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Stream Solute Export and Biogeochemistry in the Luquillo
Mountains, Puerto Rico
Richard L. BreretonWilliam H. McDowell
University of New HampshireUniversity of
New Hampshire
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El Yunque National Forest, PRThe only tropical rainforest in the U.S.
70% of runoff is withdrawn (Crook 2005)
LTER since 1988
CZO since 2009
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Tropical Montane Wet Forest
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Two Very Different Watersheds
Rio MameyesVolcaniclastic bedrock
3.5 m/y of rainLower avg. elevation
Rio Icacos/BlancoGranodiorite
bedrock4.5 m/y of rain
Higher avg. elevation
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Volcaniclastic:Rio Mameyes
Bisley watershedsQ. Sonadora
GranodioriteRio IcacosQ. Guaba
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Quebrada Sonadora - low flow
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Controls on stream chemistry
• Major ions reflect lithology and sea salt
• Dilution at high flows• Silica dilution is among highest
measured (Godsey et al. 2009)• TSS increases with flow• Biogeochemical puzzle: Carbon and
Nitrogen
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Shanley, McDowell, and Stallard 2011 “Boomerang” effect of DOC in the Icacos
0
100
200
300
400
500
1980s1990s2000s
0.1 1 10Discharge, mm hr-1
DO
C c
once
ntra
tion,
µm
ol L-1
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El Yunque, 1984
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1989 – Hurricane Hugo
(Inches of rain)
Category 4 Hurricane:226 km/hr sustained
Study Area
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Bisley Experimental WatershedsBefore Hugo After Hugo
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Georges, 1998 Category 3, 175 km/hr sustained
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Mameyes headwaters – 2005
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Weathering products and DOC:no response to hurricanes
0 50 100 150Months Since Hurricane
0
1
2
3
4
5
DO
C, m
g/L
Icacos after Georges
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Nitrate response in two small watersheds after Georges (1998-
2009)
0 50 100 150Months Since Hurricane
0
100
200
300
400
NO
3N
0 50 100 150Months Since Hurricane
0
100
200
300
400
NO
3-N
, ug/
L Volcaniclastic(Prieta)
Quartz diorite(Guaba)
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Two larger basins, Sonadora (254 ha) and Icacos (326 ha)
after Georges (1998-2009)
0 50 100 150Months Since Hurricane
0
100
200
300
400
NO
3-N
, ug/
L
RIQS
SAMPLE_ID
Pre-Hurricanes1983-1986Rio Icacos
Q. Sonadora
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What makes nitrate behave so differently from other
solutes?
Why the slower return to baseline in granodiorite
watersheds?
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Biogeochemical controlson N flux to streams
• Hydrologic flow path – groundwater? surface runoff?
• Residence time – contact with soils• What is the matrix?• Redox conditions – climate• Riparian denitrificationNO3 N2O N2
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Groundwater monitoring wells
Transects across catena
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Bisley – volcaniclastic
Riparian zone:Denitrification “hotspot”periodically anaerobic
C sourcehigh retention time
Modified from McDowell et al. 1992
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Icacos trib. – granodiorite watershed
55035
45030 20
370 20560
5560
Riparian Zone
Slope
Stream:10010
Flows into Icacos
Modified from McDowell et al. 1992
Key:NO3 NH4
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Icacos – slow recovery
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Conclusions• Luquillo stream chemistry reflects
lithology overlain by vegetation, climate, disturbance history
• Hurricanes cause dramatic shifts in forest biogeochemistry, reflected in stream nitrate peaks
• Riparian zones hold the key to understanding long-term nitrogen dynamics in the Luquillo Mountains
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Acknowledgements• Funding from NSF-LTER, NSF
Ecosystems, NSF-CZO, USFS IITF, UPR, UNH
• Collaborators include F. Scatena, A. Lugo, D. Schaefer, C. Asbury, J. Merriam, J. Potter, and others
• Field and laboratory assistance from M. Salgado, M.J. Sanchez, J. Bithorn, J. Merriam, J. Potter, J. Orlando, and others
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Questions?55035
45030 20
370 20560
5560
Riparian Zone
Slope
Stream:10010
Flows into Icacos
Modified from McDowell et al. 1992
Key:NO3 NH4