Elon M. Malkin, David H. Hollander, Ernst B. PeeblesUniversity of South Florida College of Marine Science

Parallel Nitrogen Cycles in Southwest Florida’s Tidal Rivers: Selective Remineralization of Algal Material Supports Fish Biomass

Study Questions:

•What is the source of sedimentary recalcitrant nitrogen?

•What is the source of sedimentary labile nitrogen?

•Which of these pools are associated with fish production?

Southwest Florida: Land Development & Fisheries

1. Anthropogenic Land Development:N Drains into receiving waters

2. Estuarine Dependent Juvenile Fish:Occupy Receiving Waters Positively or negatively influenced by N

3. Sedimentary N Processing:N Transformation between land and fish

Land Use

Feather Sound

Alafia River

Wildcat Creek

Curiosity Creek

McMullen Creek

Frog Creek

Myakka River

Peace River

Study Sites: 8 Watersheds

SOM vs. Sedimentary Porewater NH4+

Sediment Organic

Matter (SOM)

NH4+

Sediment Porewater

NH4+ (SedNH4

+)

NH4+

NH4+NH4

+

NH4+ NH4

+

NH4+NH4

+

NH4+

NH4+

Labile Sediment Nitrogen Pool

RecalcitrantSediment

Nitrogen Pool

0

2

4

6

8

10

12

14

16

-2 0 2 4 6 8 10 12 δ15 N SedNH 4+ , ‰ AIR

δ15

N S

OM

,

‰ A

IR r2= 0.17

SOM

SOM vs. Sedimentary Porewater NH4+

What is the Source of Recalcitrant SOM?Approach: δ15N System Averaging to Account for Advection•Sample Collection: (Primary producers, SOM)•Each primary producer δ15N value averaged across sites•Each SOM δ15N value averaged across sites•Mean values of 1oP plotted against mean SOM values

River C

River A

River B

1 2 31 average value per 1oP1 average value for SOM

1 average value per 1oP1 average value for SOM

1 2 3

1 average value per 1oP1 average value for SOM

1 2 3

Collection Sites

-2

0

2

4

6

8

10

12

2 2.5 3 3.5 4 4.5 5

δ15

N,

‰ A

IR

δ15 N Sedimentary Organic Matter, ‰ AIR

r2= 0.33

r2= 0.45POMBMA

Microalgae is not Recalcitrant N

r2= 0.83

-2

0

2

4

6

8

10

12

2 2.5 3 3.5 4 4.5 5

δ15

N,

‰ A

IR

δ15 N Sedimentary Organic Matter, ‰ AIR

r2= 0.91

Emergent VegetationUpland Plants

Vascular Plant Detritus is Recalcitrant N

What is the Source of Sedimentary Labile NH4+?

Approach: δ15N Site Specific Values to Maximize Data Points

•Site Collections (various 1OP, sedimentary NH4+)

•Site Specific Values of 1oP plotted vs. sedNH4+

River C

River A

River B

1 2 33 values per 1oP, porewater NH4

+, SOM

1 2 3

1 2 3

Collection Sites

3 values per 1oP, porewater NH4

+, SOM

3 values per 1oP, porewater NH4

+, SOM

0

2

4

6

8

10

12

14

16

-2 0 2 4 6 8 10 12 δ15 N Sedimentary Porewater NH 4+ , ‰ AIR

δ15

N,

‰ A

IR

r2= 0.07

Mangroves

Upland Plants

Upland Plant Detritus is Not Labile N

r2= 0.32

0

2

4

6

8

10

12

14

16

-2 0 2 4 6 8 10 12

r2BMA = 0.99

δ15 N Sedimentary Porewater NH 4+ , ‰ AIR

δ15

N,

‰ A

IRBMA is Labile N

BMA

0

2

4

6

8

10

12

14

16

-2 0 2 4 6 8 10 12 δ15 N Sedimentary Porewater NH 4+ , ‰ AIR

δ15

N,

‰ A

IR

r2BMA = 0.68

POM is Labile NPOM

Approach: δ13C vs. δ15N plots for all available data:

Which N Pool Supports Fish Production?

Benthic Microalgae (BMA)Particulate Organic Matter (POM)

Fish vs. Upland Plants (UP)Mangroves (MAN)Emergent Vegetation (EM)

δ1

5N

‰ A

IR

012

34567

89

10

-26 -25 -24 -23 -22

δ13C ‰ PDB

1OC (Benthic inverts)

3:1 p

er T

L

ΔN

=3‰

ΔC=1‰1OP

Stable Isotopes Link 1oP to Fish

3:1 p

er T

L

ΔC=1‰

3:1 p

er T

L

δ1

5N

‰ A

IR

012

34567

89

10

-26 -25 -24 -23 -22

δ13C ‰ PDB

2OC (fish)

1OC (Benthic inverts)

ΔN

=3‰

ΔC=1‰

1OP

ΔN

=3‰

Stable Isotopes Link 1oP to Fish

0

2

4

6

8

10

12

14

16

-30 -28 -26 -24 -22 -20

δ15

N,

‰ A

IR

δ13 C, ‰ PDB

What 1oP Support Fish Production? Fish

0

2

4

6

8

10

12

14

16

-30 -28 -26 -24 -22 -20

δ15

N,

‰ A

IR

δ13 C, ‰ PDB

Mangroves Do Not Support All Fish FishMAN

0

2

4

6

8

10

12

14

16

-30 -28 -26 -24 -22 -20

δ15

N,

‰ A

IR

δ13 C, ‰ PDB

Emergent Plants Do Not Support All Fish FishMANEM

0

2

4

6

8

10

12

14

16

-30 -28 -26 -24 -22 -20

δ15

N,

‰ A

IR

δ13 C, ‰ PDB

FishMANEMUP

Upland Plants Do Not Support All Fish

0

2

4

6

8

10

12

14

16

-30 -28 -26 -24 -22 -20

δ15

N,

‰ A

IR

δ13 C, ‰ PDB

FishMANEMUP

Vascular Plants Do Not Support All Fish

TB & CH POM

0

2

4

6

8

10

12

14

16

-30 -28 -26 -24 -22 -20

FishPOMBMA

δ15

N,

‰ A

IR

δ13 C, ‰ PDB

Microalgae Supports Fish Production

TB & CH POM

0

2

4

6

8

10

12

14

16

-30 -28 -26 -24 -22 -20

FishPOMBMAINV

δ15

N,

‰ A

IR

δ13 C, ‰ PDB

Benthic Invertebrates are Intermediates

Conclusions: Parallel Nitrogen Cycles

Vascular Plant Cycle: Recalcitrant OM is Buried

Vascular Plants

Bulk SOM

Burial

Vascular PlantsPOM

Bulk SOMLabile SOM

Recycle

SedNH4+

BMA

Burial

Microalgal Cycle: Labile OM is Remineralized

Vascular PlantsPOM

Bulk SOMLabile SOM

Recycle

SedNH4+

BMA

Burial

Foodwebs Are Associated with Labile Cycle