The Enigmatic Great Lakes Nitrogen Cycle:
Review and Insights from Isotopic Records
Nathaniel E. Ostrom, Peggy H. Ostrom and Kateri R. Salk
Department of Zoology, Michigan State University
visibleearth.nasa.gov
Outline and Objectives:
1. Review temporal concentration and isotopic records
2. Review our understanding of the Great Lakes N cycle
3. Future research directions
visibleearth.nasa.gov
Long-term trend of increasing nitrate: Lake Superior
P levels have remained low, lower than central N. Pacific gyre
NO3-/PO43- ~ 10,000; 600 times requirement for primary production
Half the watershed is the lake itself
Proposed that atm. deposition the primary cause (Ostrom et al., 1998)
Based on low d15N-NO3 ( ~ -2 ‰)
Sterner, 2011 Inland Waters 1: 29-46
Long-term trend of increasing nitrate: Lake Superior (Finlay et al., 2007)
Stable isotope data indicate atm. deposition not the source of NO3NO3- generated in the lake via nitrification
low potential for denitrification (Carlton et al., 1989)
Superior is a net NO3- producer
Lake Superior Data
(Finlay et al., 2007)
d18O-NO3
Precipitation
River Water
Lake
Water
d15N-NO3
From Kendall et al. 2007
Long-term trend of increasing nitrate: other Great Lakes
Rising NO3- evident in all of the Great Lakes
But not likely to be for the same reasons
0.04
PO43- mg/L
0.03
Lake Ontario 1970-2008
Eutrophic
0.02
Mesotrophic
0.01
Oligotrophic
0
http://www.on.ec.gc.ca/solec/nearshore-water/paper/part5.html
Lake Erie (central basin)
NO3- mg/L
NO3- mg/L
Lake Ontario 1970-2008
0.3
Dove, 2009
0.2
http://www.on.ec.gc.ca/solec/nearshore-water/paper/part5.html
Lake Ontario Sediment C and N Isotope Record
Eutrophication and recovery evident in C isotopes
definitive evidence of the success of the Clean Air and Water Act
Driver of N isotope record unclear
interpreted as an increase in denitrification toward present
lack of denitrification rate data
Hodell and Schelske (1998)
Grand Traverse Bay, Lake Michigan
85° 30’
85° 15’
45° 15’
GT3
45° 00’
GT1
44° 85’
Macrellis, 1999; McCusker et al., 1999
Grand Traverse Bay: Seasonal Progression in Temperature
1997 a cool year, 1998 El Niño
0
GT1
1997
-20
-40
-60
Depth (m)
-80
22 C
0
GT3
1997
-20
-40
-60
-80
18
14
10
-100
6
0
GT3
1998
-20
-40
-60
-80
-100
April
May
June
July August
2
Grand Traverse Bay: Station GT3 1997
Stratification well established at end of June
Chl focused at or above thermocline
SRP diminishes, NH4+ accumulates
Temperature ( °C)
Chlorophyll Fluorescence (RFU)
SRP (mM)
Ammonium (mM)
0
Depth (m)
-20
-40
-60
-80
-100
0
Depth (m)
-20
-40
-60
-80
-100
April May June July Aug.
April May June July Aug.
Grand Traverse Bay: Station GT3 1998, El Niño
Stratification well established at end of May, one month earlier
Chl focused at or above thermocline
SRP diminishes, NH4+ accumulates
Temperature ( °C)
Chlorophyl Fluorescence (RFU)
Depth (m)
0
-20
-40
-60
-80
-100
SRP (mM)
Ammonium (mM)
0
Depth (m)
-20
-40
-60
-80
-100
April May June July Aug.
April May June July Aug.
Grand Traverse Bay: Station GT3 1997
NO3- declines at surface, phytoplankton uptake
d15N: NO3- constant at ~ 2.5 ‰; NH4+ averaged 12.1 ‰
d15N-PON largely controlled by uptake of 15N depleted NO3- and enriched NH4+
Nitrate (mM)
Depth (m)
0
-20
-40
-60
-80
-100
Weighted d15N – PON (o/oo)
10
9
8
7
6
5
4
Mean = 5.6 ‰
3
d15N - NO3 (o/oo)
0
Depth (m)
-20
-40
-60
-80
-100
April May June July Aug.
7
6
5
4
3
2
1
0
Weighted d15N - NO3 (o/oo)
Mean = 2.5 ‰
April May June July Aug.
Grand Traverse Bay: Station GT3 1998, El Niño
Decline in d15N-NO3- in July-Aug.; fractionation during nitrification
average d15N-NO3- similar to 1997
d15N-PON higher by > 6 ‰ relative to 1997
Weighted d15N – PON (o/oo)
Nitrate (mM)
Depth (m)
0
15
14
13
12
11
10
9
8
-20
-40
-60
-80
-100
0
d15N - NO3 (o/oo)
Depth (m)
-20
-40
-60
-80
-100
April May June July Aug.
7
6
5
4
3
2
1
0
Mean = 12.2 ‰
Weighted d15N - NO3 (o/oo)
Mean = 3.3 ‰
April May June July Aug.
Implications to Great Lakes N Cycling
d15N of NO3- and seston very dynamic
indicates rapid turnover of PON and NO3Strong influence of the timing of spring stratification on d15N of PON
earlier stratification leads to preferential uptake of 15N enriched NH4+
Lake Ontario d15N record might be driven by climate warming
Need to know rates of NO3- and NH4+ uptake and denitrification
Hodell and Schelske (1998)
The “New” Nitrogen Cycle:
Classical view that nitrification and denitrification dominate the N cycle is changing
Anammox and DNRA increasingly being recognized as important
Anammox (ANaerobic AMMonium OXidation)
NO3- +NH4+  N2
DNRA (Dissimilatory Nitrate Reduction to Ammonium)
NO3- NH4+
N retained in ecosystem; often provides source of NH4+ to Anammox
Francis et al 2007 ISME
Journal 1, 19-27.
The “New” Nitrogen Cycle:
Anammox dominant N-loss pathway in OMZ’s off Peru, Chile and Namibia
can occur at higher O2 levels; greatly expanding area of NO3 loss
Denitrification dominates in Arabian Sea OMZ
Anammox is autotrophic and may outcompete denitrification where organic C
is less bioavailable
Francis et al 2007 ISME
Journal 1, 19-27.
Nitrogen Rate Measurements in Great Lakes very limited:
While not exhaustive, “hits” found by Web of Science:
Ecosystem and
Denitrification
Chesapeake Bay
124
Baltic Sea
141
Narragansett Bay
25
Great Lakes*
20
Nitrification
73
48
25
9
Anammox
7
20
0
0
DNRA
3
7
2
1
N2O
12
29
6
2
* Only 2 papers reporting rates of denitrification, one of DNRA, no nitrification rates
Lake Huron and Ontario, no rate data
More is known about N cycling in Lake Cadagno, Switzerland, than we do in the entire
Great Lakes (i.e. Halm et al., 2009)
Regime Change: Narragansett Bay
Shift in primary production to blue-greens
(N2 fixers)
Decrease in deposition of labile organics
Decrease in rates of denitrification
increase in DNRA and retention of N
in the ecosystem
Increase in nitrate export
Great Lakes?
N fixers are increasing
Is this changing the N cycle?
Finlay et al 2007 Ecological Applications, 17(8),
2007, pp. 2323–2332
Lane N. NATURE Vol 449 18 October 2007 778-780
and Fulweiler et al (2007) vol 448 pg. 180-182.
Summary:
NO3- is rising but likely for different reasons in different
lakes
Sediment d15N is rising: cause uncertain
Important Aspects of N cycling in the Great Lakes:
Large and predictable hypoxic zone (Lake Erie)
Hydraulic residence times from 3 to 173 y
Range of trophic states from ultra-oligotrophic (Superior)
to hyper-eutrophic (Sandusky Bay, Lake Erie)
Eutrophic to Oligotrophic conditions in the same
lake (Erie)
Marked response to climate change
Superior warming at twice the rate of the atm.
increased sediment resuspension by storms
Great Lakes could very well be a harbinger of what
is to come in the oceans
Austin and Colman 2008 Limnol. Oceanogr. 53:
2724-2730
Additional References:
Carlton et al. (1989) J. Great Lakes Res. 15: 133-140.
Dove A. (2009) Aquatic Ecosystem Health and Management 12: 281-295.
Finlay et al. (2007) Ecol. Appl. 17: 2323-2332.
Halm et al. (2009) Environmental Microbiology 11: 1945-1958.
Hodell D.A. and C.L. Schelske (1998) Limnology and Oceanography 43: 200-214.
Kendall et al. (2007) In: Stable Isotopes in Ecology and Environmental Science, R. Michener and K. Lajtha
(Eds.) Blackwell Publishing, pp. 375-449.
Macrellis A.N. (1999) Geochemical and isotope dynamics of dissolved inorganic nitrogen in Grand Traverse
Bay, Lake Michigan. Unpublished masters thesis, Geological Sciences, Michigan State University.
McCusker et al. (1999) Organic Geochemistry 30: 1543-1557.
Ostrom et al. (1998) Chemical Geology 152: 13-28.
Quinn F.H. (1992) Hydraulic residence times for the Laurentian Great Lakes. J. Great Lakes Research 18:
22-28.