Date of Award

Spring 2021

Project Type

Thesis

Program or Major

Natural Resources

Degree Name

Master of Science

First Advisor

William H McDowell

Second Advisor

Wilfred Wollheim

Third Advisor

Whendee Silver

Abstract

Surface, subsurface, and soil water chemistry of a tropical river system was studied to better understand how ecosystem compartments along hydrologic flowpaths control phosphorus dynamics, what factors control downstream phosphorus trends, and what factors account for spatial variability in PO4 concentrations in a tropical montane watershed. Interactions between ecosystem compartments and changes in the global nitrogen cycle have been well- documented and described, but much less is known in this regard about the global phosphorus [P] cycle. To enhance our understanding of interactions between ecosystem compartments, longitudinal P variability, P-processing at the interfaces, and P-cycling, I conducted a series of observations and experiments in the Icacos watershed of the Luquillo Critical Zone Observatory in Puerto Rico. Batch soil sorption experiments were used to document PO4 sorption along a riparian catena, a series of stream PO4 additions documented uptake into stream sediments from the overlying water column, and longitudinal stream transects of PO4 and the metals (Ca, Al, Fe, Mn, Mg) that may play a role in PO4 retention in stream samples were conducted to assess controls on PO4 concentrations. Results provide insight on downstream trends in PO4, spatial variability of PO4, and overall control of P nutrient dynamics in a tropical montane watershed. PO4 fraction size analysis suggests dissolved PO4 (<.2µm) is primarily taken up biotically or is abiotically converted to fine colloidal P (>.2,<.45 µm), colloidal P (>.45,<.7 µm) is captured and immobilized by bed material, and the fine colloidal fraction persists in downstream export. Stream water and groundwater chemistry suggests losses to groundwater along reaches. Soils observations and studies suggest variation in Icacos soils and evidence of highly oxidized Al- and Fe- oxide patches as one moves away from riparian areas and further in proximity of the reaches and/or further upstream in reaches. Batch soil experiments provide substantial insight on PO4 enrichment and C-Q relationships. High infiltration rates observed in the Icacos watershed in tandem with unique increasing PO4 C-Q relationships and soil sorption results suggest during times of rainfall that soils provide significant enrichment of PO¬4 to streams.

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