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Nutrient Cycling in Eutrophic Shallow Lakes and Comparable Aquatic
Ecosystems
Wayne S. Gardner and Mark J. McCarthy
Marine Science InstituteThe University of Texas at Austin
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Nutrient Cycling
Few mechanistic nutrient-cycling and food web studies have been conducted on
shallow eutrophic lakes
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Nutrient Cycling
Few mechanistic nutrient-cycling and food web studies have been conducted on
shallow eutrophic lakes
Information is needed about sources, forms, cycling rates, and fate of nutrients in lakes
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Nutrient Cycling
Few mechanistic nutrient-cycling and food web studies have been conducted on
shallow eutrophic lakes
Information is needed about sources, forms, cycling rates, and fate of nutrients in lakes
I.e. How are nutrients made available to primary and secondary producers?
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Why study nitrogen cycling?
• N often limits primary production, especially in marine systems
• Excessive N inputs can lead to eutrophication
• N compound concentrations and ratios can determine plant community structure
• N-cycling reflects community ecosystem dynamics– All organisms take up and/or release N
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Factors controlling primary and secondary production:
1. Meteorological conditions (e.g. temp., winds, and available-light)
2. Nutrient inputs, concentrations, and cycling rates
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Factors controlling primary and secondary production:
1. Meteorological conditions (e.g. temp., winds, and available-light)
2. Nutrient inputs, concentrations, and cycling rates
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Factors controlling primary and secondary production:
1. Meteorological conditions (e.g. temp., winds, and available-light)
2. Nutrient inputs, concentrations, and cycling rates
3. Food web interactions
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Factors controlling primary and secondary production:
1. Meteorological conditions (e.g. temp., winds and available-light)
2. Nutrient inputs, concentrations, and cycling rates
3. Food web interactions
4. Sediment characteristics
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Factors controlling primary and secondary production:
1. Meteorological conditions (e.g. temp., winds and available-light)
2. Nutrient inputs, concentrations, and cycling rates
3. Food web interactions 4. Sediment characteristics5. Lake Morphology
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1. Annual summer cyanobacteria blooms
2. Popular tourist destination
3. Capture and culture fisheries
4. Industrial water source
5. Drinking water source
Lake Taihu
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Outside inputs of nutrients
Stable Isotope studies of Amy Townsend-Small et al. (See
Hydrobiologia special issue)
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Stable isotope Results(Amy Town-send-Small et al.)
1. Riverine nitrogen inputs in winter are influenced by discharge of human sewage into rivers and the lake.
2. In spring, however, sewage inputs to the lake appear to be balanced by fertilizers or N2 fixation.
3. Nitrate concentrations in rivers and the lake were higher, in most cases, in spring than in winter.
4. 15N of NO3- was not correlated with NO3- concentration, indicating that concentration measurements alone are insufficient to describe N sources.
5. 15N values also show that urbanized areas of the lake have more sewage-derived N than those areas dominated by agriculture, aquaculture, or industry.
6. This result has important implications for human health, since Taihu Lake is a source of drinking and irrigation water as well as fish for human consumption.
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Stable isotope Results(Amy Town-send-Small et al.)
1. Riverine nitrogen inputs in winter are influenced by discharge of human sewage into rivers and the lake.2. In spring, however, sewage inputs to the lake appear to be balanced by fertilizers or N2 fixation. 3. Nitrate concentrations in rivers and the lake were higher, in most cases, in spring than in winter.
4. 15N of NO3- was not correlated with NO3
- concentration, indicating that concentration measurements alone are insufficient to describe N sources.
5. 15N values show that urbanized areas of the lake have more sewage-derived N than those areas dominated by agriculture, aquaculture, or industry.
6. This result has important implications for human health, since Taihu Lake is a source of drinking and irrigation water as well as fish for human consumption.
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Nutrient uptake and regeneration
Nutrients are transformed in/at the
1. Water column
2. Sediment-water interface
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Water column processes
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Phytoplankton
Micrograzers
Heterotrophicbacteria
Organic Matter
NH4+
Nitrifyingbacteria
Higher trophic
levels
NO3-
Water column processes
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Water column NH4+ uptake
dynamics relate to Chlorophyll
concentrations (e.g. Texas Estuaries)
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Comparison of NH4+ uptake rates
to Chl a concentrations in Texas estuaries
y = 0.0647x + 0.107
R2 = 0.7752p < 0.001
y = 0.0458x + 0.0066
R2 = 0.5941p <0.001
-0.5
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0 10 20 30 40
Chl a (g L-1)
Up
take
( M
h-1
)
Light
Dark
Linear (Light)
Linear (Dark)
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1. Annual summer cyanobacteria blooms
2. Popular tourist destination
3. Capture and culture fisheries
4. Industrial water source
5. Drinking water source
Lake Taihu
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Field Comparison
How do Lake Taihu water-column N fluxes compare to those in
Other shallow ecosystems?
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Water column N recycling rates in waters with different trophic status
LMI: Lake Michigan; FLB: Florida Bay; LE: Lake Erie; SBLH: Saginaw Bay, Lake Huron; MRP: Mississippi River plume; OWC: Old Woman Creek, Lake Erie; TX: Texas estuaries; TL: Taihu Lake, China; Lake
Maracaibo, Venezuela. (Red = hypereutrophic systems)
0
1
2
3
4
5
LM
I
FLB LE
SBL
H
MR
P
OW
C
TX TL
LM
V
mol
L-1
h-1
uptake rate
regeneration rate
Taihu Lake
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East Taihu Lake
(ETL)
Macrophytes
Meiliang Bay
(MB)
Algae
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Ammonium uptake and regeneration rates in Taihu Lake
Meiliang Bay
-2.6
-1.3
0
1.3
2.6
3.9
5.2
Sept-Oct May Sept-Oct May Sept-Oct May Sept-Oct May
M h
-1
Uptake(L)
Uptake(D)
Reg(L)
Reg(D)
MB-0 MB-1 MB-3 MB-7
River Inner Bay Outer Bay Main Lake
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Ammonium uptake and regeneration rates in Taihu Lake
East Taihu Lake
-0.05
0
0.05
0.1
0.15
0.2
Jan May Jan May Jan May Jan May
M h
-1
Uptake(L)
Uptake(D)
Reg(L)
Reg(D)
ETL-1 ETL-2 ETL-3 ETL-4
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Nutrient dynamics at
sediment-water interface
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Transformations within the nitrogen cycle in shallow aquatic systems(Slide modified from version of Soonmo An)
PONNO3
-
NO2-
NH4+
DON
N2
N2O
PON
DON
NO3-
NO2-
NH4+
N2
N2ONO3-
Sinking
DENITRIFICATION
Fixation
Assimilation
Excretion
Sediment
Water column
DNRA
N-dynamics (chemical perspective)
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N Transformations to remember
N-fixation: N2 Organic N (N source)
Mineralization: Organic N NH4+
Nitrification: NH4+ NO2
- NO3-
Denitrification: NO3- + OM N2 (N-sink)
DNRA: NO3- + OM NH4
+ (N-link)
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What is the fate of NO3- at
sediment-water interface?
1. Denitrification (sink)?
NO3- N2
2. Dissimilatory nitrate reduction to ammonium (DNRA; link)? NO3
- NH4+
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Intake water Collection Vessel
Pump
InflowOutflow
O-ring
Water
Intact SedimentCore
Stopper
Nutrient and gas analysis
Nutrient and gas analysis
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Intake water Collection Vessel
Pump
InflowOutflow
O-ring
Water
Intact SedimentCore
Stopper
Nutrient and gas analysis
Nutrient and gas analysis
+XS 15NO3-
Intake water Collection Vessel
Pump
InflowOutflow
O-ring
Water
Intact SedimentCore
Stopper 28, 29,30N2
14,15NH4+
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Estimation of net denitrification vs. N-fixation
• Pass water from site over core and allow to come to steady-state over night
• Measure changes in nutrients, O2, and N2:Ar as water passes over core
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September 2002
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Lake Taihu, Meiliang Bay(Sept./Oct. 2002 during Algal bloom)
Station Concentrations Fluxes(μM) (μmol N or P m-2 h-1)
NH4+ NO3
- PO43 NH4
+ DNRA NO3- N2 N-fix PO4
3-
………………… ……………………………………
River 180 14 0.18 490 250 -83 180 0 5.2
Inner Bay 3.4 6.4 0.25 40 10 5.8 77 0 1.2
Outer Bay 2.8 0.1 0.22 20 0.4 12 52 0 0.8
Main Lake 0.1 0.4 0.04 45 3.0 13 -6 14 -1.5
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Lake Taihu, Meiliang Bay(January 2004)
Station Concentrations Fluxes(μM) (μmol N or P m-2 h-1)
NH4+ NO3
- PO43 NH4
+ DNRA NO3- N2 N-fix PO4
3
…………………… ………………………………………………
River 470 97 0.59 -860 0 -230 -220? 0 -5.2
Inner Bay 280 82 0.16 -410 0 -65 420 0 -0.40
Outer Bay 100 108 0.12 -470 0 -22 230 0 -0.44
Main Lake 11 85 0.03 -120 0 5.5 86 0 0
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Lake Taihu, Meiliang Bay(May 2004)
Station Concentrations Fluxes(μM) (μmol N or P m-2 h-1)
NH4+ NO3
- PO43 NH4
+ DNRA NO3- N2 N-fix PO4
3-
………………….. ………………………………………….
River 370 66 0.64 240 0 -410 200 0 1.8 Inner Bay 44 150 0.10 -140 0 -170 330 0 -0.44Outer Bay 44 170 0.06 9 0 -130 260 0 -0.04Main Lake 0.2 180 0.09 0.6 0 -59 160 0 0.02
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Conclusions
• High levels of nitrogen are delivered to Lake Taihu from rivers and the atmoshpere
• Nitrogen sources include sewage and fertilizers
• Nutrient concentrations alone are not an adequate indicator of food-web activity
• Nutrient transformations are dynamic in shallow, eutrophic Lake Taihu
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Conclusions
• High levels of nitrogen are delivered to Lake Taihu from rivers and the atmosphere
• Nitrogen sources include different quantities of sewage and fertilizers
• Nutrient concentrations alone are not an adequate indicator of food-web activity
• Nutrient transformations are dynamic in shallow, eutrophic Lake Taihu
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Conclusions
• High levels of nitrogen are delivered to Lake Taihu from rivers and the atmosphere
• Nitrogen sources include different quantities of sewage and fertilizers
• Nutrient concentrations alone are not an adequate indicator of food-web activity
• Nutrient transformations are dynamic in shallow, eutrophic Lake Taihu
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Conclusions
• High levels of nitrogen are delivered to Lake Taihu from rivers and the atmosphere
• Nitrogen sources include different quantities of sewage and fertilizers
• Nutrient concentrations alone are not an adequate indicator of food-web activity
• Nutrient transformations are dynamic in shallow, eutrophic Lake Taihu
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Background information:
Nutrient Dynamics and Biological Structure in Shallow Freshwater and Brackish Lakes. Mortensen, E. Jeppesen, M. SØndergaard & L. Kamp Nielsen (eds.). Hydrobiologia, Vols. 275/276. 1994
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Small-volume methodologies to study N dynamics
• NH4+ concentration and atom% N-15 by
HPLC
• Other nutrients via auto analyzer
• Gases by membrane inlet mass spectrometry
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31°
2338 km2
2.0 m
Eutrophic
Latitude
Surface Area
Mean Depth
Trophic Status
Lake Taihu
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Taihu Laboratory for Lake Ecosystem Research (TLLER)
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East Taihu Lake
(ETL)
Macrophytes
Meiliang Bay
(MB)
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Lake Taihu, Meiliang Bay(May 2004)
Station Concentrations Fluxes(μM) (μmol N m-2 h-1)
NH4+ NO3
- PO43 NH4
+ DNRA NO3- N2 N-fix PO4
3-
………………….. ………………………………………….
River 370 66 0.64 240 0 -410 200 0 1.8 Inner Bay 44 150 0.10 -140 0 -170 330 0 -0.44Outer Bay 44 170 0.06 9 0 -130 260 0 -0.04Main Lake 0.2 180 0.09 0.6 0 -59 160 0 0.02
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Lake Taihu, Meiliang Bay(Sept./Oct. 2002 during Algal bloom)
Station Concentrations Fluxes(μM) (μmol N or P m-2 h-1)
NH4+ NO3
- PO43 NH4
+ DNRA NO3- N2 N-fix PO4
3-
………………… ……………………………………
River 180 14 0.18 490 250 -83 180 0 5.2
Inner Bay 3.4 6.4 0.25 40 10 5.8 77 0 1.2
Outer Bay 2.8 0.1 0.22 20 0.4 12 52 0 0.8
Main Lake 0.1 0.4 0.04 45 3.0 13 -6 14 -1.5
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Small-volume methodologies to study N dynamics
• NH4+ concentration and atom% N-15 by
HPLC
• Other nutrients via auto analyzer
• Gases by membrane inlet mass spectrometry
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Field comparison Compare sediment-water N fluxes and
transformations in two shallow ecosystems:
Florida BayTaihu Lake, China
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Florida Bay
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Florida Bay(August 2004)
Station Concentrations Fluxes(μM) (μmol N or P m-2 h-1)
Sal NH4+ NO3
- PO43 NH4
+ DNRA NO3- N2 N-fix PO4
3-
………………… ……………………………………
Rabbit 38 0.56 0.36 0.01 60 47 39 2.3 0 6.6
Murray 37 0 0.43 0.02 160 134 4.0 5.7 0 1.5
Rankin 40 1.3 0.44 0.01 340 167 1.4 40 0 0.49
Duck 41 0.97 0.47 0.02 130 102 1.3 21 0 1.9 r = 0.91
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Florida Bay (January 2005; Exp.1)
Station Concentrations Fluxes(μM) (μmol N or O2 m-2 h-1)
Sal NH4+ NO3
- NO2- NH4
+ DNRA NO3- N2 SOD NO2
-
………………… ……………………………………
Rabbit 38 0.77 0.31 0.79 40 11 1.0 156 -527 -0.8
Murray 33 0 0.26 0.48 170 54 0.8 210 -1320 1.6
Rankin 44 0.41 0.32 0.89 29 56 1.8 119 -945 1.8
Duck 41 4.26 1.19 0.96 215 82 -7.1 357 -1800 0.9 r = 0.71
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Conclusions
• Nutrient concentrations alone are not an adequate indicator of food-web activity
• Nutrient transformations are dynamic in shallow, eutrophic ecosystems and uptake rates relate to Chl conc’s
• Lake Taihu has high nutrient cycling rates that vary with sample location and season
• Denitrification rates are comparable but distinguishable among different aquatic systems
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Conclusions
• Nutrient concentrations alone are not an adequate indicator of food-web activity
• Nutrient transformations are dynamic in shallow, eutrophic ecosystems and uptake rates relate to Chl conc’s
• Lake Taihu has high nutrient cycling rates that vary with sample location and season
• Denitrification rates are comparable but distinguishable among different aquatic systems
• DNRA is important in eutrophic systems with high salinities (e.g. Florida Bay) but not in most lakes
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Acknowledgements
• Chinese Academy of Sciences• National Science Foundation, USA• National Oceanic and Atmospheric
Administration, USA• Texas Water Development Board• Colleagues at
– Nanjing Institute of Geography & Limnology (e.g. Boqiang Qin, Longyuan Yang, Zhang Lu, and Yuwei Chen)
– University of Texas Marine Science Insitute (Amy Townsend-Small, Dmitri Sobolev)
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Why study N-cycling in shallow habitats?
Shallow coastal habitats are among the most productive ecosystems in the world and are important interfaces between watersheds and open-water systems
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Chlorophyll a in Texas Estuaries
05
1015202530354045
GO
M
CC
B
NB
EM
B
NR
LC
ULM BB
Ch
l a ( g
L-1
)
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Ammonium uptake and regeneration in Texas estuaries
0.00.51.01.52.02.53.03.54.0
GO
M
CC
B
NB
EM
B
NR
LC
ULM BB
M h
-1
Uptake(L)
Uptake(D)
Reg(L)
Reg(D)
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SB2 (4.8 – 6.3)
Corpus Christi BayNRM
East Matagorda Bay
Sabine Lake
Upper Laguna Madre
Baffin Bay
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How do shallow ecosystems (e.g. < 2 m) differ from deeper ones?
1. Wind mixing- Delivers oxygen
- Prevents stratification
-Increases turbidity
Physical delivery of oxygen prevents development of bottom water hypoxia, even when there is high biological oxygen demand
—Example: Lake Taihu
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How do shallow ecosystems (e.g. < 2 m) differ from deeper ones?
2. Depositional zones different
- High sediment nutrient regeneration in regions with high organic-matter deposition
- Water column nutrient regeneration higher if labile organic matter remains suspended
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How do shallow ecosystems (e.g. < 2 m) differ from deeper ones?
3. Light penetration
Reduced by resuspension from wind mixing
Rooted plants and epiphytes- Assimilate nutrients (from sediments and
water column)- Produce labile organic compounds
Intense algal blooms - Block light from benthic plants- Increase ratio of water-column vs. sediment nutrient cycling
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How do shallow ecosystems (e.g. < 2 m) differ from deeper ones?
4. Low volume to area ratio- Magnifies effects on nutrient inputs or outflows
Lake water turnover time is shorter in shallow versus deep lakes.
More primary production per volume in shallow versus deeper lakes because light exposure occurs over a large area.
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Factors controlling primary and secondary production:
1. Meteorological conditions (e.g. temp., winds, and available-light)
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Dark
+15NH4+
Light
+15NH4+
How can we evaluate water-column N dynamics
in aquatic ecosystems? Bottle Experiments
15NH4+ additions to bottles
(light/dark incubations; HPLC analysis )
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Assumptions
• 14NH4+ and 15NH4
+ will be taken up in proportion to their concentrations
• Regenerated NH4+ is dominated by
14NH4+ (i.e. natural abundance =
99.64% 14NH4+), i.e. from organic
matter already fixed
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Meiliang Bay
(MB)
Algae
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Conclusions
• Nutrient concentrations alone are not an adequate indicator of food-web activity
• Nutrient transformations are dynamic in shallow, eutrophic ecosystems and uptake rates relate to Chl conc’s