Parallel Nitrogen Cycles in Southwest Florida’s Tidal Rivers:
Selective Remineralization of Algal Material Supports Fish
Biomass
Elon M. Malkin, David H. Hollander, Ernst B. Peebles
University of South Florida College of Marine Science
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
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?
Study Sites: 8 Watersheds
Feather Sound
Alafia River
Wildcat Creek
Curiosity Creek
McMullen Creek
Frog Creek
Myakka River
Peace River
SOM vs. Sedimentary Porewater NH4+
NH4+
Sediment Porewater
NH4+ (SedNH4+)
Labile Sediment
Nitrogen Pool
Recalcitrant
Sediment
Nitrogen Pool
Sediment Organic
Matter (SOM)
NH4+
NH4+
NH4+
NH4+
NH4+
NH4+
NH4+
NH4+
NH4+
δ15N SOM, ‰ AIR
16
14
12
10
8
6
4
2
0
SOM vs. Sedimentary Porewater NH4+
SOM
r2= 0.17
-2
0
2
4
6
8
10
δ15N SedNH4+, ‰ AIR
12
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
Collection Sites
1
River A
2
1
2
River B
1
River C
2
3
1 average value per 1oP
1 average value for SOM
3
1 average value per 1oP
1 average value for SOM
3
1 average value per 1oP
1 average value for SOM
Microalgae is not Recalcitrant N
12
δ15N, ‰ AIR
10
8
POM
BMA
r2= 0.45
r2= 0.33
6
4
2
0
-2
2
2.5
3
3.5
4
4.5
5
δ15N Sedimentary Organic Matter, ‰ AIR
Vascular Plant Detritus is Recalcitrant N
12
Emergent Vegetation
δ15N, ‰ AIR
10
8
Upland Plants
r2= 0.83
6
4
r2= 0.91
2
0
-2
2
2.5
3
3.5
4
4.5
5
δ15N Sedimentary Organic Matter, ‰ AIR
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+
Collection Sites
1
River A
1
River B
1
River C
2
2
2
3
3 values per 1oP,
porewater NH4+, SOM
3
3 values per 1oP,
porewater NH4+, SOM
3
3 values per 1oP,
porewater NH4+, SOM
δ15N, ‰ AIR
Upland Plant Detritus is Not Labile N
16
14
12
10
8
6
4
2
0
Mangroves
Upland Plants
r2= 0.07
r2= 0.32
-2
0
2
4
6
8
10 12
δ15N Sedimentary Porewater NH4+, ‰ AIR
δ15N, ‰ AIR
BMA is Labile N
16
14
12
10
8
6
4
2
0
BMA
r2BMA = 0.99
-2
0
2
4
6
8
10 12
δ15N Sedimentary Porewater NH4+, ‰ AIR
δ15N, ‰ AIR
POM is Labile N
16
14
12
10
8
6
4
2
0
POM
r2BMA = 0.68
-2
0
2
4
6
8
10 12
δ15N Sedimentary Porewater NH4+, ‰ AIR
Which N Pool Supports Fish Production?
Approach: δ13C vs. δ15N plots for all available data:
Fish vs.
Benthic Microalgae (BMA)
Particulate Organic Matter (POM)
Upland Plants (UP)
Mangroves (MAN)
Emergent Vegetation (EM)
Stable Isotopes Link 1oP to Fish
1OC (Benthic inverts)
ΔN=3‰
δ15N ‰ AIR
10
9
8
7
6
5
4
3
2
1
0
-26
1OP
-25
ΔC=1‰
-24
-23
δ13C ‰ PDB
-22
Stable Isotopes Link 1oP to Fish
1OC (Benthic inverts)
ΔN=3‰
2OC (fish)
ΔC=1‰
ΔN=3‰
δ15N ‰ AIR
10
9
8
7
6
5
4
3
2
1
0
-26
1OP
-25
ΔC=1‰
-24
-23
δ13C ‰ PDB
-22
What 1oP Support Fish Production?
Fish
δ15N, ‰ AIR
16
14
12
10
8
6
4
2
0
-30
-28
-26
-24
δ13C, ‰ PDB
-22
-20
Mangroves Do Not Support All Fish
Fish
MAN
δ15N, ‰ AIR
16
14
12
10
8
6
4
2
0
-30
-28
-26
-24
δ13C, ‰ PDB
-22
-20
Emergent Plants Do Not Support All Fish
δ15N, ‰ AIR
16
14
12
10
8
6
4
2
0
Fish
MAN
EM
-30
-28
-26
-24
δ13C, ‰ PDB
-22
-20
Upland Plants Do Not Support All Fish
δ15N, ‰ AIR
16
14
12
10
8
6
4
2
0
Fish
MAN
EM
UP
-30
-28
-26
-24
δ13C, ‰ PDB
-22
-20
Vascular Plants Do Not Support All Fish
δ15N, ‰ AIR
16
14
12
10
8
6
4
2
0
Fish
MAN
EM
UP
-30
-28
-26
-24
δ13C, ‰ PDB
-22
-20
Microalgae Supports Fish Production
δ15N, ‰ AIR
16
14
12
10
8
6
4
2
0
Fish
POM
BMA
TB & CH POM
-30
-28
-26
-24
δ13C, ‰ PDB
-22
-20
Benthic Invertebrates are Intermediates
δ15N, ‰ AIR
16
14
12
10
8
6
4
2
0
Fish
POM
BMA
INV
TB & CH POM
-30
-28
-26
-24
δ13C, ‰ PDB
-22
-20
Conclusions: Parallel Nitrogen Cycles
Vascular Plant Cycle: Recalcitrant OM is Buried
Vascular Plants
Bulk SOM
Burial
Microalgal Cycle: Labile OM is Remineralized
POM
Vascular Plants
BMA
Labile SOM
Bulk SOM
SedNH4+
Recycle
Burial
Foodwebs Are Associated with Labile Cycle
POM
Vascular Plants
BMA
Labile SOM
Bulk SOM
SedNH4+
Recycle
Burial