Phosphorus storage in coastal
sediments: Will sea-level rise
mobilize P and elevate coastal
fluxes?
Andrea Pain, Jonathan B Martin, Caitlin Young,
Moutousi Roy
University of Florida
Lentein.com
Relevance of P in coastal systems
• Limiting nutrient
• Excess P  eutrophication,
fish kills, degraded water
quality
• Indian River Lagoon: recurring
harmful algal blooms and fish
kills since 2011
• P delivery is regulated by
processes sensitive to
saltwater intrusion. What
changes can we expect with
sea level rise?
Florida Today
1
Global P cycle: reservoirs and fluxes
Sediments:
• Largest P reservoir
• Source or sink of P,
depending on balance
between P burial and
remobilization
Filippelli, 2002
2
Sedimentary P
reservoirs
Photic zone
C106N16P
Diffusion/
advection
Sediment-water interface
Org-P
Remineralization
PO4
Burial
CaCO3
Org-P
Ca-P
FeOOH
Fe-P
Stability ~ Longterm P burial
3
Sedimentary P
reservoirs
Photic zone
C106N16P
Diffusion/
advection
Sediment-water interface
Org-P
Remineralization
PO4
Burial
CaCO3
Org-P
Ca-P
FeOOH
Fe-P
Stability ~ Longterm P burial
4
Freshwater systems
Photic zone
C106N16P
• Efficient P trapping in sediment
• Low P fluxes from sediment
• P limitation of surface water
Sediment-water interface
PO4
FeOOH
Fe-P
”The iron curtain”
Oxic
zone
FeOOH
Fe-P
Diffusion,
advection
Fe2+
P
Microbially driven reactions
5
Saltwater systems
Photic zone
C106N16P
• Low trapping of P in sediment
• Higher P fluxes from sediment
• N limitation of surface water
Sediment-water interface
PO4
FeOOH
FeS
Fe-P
SO42-  HS-
Fe2+ P
Microbially driven reactions
6
Salt and P availability
• Caraco et al. (1999) Evidence for
sulphate-controlled phosphorus release
from sediments of aquatic systems.
Letters to Nature
• Blomqvist et al. (2004) Why the limiting
nutrient differs between temperate
coastal seas and freshwater lakes: A
matter of salt. Limnology and
Oceanography
Caraco et al 1999
If salt increases P release from sediments, what can we expect from large
scale changes in salinity due to sea level rise and saltwater intrusion?
7
Subterranean estuaries (STEs)
(Rotzoll and Fletcher, 2012)
• Freshwater-saltwater
gradients spanning entire
coastlines, sensitive to sea
level rise
• Even small changes in
sediment P storage could be
important if extrapolated to a
large geographical extent
• Approach: look at changes in
modern STE as a function of
salinity as an analog for future
saltwater intrusion
8
Study site: Indian
River Lagoon, FL
Sediment core locations
30 m
Salinity
25
0
20
Depth
(m)
N
10 km
10
2
Field site
15
1
5
September, 2007
0
10
0
20
Distance from shoreline (m)
30
9
Previous work
Roy et al 2011
10
Sedimentary Fe-P distribution
• Sediments leached for Fe-oxides according to SEDEX method (Ruttenberg, 1992), analyzed for
total P content to isolate Fe-bound P (Fe-P) from all other sedimentary P reservoirs
• Fe-P highest in freshest sediments, Fe-P content positively correlates with Fe-oxide content
0
4
Depth
(m)
1
3
2
2
1
00
10
20
Distance from shoreline (m)
30 Fe-P
(µmol/g)
Fe-P (µmol/g)
5
5
4.5
4
3.5
3
2.5
2
1.5
1
0.5
0
y = 0.045x + 1.7936
R² = 0.5567
0
10
20
30
Fe oxide (µmol/g)
11
Estimates of potential Fe-P losses
• Using Roy et al. (2011)
estimation of Fe-S formation
rates and assume:
1. All Fe-S originated as Feoxide
2. Fe-oxide contained Fe-P in
constant ratio (slope of
regression line)
Fe-P loss (mg/m2/year)
60
Shift to higher Fe-P loss
due to Fe-S formation in
previously freshwater
sediments
50
40
Sea level rise
30
20
10
0
0
10
20
30
Distance from shoreline (m)
Fe-P loss (mg/m2/year) = (P content of Fe oxides) * Fe-S precipitation rate
12
Conclusions and implications
1. Freshwater sediments have higher Fe-oxide content, Fe-P content
2. Fe-oxide content and Fe-P are positively correlated
3. Fe-S formation in modern STEs leads to a loss of Fe-P from sediments.
This could be analogous to what will happen due to saltwater intrusion
4. Fe-P loss reaches 50 mg/m2/year at the freshwater-saltwater mixing
zone. If all coastal sediments reached this rate (assume roughly 22.5 m of
inundated sediment, 100 km of coastline), this would equate to an
additional 112.5 kg P/year into Indian River Lagoon.
• Estimates of total P load to the lagoon: 2100-320,000 kg/year  Fe-P loss is 0.035%5.4%. A drop in the bucket, but still represents an irreversible loss of sedimentary P
5. Changes to other sedimentary P reservoirs (organic, loosely sorbed, Cabound) very likely, may increase P loss  further work needed
13
Thank you!
Acknowledgements:
Co-authors and Moutousi Roy
Saini Harshit
Water Institute &
Water Institute Graduate Fellows
National Science Foundation
St. John’s River Water Management District
14
Lentein.com
References
Blomqvist S., Gunnars A. and Elmgren R. (2004) Why the limiting nutrient differs between temperate coastal
seas and freshwater lakes: A matter of salt. Limnol. Oceanogr. 49, 2236–2241.
Caraco, N.F., Cole, J.J., Likens G. E. (1989) Caraco 1999 Evidence for sulphate-controlled phosphorus release
from sediments of aquatic systems.pdf. 1Nature 31, 316–318.
Filippelli G. M. (2002) The Global Phosphorus Cycle. Rev. Mineral. Geochemistry 48, 391–425. Available at:
http://rimg.geoscienceworld.org/cgi/doi/10.2138/rmg.2002.48.10 [Accessed October 21, 2014].
Rotzoll K. and Fletcher C. H. (2012) Assessment of groundwater inundation as a consequence of sea-level rise.
Nat. Clim. Chang. 3, 477–481. Available at: http://www.nature.com/doifinder/10.1038/nclimate1725
[Accessed March 19, 2014].
Roy M., Martin J. B., Smith C. G. and Cable J. E. (2011) Reactive-transport modeling of iron diagenesis and
associated organic carbon remineralization in a Florida (USA) subterranean estuary. Earth Planet. Sci. Lett. 304,
191–201. Available at: http://linkinghub.elsevier.com/retrieve/pii/S0012821X11000586 [Accessed March 19,
2014].
Slomp C. P. and Van Cappellen P. (2004) Nutrient inputs to the coastal ocean through submarine groundwater
discharge: controls and potential impact. J. Hydrol. 295, 64–86. Available at:
http://linkinghub.elsevier.com/retrieve/pii/S002216940400112X [Accessed March 19, 2014].
15
Sedimentary Fe:Fe-P ratio
Fe:Fe-P ratios with salinity
25
20
15
10
5
0
0
10
20
30
• Higher Fe:P ratios at lower
salinity indicate a larger
reservoir of Fe oxides available
to sorb dissolved P.
• Lower Fe:P ratios at higher
salinity indicate that Fe-oxide
reservoirs are 1) absent or 2)
more saturated with respect to P
sorption sites and have lower
potential to sorb dissolved P
Pore water salinity
16