| Abstract | Concentration-discharge (C-Q) relationships are poorly known for tropical watersheds, even
though the tropics contribute a disproportionate amount of solutes to the global ocean. The Luquillo
Mountains in Puerto Rico offer an ideal environment to examine C-Q relationships across a heterogeneous
tropical landscape. We use 10–30 years of weekly stream chemistry data across 10 watersheds to examine
C-Q relationships for weathering products (SiO2(aq), Ca21, Mg21, and Na1) and biologically controlled
solutes (dissolved organic carbon [DOC], dissolved organic nitrogen [DON], NH14, NO–
3, PO3–
4 , K1, and SO2–
4 ).
We analyze C-Q relationships using power law equations and a solute production model and use principal
component analysis to test hypotheses regarding how the structure of the critical zone controls solute generation.
Volcaniclastic watersheds had higher concentrations of weathering solutes and smaller tributaries
were approximately threefold more efficient at generating these solutes than larger rivers. Lithology and
vegetation explained a significant amount of variation in the theoretical maximum concentrations of weathering
solutes (r250.43–0.48) and in the C-Q relationships of PO3–
4 (r250.63) and SiO2(aq) (r250.47). However,
the direction and magnitude of these relationships varied. Across watersheds, various forms of N and
P displayed variable C-Q relationships, while DOC was consistently enriched with increasing discharge.
Results suggest that PO3–
4 may be a useful indicator of watershed function. Relationships between C-Q and
landscape characteristics indicate the extent to which the structure and function of the Critical zone controls
watershed solute fluxes. |
