Nutrient Cycling in Eutrophic Shallow Lakes and Comparable Aquatic Ecosystems Wayne S. Gardner and Mark J. McCarthy Marine Science Institute The University

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

Nutrient Cycling

Few mechanistic nutrient-cycling and food web studies have been conducted on

shallow eutrophic lakes

Nutrient Cycling



Information is needed about sources, forms, cycling rates, and fate of nutrients in lakes

Nutrient Cycling



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?

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

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


1. Meteorological conditions (e.g. temp., winds and available-light)


3. Food web interactions

4. Sediment characteristics


1. Meteorological conditions (e.g. temp., winds and available-light)


3. Food web interactions 4. Sediment characteristics5. Lake Morphology

1. Annual summer cyanobacteria blooms

2. Popular tourist destination

3. Capture and culture fisheries

4. Industrial water source

5. Drinking water source

Lake Taihu

Outside inputs of nutrients

Stable Isotope studies of Amy Townsend-Small et al. (See

Hydrobiologia special issue)

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.

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.

Nutrient uptake and regeneration

Nutrients are transformed in/at the

1. Water column

2. Sediment-water interface

Water column processes

Phytoplankton

Micrograzers

Heterotrophicbacteria

Organic Matter

NH4+

Nitrifyingbacteria

Higher trophic

levels

NO3-

Water column processes

Water column NH4+ uptake

dynamics relate to Chlorophyll

concentrations (e.g. Texas Estuaries)

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)

1. Annual summer cyanobacteria blooms

2. Popular tourist destination

3. Capture and culture fisheries

4. Industrial water source

5. Drinking water source

Lake Taihu

Field Comparison

How do Lake Taihu water-column N fluxes compare to those in

Other shallow ecosystems?

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

East Taihu Lake

(ETL)

Macrophytes

Meiliang Bay

(MB)

Algae

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

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

Nutrient dynamics at

sediment-water interface

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)

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)

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+

Intake water Collection Vessel

Pump

InflowOutflow

O-ring

Water

Intact SedimentCore

Stopper

Nutrient and gas analysis



Pump

InflowOutflow

O-ring

Water

Intact SedimentCore

Stopper



+XS 15NO3-


Pump

InflowOutflow

O-ring

Water

Intact SedimentCore

Stopper 28, 29,30N2

14,15NH4+

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

September 2002

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

Lake Taihu, Meiliang Bay(January 2004)


NH4+ NO3

- PO43 NH4


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

Lake Taihu, Meiliang Bay(May 2004)


NH4+ NO3

- PO43 NH4


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

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

Conclusions

• High levels of nitrogen are delivered to Lake Taihu from rivers and the atmosphere

• Nitrogen sources include different quantities of sewage and fertilizers



Conclusions





Conclusions





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

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

31°

2338 km2

2.0 m

Eutrophic

Latitude

Surface Area

Mean Depth

Trophic Status

Lake Taihu

Taihu Laboratory for Lake Ecosystem Research (TLLER)

East Taihu Lake

(ETL)

Macrophytes

Meiliang Bay

(MB)

Lake Taihu, Meiliang Bay(May 2004)

Station Concentrations Fluxes(μM) (μmol N m-2 h-1)

NH4+ NO3

- PO43 NH4


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

Lake Taihu, Meiliang Bay(Sept./Oct. 2002 during Algal bloom)


NH4+ NO3

- PO43 NH4


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

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

Field comparison Compare sediment-water N fluxes and

transformations in two shallow ecosystems:

Florida BayTaihu Lake, China

Florida Bay

Florida Bay(August 2004)


Sal NH4+ NO3

- PO43 NH4


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

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

Conclusions


• 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

Conclusions



• 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

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)

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

Chlorophyll a in Texas Estuaries

05

1015202530354045

GO

M

CC

B

NB

EM

B

NR

LC

ULM BB

Ch

l a ( g

L-1

)

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)

SB2 (4.8 – 6.3)

Corpus Christi BayNRM

East Matagorda Bay

Sabine Lake

Upper Laguna Madre

Baffin Bay

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


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


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


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.



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 )

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

Meiliang Bay

(MB)

Algae

Conclusions



Documents

Nutrient Cycling in Eutrophic Shallow Lakes and Comparable Aquatic Ecosystems Wayne S. Gardner and Mark J. McCarthy Marine Science Institute The University