecosystem services.

• Concentrations and loads of nitrogen (N)

and phosphorus (P), in both dissolved and particulate fractions, are sensitive to land cover and hydrologic variability.

• Temporal variation in N:P ratios affects lake

food webs, particularly the productivity and composition of algal communities.

• Yet little is known about riverine N:P ratios,

particularly the effects of changing land use and hydroclimatology on the mobilization and delivery of N relative to P in tributaries.

Blue-Green Algae on Grand Beach

The Issue

Satellite image of Lake Winnipeg algae bloom Lake Winnipeg

Objectives

To assess the response of N:P ratios (concentrations and loads) to changes in crop cover and hydrologic variability for streams draining the Canadian prairies.

Study Area

Seven sub-watersheds: 66 - 677 km2 Continental Climate: Long cold winters (avg -15 oC) followed by warm summers with precipitation falling predominately during summer months. Soils: clay and silt. Topography: low relief with natural land cover consisting of grasslands with permanent and temporary wetlands. Land use: row crops (small grains, canola) and livestock (cattle, poultry, swine).

Sampling and chemical measurements

Chemistry

• Grab samples for total and dissolved

P and N collected daily during the rising limb and peak of snowmelt, weekly during the falling limb and biweekly thereafter until ice cover, for two years (2013 and 2014). Discharge

• Pressure transducer in each stream

measuring water level.

• Discharge estimated from

relationship between water level at site and discharge at long-term d/s government stations. Loads

• Nutrient concentrations (measured or

linearly interpolated between sampling dates) were multiplied by daily discharge and summed by season or year.

Hydrology and Climate

• In both 2013 and 2014, runoff volume and discharge peaked during snowmelt (orange bar)

although snowmelt peaks were less in 2014 (note scale differences between years).

• 2014 also experienced more rain events, characterized by additional peaks in the hydrograph

(green square).

2013 2014

• In 2013, mean precipitation was

greatest during the spring. Runoff and discharge were greatest during snowmelt. Summer and fall were only 21%

• f annual runoff.
• In 2014, rain precipitation was

greatest in summer. Runoff and discharge were still greatest during snowmelt. Summer and fall comprised 45% of annual runoff.

Year Mean Rain precipitation (mm/day) 2 4 6 8

Snowmelt Spring Summer Fall Annual

2013 2014

Rain Precipitation

Year

Runoff (dam3/day)

200 400 600 800 1000 1200 1400 1600

Runoff

Year

Q (m3/s)

2 4 6 8 10 12 14 16 18 2013 2014

Discharge

Hydrology & Climate

2013

2013

2013 2014 2014

2014

Year

Year

Year

Nitrogen Fractions

• For TN and DIN, concentrations

were greatest during snowmelt for both 2013 and 2014 (orange bar).

• For PN, concentrations were

greatest during snowmelt and spring in 2013 (orange bars). Concentrations did not differ seasonally in 2014.

Ammonium

Dissolved N

PN (mg/L)

Season Year F SU SP SN 2014 2013 2014 2013 2014 2013 2014 2013

0.8 0.6 0.4 0.2 0.0

(d) PN

a a b b a a a a

# *

Particulate N

DIN (mg/L)

Season Year F SU SP SN 2014 2013 2014 2013 2014 2013 2014 2013

4 3 2 1

(b) TDN

a a b b b b b b

# *

Dissolved N

TN (mg/L)

Season Year F SU SP SN 2014 2013 2014 2013 2014 2013 2014 2013

4 3 2 1 a a b b b b b b

(a) TN

Total N

• Letters (a,b,c,d) above bars identify seasonal means that

differ (p<0.05) within a year; symbols (*,#) below bars identify seasonal means that differ (p<0.05) between years.

• SN = snowmelt, SP = spring, SU = summer, and F= Fall

Phosphorus Fractions

• Like N, total and dissolved P

concentrations were highest during snowmelt (orange bar).

• Like PN, PP was greatest during

snowmelt and spring in 2013 (orange bars). Concentrations showed less seasonal change in 2014.

• In 2013 (a typical snowmelt-driven

year), nutrients were largely exported in particulate forms.

• Under “wetter” (i.e., rainier)

conditions of 2014, more nutrients were exported in the dissolved form.

SRP (mg/L)

F SU SP SN 2014 2013 2014 2013 2014 2013 2014 2013

0.5 0.4 0.3 0.2 0.1

a a b b b b b b

(c) SRP

Reactive P

TP (mg/L)

Season Year F SU SP SN 2014 2013 2014 2013 2014 2013 2014 2013

1.4 1.1 0.8 0.5 0.2 a a b b b b b b

(a) TP

Total P

PP (mg/L)

Season Year F SU SP SN 2014 2013 2014 2013 2014 2013 2014 2013

0.4 0.3 0.2 0.1 0.0 a a b b a a b a

# # * * (d) PP

Particulate P

#

*

• Letters (a,b,c,d) above bars identify seasonal means that

differ (p<0.05) within a year; symbols (*,#) below bars identify seasonal means that differ (p<0.05) between years.

• SN = snowmelt, SP = spring, SU = summer, and F= Fall

d. ..

Particulate N:P Load Ratios

• Results from a two-way ANCOVA

(season and year as factors, crop cover as co-variate) showed that PN:PP ratios differed on an annual and seasonal basis.

• Annually, PN:PP ratios were greater

in 2014. Within years, values were greater during seasons when the soil was wet but not frozen (spring 2013; summer 2014) These findings suggest that under wetter soil conditions, greater quantities of PN were exported relative to PP.

• Export of PN:PP was not associated

with the extent of crop cover in the watershed.

Year PN: PP (Tonne: Tonne)

2014 2013

5 4 3 2 1

* #

Annual

PN:PP (Tonne: Tonne)

Year Season 2014 2013 F SU SP SN F SU SP SN

16 12 8 4 a b b c a b c b

* * # #

Seasonal

N: P Load Ratios

Total and Dissolved N:P Load Ratios

• Total and dissolved N:P load ratios

showed associations (p<0.05) with season and crop cover for both 2013 and 2014.

• Total N:P ratios were higher (p<0.05)

during summer and fall. DIN:SRP load ratios were lowest during fall.

• The seasonality in total and dissolved

N:P load ratios likely relates to the fact that delivery of P to streams is influenced by hydrological activity whereas N moves through the landscape in dissolved forms.

• The positive correlation between N:P

load ratios and crop cover indicates greater N loss, relative to P, under intensive crop cultivation.

DIN: SRP (Tonne: Tonne)

Year Season 2014 2013 F SU SP SN F SU SP SN

12 9 6 3

a b b c a a a b

DIN:SRP

TN:TP (Tonne: Tonne)

Year Season 2014 2013 F SU SP SN F SU SP SN

16 12 8 4 a a b b a a c a

TN:TP

Conclusions

• Dissolved versus particulate N:P load ratios responded

differently to land use and hydrologic variability: – land use was the major driver of dissolved N:P load ratios – hydrology was the main driver in particulate N:P load ratios.

• Predicting stoichiometry is important because of its strong

effects on ecological processes such as primary production.

• Improved knowledge of the dominant nutrient forms and

their transport pathways will assist in determining appropriate mitigation practices to reduce nutrient loads under a changing climate.

Acknowledgements

Collaborators

• J. Corriveau
• A. Yates
• B. Brua
• J. Culp

Technical Support

• Z. Duggan
• R. MacKay

Funding –EC Lake Winnipeg Basin Initiative –Canadian Rivers Institute (UNB) Partners –Manitoba Gov’t –Land Owners

Thank You!

Kim.Rattan@canada.ca