stream solute export and biogeochemistry in the luquillo mountains, puerto rico

Stream Solute Export and Biogeochemistry in the Luquillo

Mountains, Puerto Rico

Richard L. BreretonWilliam H. McDowell

University of New HampshireUniversity of

New Hampshire

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70% of runoff is withdrawn (Crook 2005)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

Tropical Montane Wet Forest

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

Volcaniclastic:Rio Mameyes

Bisley watershedsQ. Sonadora

GranodioriteRio IcacosQ. Guaba

Quebrada Sonadora - low flow

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

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

El Yunque, 1984

1989 – Hurricane Hugo

(Inches of rain)

Category 4 Hurricane:226 km/hr sustained

Study Area

Bisley Experimental WatershedsBefore Hugo After Hugo

Georges, 1998 Category 3, 175 km/hr sustained

Mameyes headwaters – 2005

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

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)

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

What makes nitrate behave so differently from other

solutes?

Why the slower return to baseline in granodiorite

watersheds?

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

Groundwater monitoring wells

Transects across catena

Bisley – volcaniclastic

Riparian zone:Denitrification “hotspot”periodically anaerobic

C sourcehigh retention time

Modified from McDowell et al. 1992

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

Icacos – slow recovery

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

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

Questions?55035

45030 20

370 20560

5560

Riparian Zone

Slope

Stream:10010

Flows into Icacos

Modified from McDowell et al. 1992

Key:NO3 NH4